Dissecting Multivalent Lectin-Carbohydrate Recognition Using Polyvalent Multifunctional Glycan-Quantum Dots

Article abstract of DOI:10.1021/jacs.7b05104

Multivalent protein-carbohydrate interactions initiate the first contacts between virus/bacteria and target cells, which ultimately lead to infection. Understanding the structures and binding modes involved is vital to the design of specific, potent multivalent inhibitors. However, the lack of structural information on such flexible, complex, and multimeric cell surface membrane proteins has often hampered such endeavors. Herein, we report that quantum dots (QDs) displayed with a dense array of mono-/disaccharides are powerful probes for multivalent protein-glycan interactions. Using a pair of closely related tetrameric lectins, DC-SIGN and DC-SIGNR, which bind to the HIV and Ebola virus glycoproteins (EBOV-GP) to augment viral entry and infect target cells, we show that such QDs efficiently dissect the different DC-SIGN/R-glycan binding modes (tetra-/di-/monovalent) through a combination of multimodal readouts: F?rster resonance energy transfer (FRET), hydrodynamic size measurement, and transmission electron microscopy imaging. We also report a new QD-FRET method for quantifying QD-DC-SIGN/R binding affinity, revealing that DC-SIGN binds to the QD >100-fold tighter than does DC-SIGNR. This result is consistent with DC-SIGN's higher trans-infection efficiency of some HIV strains over DC-SIGNR. Finally, we show that the QDs potently inhibit DC-SIGN-mediated enhancement of EBOV-GP-driven transduction of target cells with IC₅₀ values down to 0.7 nM, matching well to their DC-SIGN binding constant (apparent K_d = 0.6 nM) measured by FRET. These results suggest that the glycan-QDs are powerful multifunctional probes for dissecting multivalent protein-ligand recognition and predicting glyconanoparticle inhibition of virus infection at the cellular level.

Full text of DOI:10.1021/jacs.7b05104

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Article
Dissecting Multivalent Lectin-Carbohydrate Recognition
Using Polyvalent Multifunctional Glycan-Quantum Dots
Yuan Guo, Inga Nehlmeier, Emma Poole, Chadamas Sakonsinsiri, Nicole
Hondow, Andy Brown, Qing Li, Shuang Li, Jessie Whitworth, Zhongjun Li,
Anchi Yu, Rik Brydson, W. Bruce Turnbull, Stefan Pöhlmann, and Dejian Zhou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05104 • Publication Date (Web): 08 Aug 2017
Downloaded from http://pubs.acs.org on August 8, 2017
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Dissecting Multivalent Lectin-Carbohydrate Recognition Using
Polyvalent Multifunctional Glycan-Quantum Dots
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Yuan Guo†,*, Inga Nehlmeier‡, Emma Poole†, Chadamas Sakonsinsiri†,1 Nicole Hondowᶲ, Andy
Brown,ᶲ Qing Li§, Shuang Li¶, Jessie Whitworth†, Zhongjun Li§, Anchi Yu¶, Rik Brydsonᶲ,
W. Bruce Turnbull†, Stefan Pöhlmann‡, and Dejian Zhou†,*
† School of Chemistry and Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds LS2
9JT, United Kingdom.
‡ Infection Biology Unit, German Primate Center, Kellnerweg 4, 37077 Gottingen, Germany.
ᶲSchool of Chemical and Process Engineering, University of Leeds, Leeds LS2 9JT, UK.
§ Department of Chemical Biology, Peking University Health Sciences Centre, Beijing 100191, P.R. China.
¶ Department of Chemistry, Renmin University of China, Beijing 100872, P.R. China.
1 Current Address: Department of Biochemistry, Faculty of Medicine, Khon Kaen University, Khon Kaen 40002,
Thailand.
Keywords: Quantum dot, DC-SIGN, multivalency, multi-modal readout, glyconanoparticle, virus
inhibition, Förster resonance energy transfer
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Abstract
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Multivalent protein-carbohydrate interactions initiate the first contacts between virus/bacteria and target
cells which ultimately lead to infection. Understanding the structures and binding modes involved is vital
to the design of specific, potent multivalent inhibitors. However, the lack of structural information on
such flexible, complex and multimeric cell surface membrane proteins has often hampered such
endeavours. Herein we report that quantum dots (QDs) displayed with a dense array of mono-/di-
saccharides are powerful probes for multivalent protein-glycan interactions. Using a pair of closely
related tetrameric lectins, DC-SIGN and DC-SIGNR, which bind to the HIV and Ebola virus
glycoproteins (EBOV-GP) to augment viral entry and infect target cells, we show that such QDs
efficiently dissect the different DC-SIGN/R-glycan binding modes (tetra-/di-/mono- valent) through a
combination of multi-modal readouts: Förster resonance energy transfer (FRET), hydrodynamic size
measurement and transmission electron microscopy imaging. We also report a new QD-FRET method
for quantifying QD-DC-SIGN/R binding affinity, revealing that DC-SIGN binds to the QD >100 fold
tighter than DC-SIGNR. This result is consistent with DC-SIGN’s higher trans-infection efficiency of
some HIV strains over DC-SIGNR. Finally, we show that the QDs potently inhibit DC-SIGN-mediated
enhancement of EBOV-GP-driven transduction of target cells with IC50 values down to 0.7 nM, matching
well to their DC-SIGN binding constant (apparent Kd = 0.6 nM) measured by FRET. These results
suggest that the glycan-QDs are powerful multifunctional probes for dissecting multivalent protein-
ligand recognition and predicting glyconanoparticle inhibition of virus infection at the cellular level.
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Introduction
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Multivalent protein-carbohydrate interactions are widespread in biology and play a central role in many
important biological events, including viral and bacterial infection, cell-cell communication and host
immune response regulation.1-5 Such interactions initiate the first contact between pathogens (e.g. viruses
and bacteria) and target cells that ultimately leads to infection. However, monovalent protein-glycan
interactions are intrinsically weak, and hence biologically inactive. To compensate this limitation,
pathogens display arrays of specific glycans on their surface, allowing them to bind efficiently to
multimeric glycan-binding proteins (lectins) on target cell surfaces and to exploit multivalency to
enhance binding affinity and gain cell entry which ultimately lead to infection.3-6 Therefore, these
multivalent interactions are attractive targets for developing novel antiviral interventions, especially
entry inhibitors which can minimize virus resistance development.1-5,7 In this regard, the spatial- and
orientation- match between the viral surface glycans and carbohydrate recognition domains (CRDs) of
cell surface lectins is key to enhance binding affinity and specificity.8,9 Therefore, understanding the
structure and spatial arrangement of the multivalent binding partners is essential for antiviral intervention
which has been of focus of significant current research.1-5,7,10
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Synthetic glycoconjugates can block pathogen-lectin interactions whose inhibitory potency critically
depends on the spatial- and orientation- match between the multivalent binding partners.3,8-14 However,
a major challenge is the lack of structural information for many cell surface multimeric lectins, due to
the problems associated with solving the structure of such flexible, complex and multimeric membrane
proteins by X-ray crystallography. For example, despite extensive research over the past two decades,
the complete crystal structures of two important pathogen receptors, the tetrameric dendritic cell receptor,
DC-SIGN,15-20 and its closely related endothelial cell receptor DC-SIGNR21 (collectively abbreviated as
DC-SIGN/R hereafter), remain unknown apart from a structure model built upon the solution X-ray
scattering data.20 These two receptors play a key role in promoting HIV/Ebola virus (EBOV) infection
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by binding to multiple mannose-containing glycans on the virus surface.15,18,19,21-23 Interestingly, despite
sharing 77% amino acid identity, an overall tetrameric structure,24,25 and identical individual CRD-
mannose binding motifs,17 these two receptors can differentially augment viral infectivity. For example,
DC-SIGN is more effective in augmenting the infectivity of some HIV strains than DC-SIGNR,18,23 while
only DC-SIGNR, but not DC-SIGN, can effectively promote West Nile virus infection.26 Given their
close similarity, such differences must result from their different multivalent binding properties, arising
presumably from the different spatial and orientation arrangements of their four CRDs which have been
shown to be flexibly linked to the neck domain.27 These observations make DC-SIGN/R an ideal pair of
model multimeric proteins to investigate how subtle structural differences influence multivalent protein-
glycan interactions. Unfortunately, the widely used biophysical techniques (e.g. isothermal titration
calorimetry (ITC)28,29 and surface plasmon resonance (SPR)30), although powerful in providing
quantitative binding affinities, kinetics, and thermodynamics, cannot reveal the structural information
(e.g. binding mode, binding site distance and orientation) which is key to the design of potent multivalent
inhibitors.1,8,9,11 Therefore, there appear to be a clear capability gap of current methods in dissecting such
multivalent lectin-glycan interactions.
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Herein we propose that this capability gap may be addressed by developing a polyvalent glycan-quantum
dot (QD-glycan)-based multimodal readout strategy to fully exploit multivalency and QD’s unique
properties. First, the QD’s unique, size-dependent, strong and stable fluorescence31-33 can be harnessed
for binding quantification via a Förster resonance energy transfer (FRET) based ratiometric readout.34-38
Compared to other methods (e.g. SPR and ITC), the QD-FRET readout has the advantages of rapid,
separation-free detection in solution, high sensitivity, and a ratiometric readout signal with self-
calibration function, making it much less sensitive to instrument noise and signal fluctuation, allowing
for highly robust, accurate detection.34-37 Indeed, the QD-FRET technique has been widely employed to
address broad biological and biomedical problems, e.g. bio-/enzymatic-/intracellular- sensing, immuno-
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assays, cell monitoring and tracking,31-46 and more recently to probe multivalent protein-glycan
interactions.47 Second, the solid nanoscale core of the QD can be decorated with polyvalent specific
glycan ligands to enhance binding affinity by exploiting multivalency. Third, the QD-protein binding
can be directly monitored in solution by dynamic light scattering via binding induced hydrodynamic size
changes. Finally, the high contrast of the QD core in scanning transmission electron microscopy (STEM)
can be harnessed to directly visualize binding-induced particle arrangements so as to probe the exact
binding mode. Despite extensive research, most QD-FRET work reported so far have only utilized the
fluorescence property of the QD, hence the unique multi-functionality of the QD probe has not been fully
exploited. For example, using the QD-FRET readout strategy, we have recently found that compact
polyvalent mono-mannose-capped QDs (QD-Man) specifically bind to DC-SIGN, but not to DC-SIGNR.
We have also proposed that the four CRDs face upwards in DC-SIGN, but point sideways in DC-SIGNR,
making the latter unable to bind multivalently (>2) to one QD.47 However, QD-Man failed to differentiate
binding of DC-SIGNR and monovalent CRD (SI, Figure S1),47 possibly due to the fact that the individual
CRD-mannose binding is too weak to measure at low concentrations. Therefore, the overall QD-Man-
DC-SIGN/R binding modes remain unclear. Herein we solved this problem by increasing the individual
CRD binding affinity of the glycan displayed on the QD and by developing a novel multimodal readout
strategy comprising FRET, hydrodynamic size measurement and S/TEM imaging to fully exploit the
unique multi-functionality of the glycan-QD. We further show that there is a good correlation between
the QDs’ DC-SIGN/R binding affinity and their virus inhibition potency.
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Results and Discussion
Glycan ligand design and synthesis. To increase individual CRD-glycan binding affinity, manno-
pyranosyl-α-1,2-manno-pyranose (DiMan) was coupled to the terminal end of the dihydrolipoic acid-
oligo(ethylene glycol)- based multifunctional ligands48,49 (abbreviated as DHLA-EGn-DiMan hereafter,
where n = 3 or 11 stands for a uniform linker containing 3 or 11 EG units, respectively) using the route
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described in Scheme 1. For comparison, their mono-mannosyl equivalent ligands (e.g. DHLA-EGn-Man,
n =3 or 11) were also synthesised as described previously.47 Individual DiMan-CRD binding is ~4 times
as strong as that of Man-CRD (Kd ~0.9 versus 3.5 mM),29 allowing us to investigate how individual
CRD-glycan affinity contributes to the overall QD-glycan-DC-SIGN/R multivalent binding.
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Scheme 1. The synthetic route to DHLA-EGn-DiMan (where n = 3 or 11). Reaction conditions were: (i)
DCC/DMAP, DCM; (ii) triphenyl-phosphine, EtOAc/H2O; (iii) DCC/DMAP, DCM; (iv) BF3.OEt2,
DCM, molecular sieves, (v) NaN3/TBAI, DMF; (vi) NaOMe, MeOH, then Amberlite H+ resin; (vii)
EtOH; (viii) TCEP.HCl, CHCl3/EtOH/H2O.
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Briefly, Lipoic acid (LA) was first coupled to NH2-EGn-N3 (n = 3, 11) to form LA-EGn-N3 (step i). It was
then reduced to LA-EGn-NH2 by triphenyl-phosphine (step ii), and then coupled to cyclooctyne-COOH
to form LA-EGn-cyclooctyne (step iii). Meanwhile, 3,4,6-tri-O-acetyl-2-O-(2,3,4,6-tetra-O-acetyl--D-
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mannopyranosyl)--D-mannopyranosyl-trichloroacetimidate was reacted with 2-[2-(2-chloroethoxy)
ethoxy]ethanol to introduce a EG2 linker (step iv), which was then treated with NaN3 to convert the linker
terminal hydroxyl group into an azide (step v), after removal of the acetyl protection groups, the azide
modified glycan, (1-azido-3,6-dioxaoct-8-yl-2-O--D-manno-pyranosyl--D-mannopyranoside), was
obtained (step vi). After that, the azide modified glycan was coupled to LA-EGn-cyclooctyne via the Cu-
free “click” chemistry to give LA-EGn-glycan (step vii), and reduction by tris(2-carboxyethyl)phosphine
(TCEP) gave the desired final DHLA-EGn-Glycan ligand. Details of the synthesis procedures and their
spectroscopic data are given in the Supporting Information (SI). Each DHLA-EGn-glycan ligand contains
three different functional domains: a DHLA for robust chelative QD capping;48,50 a hydrophilic, flexible
EGn linker for imposing high water-solubility, stability and resistance against non-specific adsorption as
well as for tuning the inter-sugar spacing; and a terminal glycan for specific protein binding.47
Preparation and characterization of glycan-QDs. The DHLA-EGn-glycan ligands, after deprotonation
by NaOH, were directly used to initiate cap-exchange with a commercial hydrophobic CdSe/ZnS QD
(EM ~560 nm) in a homogenous solution using our recently developed highly efficient cap-exchange
method.47 Details of the cap-exchange procedures were given in the SI, part 3. All of the resulting DHLA-
EGn-glycan capped QDs (abbreviated as QD-EGn-glycan hereafter) formed highly stable dispersions in
aqueous media, and displayed no noticeable changes in appearance or fluorescence over times >1 month.
Moreover, the QDs were compact and uniform in size, displaying a small hydrodynamic diameter (Dh)
of 8.3 and 9.5 nm for QD-EG3-DiMan and QD-EG11-DiMan, respectively (Figure S2),51 suggesting the
formation of isolated, aggregation-free QD dispersions.52-55 Importantly, the QDs were densely capped
with the glycan ligands (glycan valency >220), which would be difficult to achieve by other methods
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(e.g. post cap-exchange chemical coupling). Using the Dh values and corresponding glycan valencies,
the average inter-glycan distances (d) of the QD-EGn-glycans were estimated in the range of 0.9-1.3 nm
(Table 1, see SI Section 6.1 for calculation method). Interestingly, this distance matches well to the
average inter-glycan sequon distance (~1.2 nm) found on the HIV surface heavily glycosylated
glycoprotein, gp120.56,57 Moreover, the QD surface glycan density and inter-glycan distance (d) can be
readily tuned by varying the linker length and diluting the DHLA-EGn-glycan ligand using an inert
hydrophilic spacer ligand, DHLA-zwitterion, during the cap-exchange process (see Figure 1).
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Table 1. Summary of the chemical and physical parameters of the QD-EGn-glycan conjugates.
QD surface Ligands Glycan
Dh (nm)
Inter-glycan
spacing (nm)
0.98±0.11
Glycan footprint on
QD surface (nm2)
0.75±0.16
valency
DHLA-EG3-Man
DHLA-EG11-Man
DHLA-EG3-DiMan
DHLA-EG11-DiMan
330±70
222±62
369±38
281±25
8.9±0.1
9.6±0.2
8.3±0.1
9.5±0.1
1.29±0.36
0.86±0.09
1.13±0.10
1.30 ±0.36
0.59 ± 0.06
1.01±0.09
Figure 1. (A) Schematic showing our approach to quantify QD-glycan-DC-SIGN/R multivalent binding
by QD-sensitised dye FRET mechanism. (B, C, D) Schematic presentation of tuning the QD surface
glycan valency and inter-glycan distance (d) via EG linker length (n = 3 for B; n = 11 for C) and glycan
dilution with an inert DHLA-zwitterion spacer ligand (D).
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Differentiating QD-DiMan-DC-SIGN/R binding modes by FRET. The DC-SIGN/R proteins were
expressed and labeled with an Atto-594 dye at a site-specifically introduced cysteine residue on the CRD
(SI part 4).47 The chosen mutation residue (Q274 in DC-SIGN and R287 in DC-SIGNR) is located out
with the glycan binding sites, minimizing any possible interference with CRD glycan binding. The QD
emission has good overlap with the Atto-594 absorption, ensuring that efficient FRET can occur (Förster
radius R0 = ~4.0 nm, Figure S3), but has minimal overlap of dye emission spectra, allowing for easy
separation of the QD and dye FRET signal without the need of spectral deconvolution. We first screened
the QD-glycan-DC-SIGN/R binding by titrating different amounts of labeled proteins into a fixed
concentration of the QD-glycan (40 nM) in a binding buffer (20 mM HEPES, 100 mM NaCl, 10 mM
CaCl2, pH 7.8). The resulting fluorescence spectra were shown in Figure 2 (all spectra have been
corrected by dye direct excitation background). Similar to QD-Man, incubation of QD-DiMan with the
labeled DC-SIGN resulted in significant quenching of QD fluorescence (EM ~554 nm) and an concurrent
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enhancement of the Atto-594 FRET signal (EM ~626 nm), consistent with a QD-sensitised Atto-594
FRET mechanism (Figure 2). Moreover, the FRET signal was found to be strongly Ca2+-dependent and
was completely diminished in the absence of Ca2+ (SI, Figure S4). This observation is fully consistent
with the Ca2+-dependency of the DC-SIGN-glycan binding.17,24 Despite such similarities, three major
differences between QD- DiMan and QD-Man binding to DC-SIGN/R were observed:
(1) Most significantly, binding of DC-SIGNR to QD-DiMan produced notable FRET signals which were
markedly higher and well-separated from those of the monovalent CRDs (Figure 2 and SI, Figure S5), a
sharp contrast to that of QD-Man where signals obtained from DC-SIGNR and monovalent CRD binding
were equally weak and indistinguishable from non-specific adsorption background (SI, Figure S1).47
Moreover, the apparent FRET ratios (I626/I554) obtained from the monovalent CRD-QD-DiMan binding
were still indistinguishable from the background, suggesting that monovalent binding is too weak (Kd
~0.9 mM)29 to measure with 40 nM QD. Given that the I626/I554 ratio is linearly correlated to the number
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of acceptors (proteins) bound to each QD in the absence of other quenching effect (see SI, section 5.5),47
this result implies that DC-SIGNR-QD-DiMan binding is multivalent, and not monovalent, otherwise
similar FRET ratios would have been expected.
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DC-SIGN
DC-SIGNR
CRD
2.0
1.5
1.0
0.5
0.0
2.0
1.5
1.0
0.5
0.0
2.0
1.5
1.0
0.5
0
(A)
(B)
(C)
(D)
4
3
2
1
0
0.3
0.6
1.3
1.6
3.1
4.7
6.3
12.5
18.8
0
0
2.1
4.2
8.4
12.6
16.8
0.6
1.3
DC-SIGN
DC-SIGNR
DC-SIGN CRD
DC-SIGNR CRD
2.5
3.1
4.7
6.3
12.5
18.8
0.0
500
550
600
650
700
750
500
550
600
650
700
750 500
550
600
650
700
750
0
5
10
15
20
nm
Protein/QD Ratio
/nm
nm
2.0
1.5
1.0
0.5
0.0
2.0
1.5
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DC-SIGN
DC-SIGNR
DC-SIGN CRD
DC-SIGNR CRD
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600nm
Protein/QD Ratio
nm
nm
Figure 2. Dye-direct excitation background-corrected fluorescence spectra of QD-DiMan (100% glycan
density) after binding to Atto-594-labeled proteins at different protein:QD ratios (PQR): QD-EG11-
DiMan + DC-SIGN (A); QD-EG11-DiMan + DC-SIGNR (B); QD-EG11-DiMan + DC-SIGN CRD (C);
QD-EG3-DiMan + DC-SIGN (E); QD-EG3-DiMan + DC-SIGNR (F), QD-EG3-DiMan + DC-SIGN
CRD (G), and the resulting I626/I554 ratio v.s. PQR relationship for QD-EG11-DiMan (D) and QD-EG3-
DiMan (H). DC-SIGNR binding data are fitted by Hill’s equation, y = Rmax × xn/(kn + xn), where Rmax is
the maximum I626/I554 ratio, k is the PQR value that gives 50% Rmax, and n is the Hill coefficient. The
best fit parameters are Rmax = 2.6±0.9, k = 17.2±6.1, n = 1.5±0.1 and R2 = 0.9984 for QD-EG11-DiMan
and Rmax =1.7±0.2, k = 9.7±1.5, n = 2.4±0.4 and R2 = 0.9991 for QD-EG3-DiMan.
(2) Binding of DC-SIGN to QD-DiMan still produced a much stronger FRET signal than that of DC-
SIGNR (Figure 2), suggesting that DC-SIGN must display a higher binding multivalency to one QD than
DC-SIGNR. These results are not unexpected, and in fact, they are fully consistent with our proposed
QD-DC-SIGN/R binding models.47 The four side-way facing CRDs in DC-SIGNR may split into two
pairs and bind divalently with two different QDs. This binding mode should result in positive binding
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cooperativity. Fitting the DC-SIGNR binding curves by the Hill’s function: y = Rmax × xn/(kn + xn), where
Rmax is the maximum I626/I554 ratio, k is the protein:QD ratio (PQR) that gives 50% Rmax, and n is the Hill
coefficient, indeed revealed that the n for both QD-EG11-DiMan (1.5±0.1) and QD-EG3-DiMan (2.4±0.4)
were > 1, clearly confirming positive binding cooperatively (Figure 2D/2H). In contrast, the four
upwardly facing CRDs in DC-SIGN may bind tetravalently to a single QD which should produce no
binding cooperativity (n ≤ 1). Indeed, a similar Hill’s fit of the DC-SIGN binding curves with QD-DiMan
with 25% glycan density revealed the n to be 0.85±0.15 for QD-EG11-DiMan and 1.0±0.3 for QD-EG3-
DiMan, confirming no binding cooperativity (Figure 3). Here the QD surface glycan density used in DC-
SIGN binding was diluted to 25% by DHLA-zwitterion ligand to avoid FRET quenching observed with
100% glycan density QDs at high PQRs (see Figure 2D/2H and the next section). Therefore, the different
binding multivalency modes of DC-SIGN/R have been successfully differentiated via polyvalent QD-
DiMan binding and a ratiometric FRET readout strategy.
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(3) Interestingly, unlike the QD-Man-DC-SIGN binding, where the apparent FRET ratio (I626/I554)
followed a typical binding pattern with increasing PQR before reaching saturation (Figure S1), the QD-
DiMan-DC-SIGN interaction exhibited a distinct two-stage response (Figure 2I/J). The I626/I554 ratio
initially increased roughly linearly with the PQR at < 6 as expected for a single QD-donor in a FRET
interaction with N identical receptors model; however, the I626/I554 ratio then decreased with the
increasing PQR at >6. Using the surface areas calculated from the Dhs of the QDs and the DC-SIGN
head footprint, the number of DC-SIGN molecules that can be packed onto the QD surface without
crowding was estimated as ~6/~8 for QD-EG3-DiMan/QD-EG11-DiMan, respectively (Figure S6). These
numbers approximately match the critical PQR (the turning point on the FRET response curve),
suggesting that surface crowding is responsible for the observed FRET decrease. The quenching is likely
due to crowding-induced reorganisation of the QD-bound DC-SIGNs via their flexible neck region27 that
brings the dyes in proximity to each other and causes mutual quenching. This assignment was supported
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by that no quenching was observed for the labeled DC-SIGN only under equivalent concentrations in the
absence of QD-DiMan (Figure S7). Consistent with these results, the fluorescence lifetime of the QD-
EG11-DiMan (14.75 ns) was reduced to 7.75 ns and further to 1.76 ns as PQR increased from 3 to 10.
Meanwhile, the dye lifetime (3.48 ns for protein only) was increased to 8.09 ns at PQR = 3, but then
decreased to 4.00 ns at PQR =10 (Figure S8).58 Interestingly, diluting the QD surface DHLA-EGn-glycan
density to 25% with DHLA-ZW removed the two stage behaviour and the binding curves returned to
their normal shape without FRET quenching at high PQRs (Figure 3C/3F). However, it also produced
significantly lower (~4 fold) FRET ratios at saturation (Figure 3C/3F), suggesting a significantly reduced
DC-SIGN binding capacity for the 25% glycan-QD. This result further supports the proposal of surface
crowding induced CRD reorganisation being responsible for the FRET quenching observed with the
100% glycan-QDs under high PQRs.
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2.0
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DC-SIGN
DC-SIGNR
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Protein/QD Ratio
nm
/nm
Figure 3. Dye-direct excitation background-corrected fluorescence spectra of QD-EGn-DiMan (with
25% glycan density diluted by DHLA-ZW ligands) after binding to Atto-594-labeled proteins at different
PQRs: QD-EG11-DiMan + DC-SIGN (A); QD-EG11-DiMan + DC-SIGNR (B); QD-EG3-DiMan + DC-
SIGN (D); QD-EG3-DiMan + DC-SIGNR (E) and the I626/I554 ratio v.s. PQR relationship for QD-EG11-
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DiMan (C) and QD-EG3-DiMan (F). The DC-SIGN binding data were fitted to the Hill’s equation, giving
Rmax = 1.2±0.2, k = 4.9±2.1, n = 0.85±0.15, and R2 = 0.9918 for QD-EG11-DiMan and Rmax =5.4±1.6, k
= 4.4±3.1, n = 1.0±0.3, and R2 = 0.9994 for QD-EG3-DiMan.
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Using the FRET efficiency obtained from the QD quenching (e.g. E = 1 – I/I0, where I and I0 are the
fluorescence intensities of the QD with and without the protein, respectively) and a single QD in FRET
interaction with N identical acceptor model, E = 1/[1 + (r/R0)6/N],34 the average QD-dye distance r was
calculated to be ~5.2 and ~5.7 nm for DC-SIGN binding to QD-EG3-DiMan and QD-EG11-DiMan,
respectively (SI, Figure S3C & D). Both r values were ~1 nm longer than the hydrodynamic radii of the
corresponding QD-EGn-glycans (e.g. ~4.2 and ~4.8 nm). This result is not unreasonable considering the
distance between the dye labeling position and the glycan binding site, as well as the flexible nature of
the EGn linker which may become more extended upon protein binding. However, the equivalent FRET
efficiency v.s. dye:QD ratio responses for DC-SIGNR binding to QD-DiMan were S-shaped and could
not be fitted by the single QD in FRET interaction with N identical acceptor model (SI, Figure S3C/D).
The relatively weak binding between DC-SIGNR and QD-DiMan (>100 fold weaker than that of DC-
SIGN equivalent, see the next section) and positive binding cooperatively may have led to the S-shape
response curve: presumably because DC-SIGNR added under low PQRs was unable to bind efficiently
to QD-DiMan to produce efficient FRET at the early stages of titration.
Quantifying QD-glycan-DC-SIGN/R binding affinity by FRET. The different QD-binding modes and
multivalency exhibited by DC-SIGN/R should result in differing binding affinities (Kds). Theoretically,
the I626/I554 ratio is linearly correlated to the numbers of acceptors (proteins) bound to the QD, making it
a reliable signal for quantifying the proportion of the bound QD-protein complexes in a QD/protein
mixture (SI part 5.4).47,59 Here we have developed a new method by simultaneously changing the
QD/protein concentration whilst keeping PQR fixed at 1 for DC-SIGN (to avoid the FRET quenching at
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high PQRs) or 10 for DC-SIGNR (to compensate the low FRET ratio at PQR = 1, Figure S9). Under
such conditions, the I626/I554 intensity ratio can provide a true reflection of the fraction of bound QD-
protein complexes within the QD-protein mixture. The experiments were performed in the binding buffer
containing 1 mg/ml of bovine serum albumin (BSA) to minimise the possible non-specific adsorption of
QD/protein on surfaces which were non-negligible at low concentrations (<10 nM).59 The resulting
fluorescence spectra revealed that both the dye FRET and QD fluorescence signals increased with the
increasing concentration (SI, Figure S9). However, the former increased faster than the latter, giving an
increased I626/I554 ratio with the increasing concentration. The resulting I626/I554 ratio-concentration
relationships were fitted by the Hill’s equation to derive the apparent dissociation constants (Kds, Figure
4). The parameters derived from the best fits were summarized in Table 2.
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(E)
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0.2
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0.0
Apparent Kd = 35±7 nM
Apparent Kd = 0.61±0.07 nM
Apparent Kd = 62±8 nM
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(F)
Apparent Kd = 633±77 nM
Apparent Kd = 714±18 nM
Apparent Kd = 2.1±0.5 nM
0
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2000
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C/nM
C/nM
C/nM
Figure 4. Relationship between the I626/I554 ratio and protein concentration for a fixed protein:QD molar
ratio of 1:1 for DC-SIGN and 10:1 for DC-SIGNR. (A): DC-SIGN + QD-EG3-Man; (B) DC-SIGN +
QD-EG11-Man; (C) DC-SIGN + QD-EG3-DiMan; (D) DC-SIGN + QD-EG11-DiMan; (E) DC-SIGNR +
QD-EG3-DiMan; and (F) DC-SIGNR + QD-EG11-DiMan. Data were fitted by the Hill’s equation, Y =
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n
Rmax×Cn/[Kd +Cn], where Rmax, Kd, n and C are the maximum I626/I554 ratio, apparent Kd, Hill coefficient,
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and protein concentration, respectively. The fitting parameters were summarised in SI, Table S1.
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Table 2. Key chemical and biophysical parameters of the QD-EGn-glycans and their binding affinities
with DC-SIGN/R measured by FRET.
QD Surface Ligands
Glycan
valency (N) DC-SIGN (nM)
Apparent Kd
Apparent Kd
DC-SIGNR (nM)
/
Enhancement
factor
100,000
/N
~300
DHLA-EG3-Man
DHLA-EG11-Man
DHLA-EG3-DiMan
DHLA-EG11-DiMan
330±70
222±62
369±38
281±25
35±7
714±18
0.61±0.07
2.1±0.5
/
4,900
~22
62 ± 8
633±77
1,480,000
430,000
~4,000
~1,500
* DC-SIGN affinity enhancement factor is calculated by:  Kd (CRD-Glycan)/Apparent Kd (DC-SIGN-QD), where Kd CRD-
Man and Kd CRD-DiMan are 3.5 and 0.9 mM, respectively.29
Four notable findings are revealed by Table 2. First, a polyvalent display of DiMan on the QD greatly
enhanced its affinity for DC-SIGN: a remarkably low apparent Kd of 610 pM was achieved with QD-
EG3-DiMan, translating to a massive ~1.5 million fold affinity enhancement ( over the monovalent
CRD-DiMan binding (Kd ~0.9 mM),29 and a normalised per sugar enhancement factor, /N, of ~4000.
Second, although a polyvalent display of Man on the QD also enhanced its DC-SIGN affinity, the level
of enhancement was significantly lower than that of the DiMan equivalent (< 1/10 in /N terms). This
difference may be due to the extended binding surface of the DC-SIGN CRD which contains both
primary and secondary binding sites.17,24 For QD-Man, it may bind mainly to the primary site whereas
QD-DiMan may bind to both primary and secondary sites, leading to greater affinity enhancement. Third,
the apparent Kd for QD-DiMan binding to DC-SIGN was found to be >100 fold lower than that to DC-
SIGNR, suggesting that DC-SIGN’s binding affinity is >100 fold stronger than that of DC-SIGNR. Given
that each HIV surface gp120 trimer spike is densely coated with mannose containing glycans57 and is of
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comparable size (~12 nm)60 to a QD-DiMan, this result thus provides a plausible explanation why DC-
SIGN has been found to be more effective in trans-infecting some HIV strains than DC-SIGNR.23
Moreover, this result explains the reason why wild-type DC-SIGNR was unable to compete off DC-
SIGN from binding to QD-DiMan observed in the next section. Finally, the flexible EG linker also had
a significant impact on the overall binding affinity: increasing the linker length from 3 to 11 EG units led
to >3 fold lower affinity. This is presumably because the longer the EG linker the more flexible and
disordered the terminal glycans will be, hence there is a greater entropic penalty to pay upon DC-SIGN
binding. Nevertheless, a suitable EG linker is essential to impose high QD stability in aqueous media and
to minimise non-specific interactions with non-target proteins.
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Confirming QD-DC-SIGN/R binding specificity using wild-type receptor competition. A FRET
competition experiment using unlabeled wild-type proteins was further employed to confirm that the
labeled DC-SIGN/R (containing a site-specific cysteine mutation and Atto-594 labeling, see SI, section
4)-QD binding truly reflected wild-type protein binding properties. The experiment was performed on
the QDs with 25% glycan density to overcome the FRET quenching problem observed with 100%
glycan-QD (Figure S10A). As wild-type protein:labeled DC-SIGN ratio (WLR) increased, the FRET
signal reduced progressively while the QD fluorescence correspondingly recovered (Figure 5A),
confirming that wild-type DC-SIGN successfully displaced labeled DC-SIGNs from binding to the QD.
In contrast, wild-type DC-SIGNR caused no apparent changes to either the QD or the FRET signal
(Figure 4B), suggesting no binding competition occurred. These results indicate that wild-type and
labeled DC-SIGN molecules must bind to the same sugar sites (same binding mode) on the QD surface,
whereas DC-SIGNR may be too weak to displace the labeled DC-SIGN from binding to the QD. Their
different competition efficiencies were clearer in the normalised I626/I554 versus WLR plots (Figure 4C),
where DC-SIGNR gave no apparent changes but DC-SIGN yielded significantly reduced FRET ratios.
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This result is not unexpected because DC-SIGNR-QD-DiMan binding is >100 fold weaker than that of
the equivalent DC-SIGN interaction (see previous section, Table 2).
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nm
WLR
Figure 5. Dye-direct excitation background corrected fluorescence spectra of QD-EG3-DiMan(25%)/
DHLA-ZW(75%) + Atto-594 labeled DC-SIGN (PQR = 12.5 to ensure saturate protein binding) after
mixing with different amounts of wild-type DC-SIGN (A) or DC-SIGNR (B). The QD-only fluorescence
spectra in the absence of proteins are also displayed for comparison (shown in black open circles). (C)
Plots of the corresponding normalized I626/I554 ratio versus wild-type protein:labeled DC-SIGN molar
ratio (WLR) fitted by a competitive binding model.
The relative affinity between wild-type and labeled DC-SIGN for the QD-DiMan binding was further
evaluated by a simple competitive model, F = IR50/[IR50 +CWT/CLP], where F is the FRET ratio in the
presence of wild-type protein normalised by that without, CWT and CLP are wild-type and labeled protein
concentrations, respectively, and IR50 is the molar ratio of wild type DC-SIGN:labeled DC-SIGN
required to reduce F by 50%. An IR50 value of 1 indicates that both proteins bind to the QD with equal
affinity, while an IR50 value of < 1 indicates that the labeled protein binds more weakly than wild-type
protein. Fitting the data using this model gave an IR50 value of 0.88 and 0.37 for QD-EG11-DiMan and
QD-EG3-DiMan, respectively (Figure 5, and SI, Figure S10). Both IR50 values were <1, indicating that
the site-specific mutation and dye-labeling in DC-SIGN weakened its binding affinity with the QDs. This
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effect was more pronounced for QD-EG3-DiMan, presumably because its shorter EG3 linker may limit
the terminal sugar’s ability to re-organise and fit perfectly within the protein’s binding pockets.
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Figure 6. Hydrodynamic diameter histograms of QD-EG11-DiMan before (A) and after binding to wild-
type DC-SIGN (B) or DC-SIGNR (C) measured by dynamic light scattering (DLS). Insets show
schematics of the QD and/or QD-protein assemblies. Cryo-TEM (contrast inverted HAADF STEM)
images of the QD-EG11-DiMan before (D) and after binding to wild-type DC-SIGN (E) or DC-SIGNR
(F). Consistent with the Dh values shown in (A), both isolated and clustered QDs are found in the
corresponding TEM image (D) for QD-EG11-DiMan. Histograms of nearest neighbour distance (NND)
distributions measured from the TEM images of QD-EG11-DiMan before (G) and after binding to wild-
type DC-SIGN (H) or DC-SIGNR (I). The distribution histograms were fitted by Gaussian function with
fitting parameters shown in each graph. All samples were measured in binding buffer.
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Differentiating QD-wild type protein binding modes by DLS, TEM and fluorescence quenching.
The hydrodynamic size (Dhs) of the QD-protein assemblies provided further support for the different
binding modes of DC-SIGN/R.51 Binding of wild-type DC-SIGN with QD-EG11-DiMan (PQR = 12.5)
gave only a single, narrowly distributed species with a Dh of ~42 nm (Figure 6B). This value was
significantly bigger than that of isolated QDs (Dh ~ 8.8 nm in binding buffer, Figure 6A) or wild-type
DC-SIGN (Dh ~14 nm, Figure S11), suggesting that all DC-SIGNs were bound to the QD and formed a
uniform QD-protein assembly. In contrast, binding of wild-type DC-SIGNR gave a bimodal distribution
with Dhs of ca. 124 and 205 nm (Figure 6C), respectively. Moreover, almost identical Dh distributions
were also observed with its equivalent QD-EG3-DiMan interaction (SI, Figure S2D, Table S2). These Dh
values were too large to be isolated individual QD-protein assemblies, a strong indication of QD
agglomeration via QD/DC-SIGNR inter-linking. This hypothesis was confirmed by two dimensional
imaging of the QD dispersions via rapid plunge freezing and subsequent TEM/STEM imaging at low
magnification after analytical confirmation of the QD size and contrast level in these images 61 (we term
this cryo-snapshot TEM/STEM, SI, part 6), where the high contrast of the QD was employed as the
differentiating modality. Figure 6D shows that the QDs clustered in the binding buffer, possibly due to
weak binding between Ca2+ ions and QD-surface DiMan ligands since QD-DiMan appeared as isolated
particles in pure water without Ca2+ (Figure S12). However, the QD-DiMan clusters were completely
dispersed upon binding with wild-type DC-SIGN, revealing only isolated QDs (Figure 6E); whereas
binding of wild-type DC-SIGNR produced more aggregated QDs (Figure 6F). The different binding
behaviours were further supported by a nearest neighbour particle distance (NND) analysis of TEM
images (see SI, part 6).61 A large NND of ~46 nm was found for DC-SIGN bound QDs, which was >3
times that of DC-SIGNR bound QDs (~14 nm) or clustered QDs in binding buffer (~12 nm, see Figure
6G/H/I). These results agree excellently with the DLS results and our proposed DC-SIGN/R binding
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modes. The strong tetravalent binding of DC-SIGN with one QD should produce isolated QDs,
preventing them from getting close to each other and hence a large NND; whereas the bis-divalent
binding of DC-SIGNR with two different QDs should lead to QD inter-linking and a small NND.
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The postulated DC-SIGN/R-QD binding modes were further supported by the different fluorescence
quenching behaviors by DHLA-EG3-DiMan coated gold nanoparticles (GNP-EG3-DiMan, Figure S13).
GNP was chosen here because its efficient universal fluorescence quenching can extend beyond the
traditional FRET distance limit of ~10 nm.62,63 Here a 605 nm emitting QD was used to minimise the QD
fluorescence reduction due to absorption of GNP at the excitation wavelength (EX = 590 nm). Mixing
GNP-EG3-DiMan (5 nM) with QD-EG3-DiMan (10 nM) in binding buffer gave almost the same
fluorescence as the QD alone, suggesting minimal QD-GNP cross-linking. Addition of wild-type DC-
SIGNR to the QD-GNP mixture significantly quenched the QD fluorescence, whereas introduction of
wild-type DC-SIGN increased the QD fluorescence considerably (Figure S14). These results matched
well to that expected from the DC-SIGN/R binding modes: cross-linking by DC-SIGNR should lead to
QD/GNP assembly and QD fluorescence quenching by the proximal GNPs; whereas the strong
tetravalent binding of DC-SIGN to one QD or GNP should not only prevent any GNP or QD assembly,
but also break up any pre-assembled QD clusters in binding buffer (see Figure 6D), resulting in higher
fluorescence over the QD-only sample
Inhibiting DC-SIGN/R-mediated augmentation of EBOV-GP-driven viral entry. The strong DC-
SIGN binding affinity afforded by QD-EGn-DiMan suggests that these QDs could effectively block DC-
SIGN-mediated virus infection. To investigate this potential, a murine leukemia virus (MLV)-based
vector bearing the EBOV-GP was employed to deliver the luciferase gene into human embryonic kidney
cells (293T) previously transfected to express DC-SIGN/R.47 The virus particles can bind to cell surface
DC-SIGN/R via incorporated EBOV-GPs on their membrane surface to enhance cell uptake and gene
transduction. As expected, DC-SIGN/R expression in cells greatly increased the efficiency of EBOV-
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GP-driven gene transduction. Pre-treatment of cells with QD-EGn-DiMan greatly reduced the gene
transduction of DC-SIGN-positive cells down to the low nM range, indicating high inhibition potency
(Figure S15). The normalised inhibition data were fitted by an inhibition model, giving an IC50 of 0.7±0.2
and 1.4±0.1 nM for QD-EG3-DiMan and QD-EG11-DiMan, respectively (Figure 7). Such low IC50 values
place them among the most potent glyconanoparticle inhibitors against EBOV-GP-driven transduction
of host cells. In fact, their inhibition potency is comparable to the giant globular multivalent
glycofullerenes (IC50 = 0.667 nM)14 and the virus-like glycodendri-nanoparticles (IC50 = 0.91 nM).13
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1.4
DC-SIGN
DC-SIGNR
DC-SIGN control
DC-SIGNR control
1.4
1.2
1.0
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DC-SIGNR control
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IC50=0.7±0.2 nM
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[QD]/nM
[QD]/nM
Figure 7. Normalised luciferase activities of the DC-SIGN- or DC-SIGNR- expressing 293T cells as a
function of the pre-treatment QD-EG3-DiMan (A) or QD-EG11-DiMan (B) concentration. The data for
particles bearing the Ebola virus glycoprotein (EBOV-GP) are shown in open circles while the results
obtained with control particles bearing the vesicular stomatitis virus glycoprotein (VSV-G) are shown in
triangles. The luciferase activities, after subtraction by their corresponding pcDNA control background,
were normalised by the respective values in the absence of the QDs (SI, Figure S15). Data were fitted by
a competitive binding model, F =IC50/(IC50 + CQD), where F is the normalized luciferase activity, CQD
is the QD concentration, and IC50 is the concentration that gives 50% inhibition.47 The gene transduction
driven by a control vector bearing VSV-G, which cannot use DC-SIGN/R to augment cell entry, was
unaffected by the QD treatment, confirming that the specific QD-DC-SIGN/R binding was responsible
for the observed inhibition.
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Interestingly, these IC50 values roughly matched their apparent binding Kds with DC-SIGN (i.e. 0.61 and
2.1 nM) measured by QD-FRET (Table 2). Moreover, the gene transduction of DC-SIGNR-expressing
cells was reduced only marginally by treatment with 80 nM QD-EG11-DiMan (~10%) but it was more
pronounced with QD-EG3-DiMan (~50%). The inhibition potencies obtained here again roughly matched
those expected from their respective DC-SIGNR binding apparent Kds (i.e. ~633 and ~62 nM) measured
by FRET. The good match between the apparent Kd and IC50 values demonstrated that our FRET based
Kd measurement could serve as a viable, rapid method for predicting virus inhibition potency of
glyconanoparticles at the cellular level. Although the toxic cadmium content can prevent the current QD-
glycans from being used for treatment and prevention of EBOV infection, replacing the CdSe/ZnS QD
with other biocompatible, non-toxic nanoparticles (e.g. gold, Cd-free QD) should overcome this problem,
where nanoparticles displayed with similar polyvalent glycan ligands could be used as potent, specific
virus inhibitors and therapeutic reagents.
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Conclusions
We have demonstrated that compact QDs displaying dense polyvalent DHLA-EGn-DiMan ligands are
powerful probes for dissecting multivalent protein-glycan interactions via multimodal readout strategies
(FRET, particle size analysis, TEM imaging and GNP-based fluorescence quenching). Unlike most other
glycoconjugates which were constructed on passive scaffolds, the unique properties of QD (e.g.
fluorescence, size and inherent TEM contrast) have been fully exploited for the purpose of multimodal
readout for the first time. Significantly, we have revealed that DC-SIGN binds tetravalently to a single
QD, whereas DC-SIGNR binds divalently to two different QDs. The different binding modes, arising
from the different CRD spatial arrangements, yields >100 fold tighter QD-DiMan binding affinity for
DC-SIGN over DC-SIGNR which also help to explain why DC-SIGN is more effective in trans-infecting
some HIV strains that DC-SIGNR. Moreover, a new QD-FRET-based ratiometric method has been
developed to quantify the apparent QD-protein binding Kds. An impressively low Kd (~610 pM) and a
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per glycan affinity enhancement factor (/N) of ~ 4000 have been attained with QD-DiMan. Importantly,
QD-DiMan was found to potently inhibit DC-SIGN-mediated augmentation of EBOV-GP-driven
infection of host cells with an IC50 of ~0.7 nM, placing it among the most potent inhibitors against the
EBOV-GP driven virus infections.10,13,14 Moreover, this IC50 value also matches well to its DC-SIGN
binding apparent Kd measured by the ratiometric QD-FRET readout strategy. Together, these results
demonstrate that the QD-FRET-based affinity measurement developed herein could serve as a robust,
rapid, and sensitive method for predicting glyconanoparticle inhibition potencies against EBOV-GP
driven virus infections at the cellular level.
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Experimental Section
Materials
A CdSe/ZnS core/shell QD (EM ~560 nm) was purchased from PlasmaChem GmbH (Berlin, Germany).
The QD was supplied as dry powders and capped with mixed ligands of trioctylphosphine oxide (TOPO),
hexadecylamine and oleic acid. A CdSe/ZnSe/ZnS core/shell/shell QD capped with mixed ligands of
TOPO and trioctylphospine (EM ~605 nm) in toluene was purchased from STREM chemicals UK Ltd.
O-(2-Aminoethyl)-O′-(2-azidoethyl)decylethylene glycol (N3-EG11-NH2, >95% oligomer purity) was
purchased from Polypure Plc (Norway). Azido-3,6,9-trioxyundecan-1-amine (N3-EG3-NH2, >90%
monomer purity), N,N-dimethyl-1,3-propanediamine (>99%), 1,3-propane-sultone (> 99%), lipoic acid
(LA, >99%), triphenylphosphine (>98.5%), dicyclohexyl-carbodiimide (DCC, >99%), dimethylamino-
pyridine (DMAP, >99%), tris(2-carboxyethyl)phosphine hydrochloride (TCEP.HCl, >98%), and other
chemicals were purchased from Sigma-Aldrich UK Ltd (Dorset, UK). Solvents were obtained from
Fisher Scientific (Loughborough, UK). Ultra-pure water (resistance >18.2 MΩ.cm) purified by an ELGA
Purelab classic UVF system, was used for all experiments and making buffers.
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Preparation of QD-EGn-glycan (n = 3 or 11).47 1 nmol CdSe/ZnS QD in 0.2 mL toluene was first
precipitated by 1 mL ethanol followed by centrifugation to remove any free ligands. The QD pellet was
dissolved in CHCl3 (50 µL), then DHLA-EGn-glycan ligand (0.80 µmol in CHCl3) pre-deprotonated by
NaOH (8.0 µL, 0.10 M in EtOH) and MeOH were added to make a homogenous solution (CHCl3:MeOH
= 1:1 v/v). The resulting solution was wrapped in an aluminium foil and stirred at room temperature (RT)
for 30 min. Hexane was then added until the solution became cloudy. The mixture was centrifuged at
10,000 g for 5 min, where all the formed QD-EGn-glycan pelleted. After removal of the clear supernatant,
the pellet was dissolved in 100 µL of pure H2O and transferred to a 30 KDa MWCO spin column and
washed with H2O (3 × 100 µL) to remove any unbound free ligands, yielding the QD-EGn-glycan stock.
The QD concentration was determined by its first exciton peak absorbance at 546 nm (= 1.3×105 M-
1·cm-1 ) using the Beer-Lambert law.47
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Fluorescence spectroscopy. All fluorescence spectra were recorded on a Cary Eclipse fluorometer using
a fixed excitation wavelength, EX, of 450 nm, corresponding to the absorption minimum of Atto-594 to
minimise the direct excitation background. The measurements were performed in a binding buffer (20
mM HEPES pH 7.8, 100 mM NaCl, 10 mM CaCl2) containing 10 g/mL of a His6-Cys peptide which
we found to improve the QD stability and reduce non-specific adsorption.47,59 The labeled proteins were
mixed with the QD at RT for 20 min before fluorescence spectra were recorded. Binding of labeled
monomeric DC-SIGN or DC-SIGNR CRD with the QDs was performed the same way. For apparent Kd
measurement, a series of samples containing different concentrations of the QD/labeled proteins (but
with a fixed PQR of 1 for DC-SIGN or 10 for DC-SIGNR) were prepared in the same binding buffer as
above but containing 1 mg/mL of BSA to reduce non-specific adsorption. The samples were incubated
at RT for 20 min before fluorescence spectra were recorded. Adjustments of the PMT voltages and
EX/EM slit widths were used to compensate the low fluorescence signal at low concentrations. Although
this may affect the absolute fluorescence intensity, the FRET ratio is not affected due to its ratiometric
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nature. All fluorescence spectra were corrected for the dye direct excitation background by subtracting
the corresponding fluorescence spectrum of the same concentration labeled protein only recorded under
identical conditions.
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Data Fitting. Direct excitation background corrected fluorescence peak intensities at 554 nm (QD) and
626 nm (Atto-594 FRET) were used to calculate the apparent FRET ratio, I626/I554. The I626/I554 ratio
versus protein concentration plots were fitted to the Hill’s equation to derive the apparent Kd:
୬
଺ଶ଺
(1)
୬
୬
ହହସ
ୢ
Where Rmax is the saturated FRET ratio, Kd is the apparent dissociation constant, [C] is the protein
concentration and n is Hill’s coefficient. Iterative fittings were used to yield the best fit (R2) for Rmax and
Kd determination. The relative binding affinity between wild-type and labeled DC-SIGN for the QD-
DiMan was analysed by a simple competitive model, F = IR50/[IR50 +CWT/CLP].
Where F is the apparent FRET ratio in the presence of wild-type protein normalised by that without, CWT
and CLP are wild-type and labeled protein concentrations, respectively. Iterative fittings were used to
yield the best fit (R2) for the IR50 determination.
STEM imaging. Three QD samples (QD-EG11-DiMan, QD-EG11-DiMan + wild-type DC-SIGN and
QD-EG11-DiMan + wild-type DC-SIGNR) were prepared in binding buffer with CQD = 40 nM and Cprotein
= 1.5 M. 3.5 L of the QD sample was placed onto a plasma-cleaned TEM grid with a continuous
carbon support film, blotted, and plunge frozen into liquid ethane. The TEM grids were then warmed to
RT over several min by placing them in the liquid nitrogen cooled storage container in a rotary pumped
vacuum desiccator. The samples were analysed using an FEI Titan Cubed Themis 300 G2 S/TEM
equipped with FEI SuperX energy dispersive X-ray (EDX) spectrometers. The samples were imaged
using high angle annular dark field scanning transmission electron microscopy (HAADF STEM)
mode,61,64 which provides atomic number contrast (≈Z1.7), thereby permitting imaging of the high atomic
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number quantum dots (brighter) on the low atomic number background (darker). A series of images at
the same magnification were recorded for each sample which were then analysed in MATLAB to
measure the nearest neighbour distances (NNDs). Histograms of NNDs for each image were produced.
The combined histograms were plotted as a percentage of the total population and fitted by Gaussian
distribution.
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Inhibition of DC-SIGN/R-mediated augmentation of EBOV-GP-driven transduction.47 The
experiments were performed using human embryonic kidney 293T cells. Target 293T cells seeded in 96-
well plates were transfected with plasmids encoding DC-SIGN or DC-SIGNR or control transfected with
empty plasmid (pcDNA). The cells were washed at 16 h post transfection and further cultivated at 37°C,
5% CO2 in Dulbecco's modified eagle medium (DMEM) containing 10% fetal bovine serum (FBS). At
48 h post transfection, the cells were exposed to twice the final concentration of QD-DiMan inhibitor in
DMEM supplemented with 10% FBS for 30 min in a total volume of 50 µl. Thereafter, the cells were
inoculated with 50 µl of preparations of MLV vector particles encoding the luciferase gene and bearing
either EBOV-GP or the vesicular stomatitis virus glycoprotein (VSV-G) as control. Binding of QD-
DiMan to DC-SIGN/R on the surface of 293T cells can block the interaction of these lectins with the
EBOV-GP on the particle surface, reducing the cellular uptake of vector particles and thus reducing
transduction efficiency. At 6 h post inoculation, 100 µl of fresh DMEM culture medium was added and
the cells incubated for another 72 h. Thereafter, luciferase activities in cell lysates were determined using
a commercially kit (PJK), following the manufacturer’s instructions, as described in our previous
publication.47
ASSOCIATED CONTENT
Supporting Information
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The Supporting Information is available free of charge on the ACS Publications website at
http://pubs.acs.org.
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Experimental details which include the detailed synthesis and spectroscopic characterization of the
DHLA-EGn-DiMan ligands, DC-SIGN/R production and labeling; lifetime measurement, STEM and
DLS measurements, calculation of inter-glycan distance, virus inhibition experiment, supporting tables
showing the physical parameters of the QD-EGn-glycan, fitting parameters, and supporting figures
showing the hydrodynamic size distributions of the proteins and QDs with and without proteins; QD-
Atto594 Förster radius (R0) and average QD-dye distance (r); Ca2+-dependence of DC-SIGN/R QD
binding; lifetime decay curves; GNP-DiMan hydrodynamic size and TEM image; QD fluorescence
quenching by GNP in the presence of DC-SIGN/R, and original virus inhibition data.
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AUTHOR INFORMATION
Corresponding authors:
* Email: y.guo@leeds.ac.uk (Y.G.), fax: +44-113-3436565.
* Email: d.zhou@leeds.ac.uk (D.Z.), tel: +44-113-3436230; fax: +44-113-3436565.
ORCID
Yuan Guo: 0000-0003-4607-7356
Bruce Turnbull: 0000-0002-7352-0360
Stefan Pöhlmann: 0000-0001-6086-9136
Dejian Zhou: 0000-0003-3314-9242
Author Contributions
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Y.G., D.Z., and S.P. conceived research. Y.G., I.N., E.P., C.S., N.H., Q.L., S.L., J.W. and A.B. performed
research. Y.G., D.Z., N.H., E.P., A.Y., S.P. and W.B.T. analysed data. D.Z. and Y.G. wrote the paper.
All authors have discussed the results and approved the final version of the paper.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the Wellcome Trust (Grant No: 097354/Z/11/Z), BBSRC (Grant No: BB/M005666/1) and the
University of Leeds for funding this project. Y.G. thanks the Wellcome Trust for supporting a career re-
entry fellowship (Grant No: 097354/Z/11/Z). E.P. thanks the University of Leeds and EPSRC for funding
her a DTA PhD studentship. C.S. thanks Khon Kaen University and its Faculty of Medicine for providing
a PhD studentship. We also thank Dr M. Rowan Brown (Swansea University, UK) for the development
of MATLAB scripts that enabled automation of nearest neighbour measurements in TEM images.
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