A bis(phosphine)-modified peptide ligand for stable and luminescent quantum dots in aqueous media

Article abstract of DOI:10.1055/s-0033-1339340

We describe a new class of ligands for semiconductor nanoparticles (quantum dots = QDs), which bind well and allow for their facile dissolution in aqueous solution. As a proof of principle, we have designed and synthesized a novel bis(phosphine)-modified

Full text of DOI:10.1055/s-0033-1339340

2426▌
PAPER
paper
A Bis(phosphine)-Modified Peptide Ligand for Stable and Luminescent
Quantum Dots in Aqueous Media
Bis(phosphine)-Modified Peptide Ligand
Michael E. Jung,* Michael Trzoss, James M. Tsay, Shimon Weiss
Department of Chemistry and Biochemistry, California NanoSystems Institute, and Department of Physiology,
University of California at Los Angeles, Los Angeles, CA 90095-1569, USA
Fax +1(310)2063722; E-mail: jung@chem.ucla.edu
Received: 01.05.2013; Accepted after revision: 06.06.2013
ctylphosphine oxide coating. This effect of the thiol
groups has also been described by Huang and Tomalia7
during their studies of thiol-modified dendrons as ligands
for quantum dots. However, when they used phosphine-
Abstract: We describe a new class of ligands for semiconductor
nanoparticles (quantum dots = QDs), which bind well and allow for
their facile dissolution in aqueous solution. As a proof of principle,
we have designed and synthesized a novel bis(phosphine)-modified
peptide (BPMP) and shown that it has the ability to solubilize quan- modified dendrons as ligands, the resulting quantum dots
tum dots in aqueous media. We further showed that the correspond-
ing phosphine oxide derivatives of these new ligands are less good
at solubilizing the quantum dots. These new bis(phosphine)-modi-
served good quantum yields. Prompted by these results,
fied peptide ligands are easy to prepare and may well replace thiol-
containing binding sequences in functionalized peptides for quan-
again showed the desired high quantum yields. Kim and
Bawendi8 used oligomeric phosphine ligands and also ob-
we decided to design and synthesize a novel bis(phos-
phine)-modified peptide (BPMP) and study its ability to
solubilize quantum dots in aqueous media. Bis(phos-
phine)-modified peptides could replace thiol-containing
binding sequences in functionalized peptides for quantum
dot coating, potentially resulting in quantum dots with
higher quantum yields.
tum dot coating, potentially resulting in quantum dots with higher
quantum yields.
Key words: bis(phosphine)-modified peptides, nanoparticles,
quantum dots, quantum dot stabilization, ligands
Several phosphine-containing amino acids and peptides
have been prepared and studied, many by the Gilbertson
group.9 We initially focused our attention on the bis(phos-
phine) modified tetrapeptide 1 which would contain what
we hoped would be the necessary segments to insure both
good binding to the quantum dots and solubility (Scheme
1). Thus we proposed to couple the free acid functionality
of the doubly protected tetrapeptide 2 (PG1NH-Gly-Gly-
Glu-Gly-OPG2) with the known bis[2-(diphenylphosphi-
no)ethyl]amine (3)10 to give via a simple amide formation,
after deprotection, the bidentate phosphine-modified tet-
rapeptide. This compound would have the required quan-
tum dot binding domain and a solubilizing hydrophilic
domain linked by a small spacer unit ligand. We report
herein the synthesis and quantum dot binding and solubi-
lizing properties of the modified peptide 1.
Semiconductor nanocrystals (quantum dots = QDs) ex-
hibit unique optical properties that are desirable for appli-
cations in biological imaging.1 Quantum dots are highly
photostable and show broad absorption spectra. Their nar-
row emission bands can be tuned by altering the particle
size. Various synthetic routes provide access to monodis-
persed quantum dots with very good control over their
size dispersion. These quantum dots are typically coated
with trioctylphosphine oxide (TOPO) and are highly lipo-
philic.2 The challenge that remains for applications in bio-
logical systems is the choice of appropriate ligands that
can either replace or interact with the trioctylphosphine
oxide coating to generate water-soluble quantum dots,
that are still monodispersed and show comparable high
photochemical quantum yields. Several approaches to sol-
ubilize quantum dots in aqueous media have been report-
ed in the literature.3 The traditional approaches use mono-
or polyfunctional ligands containing thiol or phosphine
groups to replace the trioctylphosphine oxide coating on
the surface of quantum dots.4 The Weiss group has devel-
oped peptides containing a specially designed binding
sequence with several cysteine residues.5 These polypep-
tide-based ligands can be easily conjugated, for example,
onto dyes, and the resulting quantum dots can be used for
biological imaging or for the generation of singlet oxygen
in living cells.6 The drawback of this approach is the lower
quantum yield of the peptide-coated quantum dots which
use thiol ligands compared to quantum dots with a trio-
The protected peptide 2 could arise by simple peptide cou-
pling reactions of the following three fragments: the N-
protected diglycine 4, glutamic acid 5-benzyl ester 5, and
the simple glycine ester 6 (Scheme 2). This approach
would allow one to easily choose the two protecting
groups PG1 and PG2 to facilitate further couplings of the
phosphine-modified peptide onto functionalized peptides
for future applications. In this initial study we equipped
both terminal ends of the peptide 9 with acid-labile pro-
tecting groups (Boc for the amine and a tert-butyl ester for
the carboxylic acid) to allow simultaneous deprotection
after coupling to the bis(phosphine)amine 3 without the
need of an additional purification step for the phosphine-
modified peptide.
SYNTHESIS 2013, 45, 2426–2430
Advanced online publication: 17.07.2013
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DOI: 10.1055/s-0033-1339340; Art ID: SS-2013-M0320-OP
© Georg Thieme Verlag Stuttgart · New York

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Bis(phosphine)-Modified Peptide Ligand
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afforded the desired amide 11 in 77% yield. Finally, de-
protection of both the N-Boc group and the tert-butyl ester
occurred in quantitative crude yield by treatment of 11
with trifluoroacetic acid in dichloromethane to give the
modified tetrapeptide 1. This bis(phosphine)-modified
peptide seems to be similar to triphenylphosphine in its
propensity toward oxidation and thus is fairly stable to ox-
idation if normal precautions are taken.
QD
Ph2P
bidentate ligand
PPh2
N
O
spacer
CO2H
O
H
H
N
N
H2N
N
H
O
O
hydrophilic domain
CO2R
RNH
BocNH
O
H
H
H
O
H
1
N
CO2H
b,c
N
N
N
H
O
O
CO2t-Bu
7 R = H
Ph2P
PPh2
+
9 R = Bn
10 R = H
a
d
8 R = Boc
CO2H
N
H
PPh2
PPh2
3
O
N
O
H
H
N
N
CO2PG2
PG1NH
N
e
H
BocNH
O
O
O
H
O
H
H
N
2
N
CO2t-Bu
N
H
Scheme 1 Design of bis(phosphine)-modified peptide 1
O
11
PPh2
PPh2
O
N
CO2H
H2N
f
O
H
H
H
N
N
CO2H
O
N
H
H
H
O
H
O
N
N
CO2PG2
O
PG1-NH
N
H
1
O
Scheme 3 Reagents and conditions: (a) Boc2O, Et3N, dioxane, 88%;
(b) 1. ClCO2Et, DIPEA, THF, 0 °C; 2. 5, DIPEA; (c) 1. HOBt,
EDC·HCl, DMF; 2. DIPEA, NH2CH2CO2t-Bu, 73% (2 steps, from 8);
(d) H2, Pd/C, EtOAc, 88%; (e) 1. HOBt, EDC·HCl, CH2Cl2; 2. 3,
77%; (f) TFA, CH2Cl2, 96%.
2
CO2Bn
+
H
N
CO2PG2
CO2H
+
H2N
H
PG1-NH
H2N
CO2H
O
To compare the efficiency of binding of these phosphine-
modified peptides to the well-known phosphine oxide-
modified systems (e.g., TOPO), we decided to prepare the
bis(phosphine oxide) of the modified tetrapeptide. The
protected bis(phosphine) 11 was treated with molecular
oxygen in the presence of low-level ultraviolet light at
room temperature for three hours to give the correspond-
ing bis(phosphine oxide) 12 in quantitative yield (Scheme
4). Removal of the N-Boc and tert-butyl ester protecting
groups with trifluoroacetic acid gave the desired bis(phos-
phine oxide) 13 in quantitative yield.
4
5
6
Scheme 2 Retrosynthesis of acid 2
Commercially available diglycine 7 (glycinyl glycine)
was protected as the N-Boc carbamate 811 (Boc-Gly-Gly)
in 88% yield by treatment with di-tert-butyl dicarbonate
(Boc2O) and triethylamine (Scheme 3). Reaction of 8 with
ethyl chloroformate in the presence of N,N-diisopropyl-
ethylamine followed by addition of glutamic acid 5-ben-
zyl ester 5 afforded the expected amide, which was used
directly without extensive purification. Reaction of this
acid with 1H-1,2,3-benzotriazol-1-ol (HOBt) and 1-(3-di-
methylaminopropyl)-3-ethylcarbodiimide in N,N-dimeth-
ylformamide using N,N-diisopropylethylamine and then
addition of glycine tert-butyl ester furnished the desired
fully protected tripeptide 912 in 73% yield over the two
steps. Hydrogenolysis of the benzyl ester of 9 using palla-
dium on carbon in ethyl acetate gave the acid 10 in 88%
yield. Reaction of this acid 10 with 1H-1,2,3-benzotri-
azol-1-ol and 1-(3-dimethylaminopropyl)-3-ethylcarbodi-
imide in dichloromethane followed by addition of bis[2-
(diphenylphosphino)ethyl]amine (3) followed by workup
Tributylphosphine (TBP)/trioctylphosphine oxide coated
quantum dots could be transferred from organic solvents
to water with the bidentate bis(phosphine)-modified pep-
tide 1 described above by using methods similar to those
developed by Chan et al., who solubilized CdSe/ZnS
quantum dots with monothiol ligands.13 While the
bis(phosphine)-modified peptides could solubilize the
CdSe/ZnS quantum dots in water and buffer, the phos-
phine oxide peptide 13 could not under similar conditions
(see experimental section), suggesting that the phosphine
oxide peptides did not effectively replace the bound trioc-
tylphosphine oxide/tributylphosphine ligands. It is impor-
tant to note that the bis(phosphine)-modified peptide
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2013, 45, 2426–2430

2428
M. E. Jung et al.
PAPER
over 4 Å MS. i-Pr2NH was distilled from NaOH and MeOH was
distilled from Mg turnings under an argon atmosphere. Commercial
t-BuOOH was dried over 4 Å powdered MS for 30 min prior to use.
All other solvents or reagents were purified according to literature
procedures. 1H NMR and 13C NMR spectra were obtained on ARX-
400, ARX-500, or Avance-500 spectrometers. IR spectra were re-
corded on Nicolet 501 or Nicolet AVATAR 370 instruments using
liquid films (neat) or in CDCl3 soln on NaCl plates, and only the sig-
nificant absorption bands are recorded (in cm–1). MS was carried
out on a Waters LCT Premier HRMS using ESI-TOF. TLC was car-
ried out using precoated silica gel sheets (Merck 60 F254). Visual
detection was performed with UV light, p-anisaldehyde stain,
KMnO4 stain or I2. Flash chromatography was performed using
SilicaFlash™ P60 (60 Å, 40–63 mm) silica gel from SiliCycle Inc.
with compressed air. The QDs were purchased from Quantum Dot
Corp. (now a subsidiary of Life Technologies) and consisted of
CdSe/ZnS core/shell and a proprietary amphiphilic polymer coat.
The coat protects the QDs from environmental influences and pro-
vides stability in H2O. These QDs were further modified by conju-
gation to streptavidin (QDot 605 streptavidin conjugate, 1000-1).16
The buffer is proprietary.
PPh2
PPh2
O
N
O
H
O
a
H
H
N
N
CO2t-Bu
BocN
H
N
H
O
11
POPh2
POPh2
O
N
O
H
H
O
H
N
N
CO2R2
R1N
H
N
H
O
12 R1 = Boc R2 = t-Bu
13 R1 = R2 = H
b
Scheme 4 Reagents and conditions: (a) O2, UV (unfiltered Hg
lamp), 23 °C, 3 h, quant.; (b) TFA, CH2Cl2, 95%.
2-[2-(tert-Butoxycarbonylamino)acetamido]acetic Acid (8)11
Et3N (3.16 mL, 22.71 mmol) and Boc2O (3.63 g, 16.65 mmol) were
added to a suspension of diglycine (7; 2.00 g, 15.14 mmol) in a mix-
ture of dioxane (60 mL) and H2O (10 mL) at 0 °C. The mixture was
stirred at 23 °C for 16 h, diluted with H2O (250 mL), and acidified
to pH 3 by addition of solid KHSO4. The mixture was extracted with
EtOAc (5 × 50 mL), the combined organic phases were dried
(Na2SO4), and all solvents were removed under reduced pressure;
this gave 8 as a white solid, which was used without further purifi-
cation.
coating yielded smaller-sized quantum dots than commer-
cially available quantum dots and phyochelatin-related
peptide-coated quantum dots. The bis(phosphine)-modi-
fied peptide-coated quantum dots were stable for more
than three months. Extraction of the diffusion constant of
the bis(phosphine)-modified peptide quantum dots from
the analysis of the fluorescence correlation spectroscopy
(FCS) data (not shown) indicated a final hydrodynamic
diameter of ~8 nm.14 Meanwhile, little aggregation was
detected, making these probes potentially useful for sin-
gle-molecule tracking studies. Due to the smaller sizes of
the bis(phosphine)-modified peptide quantum dots, they
may also be useful in certain applications for biological
imaging (e.g., in vivo studies and cellular entry). A quan-
tum yield decrease of ~40% occurred due to the exchange,
suggesting that the ZnS surfaces of the quantum dots may
be incompletely passivated by the bis(phosphine)-modi-
fied peptides. It should be pointed out that the original
quantum dots before the bis(phosphine) coating had a
quantum yield of ~20%. Peptides with additional phos-
phines for multidentate ligands may increase the quantum
yields of the quantum dots and are currently being inves-
tigated.
Yield: 3.10 g (13.35 mmol, 88%).
1H NMR (400 MHz, DMSO): δ = 12.55 (br s, 1 H), 8.03 (t, J = 5.5
Hz, 1 H), 6.96 (t, J = 5.7 Hz, 1 H), 3.75 (d, J = 5.7 Hz, 2 H), 3.56 (d,
J = 5.8 Hz, 2 H), 1.37 (s, 9 H).
N-tert-Butylcarbamoylglycinylglycinyl Glutamate 5-Benzyl
Ester (9)
ClCO2Et (0.96 mL, 10.08 mmol) was added dropwise to a soln of 8
(1.80 g, 7.75 mmol) and DIPEA (1.73 mL, 10.85 mmol) in THF (50
mL) at 0 °C. The mixture was stirred for 1 h while warming up to
10 °C. The mixture was cooled to –15 °C and a precooled (–15 °C)
suspension of 5 (3.31 g, 13.95 mmol) in a mixture of THF (50 mL)
and DIPEA (2.31 mL, 13.95 mmol) was added via a Teflon cannula.
The mixture was stirred for 2 h while warming to 23 °C and then for
a further 14 h at 23 °C. H2O (300 mL) was added and the mixture
was acidified to pH 5 by the addition of solid KHSO4. The mixture
was extracted with EtOAc (5 × 50 mL), the combined organic phas-
es were washed with sat. aq NaCl (2 × 50 mL) and dried (Na2SO4)
and all solvents were removed under reduced pressure. The crude
residue was dissolved in DMF (40 mL); then HOBt (1.57 g, 11.60
mmol) and EDC·HCl (2.37 g, 12.38 mmol) were added and the mix-
ture was stirred for 1 h at 23 °C. DIPEA (2.11 mL, 12.76 mmol) and
NH2CH2CO2t-Bu·HCl (2.08 g, 12.38 mmol) were added and the
mixture was stirred for 16 h at 23 °C. H2O (200 mL) and EtOAc
(200 mL) were added and the layers were separated. The aqueous
layer was extracted with EtOAc (2 × 100 mL), the combined organ-
ic layers were washed with sat. aq NaCl (5 × 60 mL) and dried
(Na2SO4), and all solvents were removed under reduced pressure.
The residue was purified by flash column chromatography (silica
gel, EtOAc–acetone, 8:2); this gave 912 as a semi-solid gum.
We have prepared the bis(phosphine)-modified tetrapep-
tide 1 as a model bis(phosphine)-modified peptide and
shown that it readily solubilizes quantum dots to give a fi-
nal particle with a hydrodynamic diameter of ~8 nm.
These are smaller-sized quantum dots than commercially
available ones. We have also shown that the correspond-
ing bis(phosphine oxide) tetrapeptide 13 did not solubilize
quantum dots as well.15 Further work on the preparation of
novel bis(phosphine)-modified peptides and their use
with quantum dots is under way.
Yield: 3.21 g (5.68 mmol, 73% over 2 steps).
All reactions were carried out under an argon atmosphere unless
otherwise specified. THF and Et2O were distilled from benzoqui-
none ketyl radical under an argon atmosphere. CH2Cl2, toluene,
benzene, pyridine, Et3N, and DIPEA were distilled from CaH2 un-
der an argon atmosphere. DMSO was distilled over CaH2 and stored
1H NMR (400 MHz, CDCl3): δ = 7.89 (br d, J = 7.6 Hz, 1 H), 7.80
(br s, 1 H), 7.71 (br s, 1 H), 7.36–7.30 (m, 5 H), 5.99 (br s, 1 H), 5.10
(s, 2 H), 4.77–4.69 (m, 1 H), 4.08–3.84 (m, 6 H), 2.55–2.45 (m, 2
H), 2.25–2.00 (m, 2 H), 1.45 (s, 9 H), 1.43 (s, 9 H).
Synthesis 2013, 45, 2426–2430
© Georg Thieme Verlag Stuttgart · New York

PAPER
Bis(phosphine)-Modified Peptide Ligand
2429
13C NMR (100 MHz, CDCl3): δ = 172.6, 171.1, 170.0, 168.9, 168.3,
155.9, 135.5, 128.1, 127.85, 127.8, 81.4, 79.4, 66.0, 52.0, 43.6,
42.7, 41.6, 30.0, 28.0, 27.6, 27.2.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C27H40N4O9: 587.2687;
found: 587.2704.
tert-Butyl 12-(3-{Bis[2-(diphenylphosphoryl)ethyl]amino}-3-
oxopropyl)-2,2-dimethyl-4,7,10,13-tetraoxo-3-oxa-5,8,11,14-
tetraazahexadecan-16-oate (12)
O2 was bubbled for 5 h through a soln of 11 (300 mg, 0.334 mmol)
in THF (30 mL) while the mixture was irradiated with UV light (Hg
lamp). The solvent was removed under reduced pressure; this gave
12.
12-{[(2-tert-Butoxy-2-oxoethyl)amino]carbonyl}-2,2-dimethyl-
4,7,10-trioxo-3-oxa-5,8,11-triazapentadecan-15-oic Acid (10)
Pd/C (5%, 250 mg) was added to a soln of 9 (2.50 g, 4.42 mmol) in
EtOAc (150 mL) and H2 was slowly bubbled through the stirred
soln at 23 °C for 48 h. THF (60 mL) was added to dissolve the pre-
cipitate formed and the mixture was filtered through Celite. After
evaporation of all solvents under reduced pressure, Et2O (200 mL)
was added and the product was isolated by filtration to give 10 as a
white solid.
Yield: 311 mg (0.334 mmol, quant).
1H NMR (400 MHz, CDCl3): δ = 8.05 (br s, 1 H), 7.87 (br d, J = 6.4
Hz, 1 H), 7.69–7.63 (m, 8 H), 7.50–7.32 (m, 13 H), 5.96 (br s, 1 H),
4.39 (ddd, J = 8.1, 7.9, 3.9 Hz, 1 H), 4.24 (dd, J = 6.4, 6.4 Hz, 1 H),
4.08 (dd, J = 17.0, 5.9 Hz, 1 H), 3.86–3.71 (m, 3 H), 3.64 (dd,
J = 6.2, 6.2 Hz, 1 H), 3.55–3.35 (m, 4 H), 2.61–2.49 (m, 4 H), 2.32–
2.12 (m, 2 H), 2.07–1.02 (m, 2 H), 1.35 (s, 9 H), 1.34 (s, 9 H).
31P NMR (162 MHz, CDCl3): δ = 30.41, 30.22.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C48H61N5O10P2:
Yield: 1.84 g (3.87 mmol, 88%).
1H NMR (400 MHz, DMSO): δ = 12.08 (br s, 1 H), 8.23 (br s, 1 H),
8.01 (br d, J = 7.9 Hz, 1 H), 7.96 (br s, 1 H), 6.99 (br s, 1 H), 4.35–
4.24 (m, 1 H), 3.81–3.51 (m, 6 H), 2.30–2.20 (m, 2 H), 1.98–1.88
(m, 1 H), 1.80–1.69 (m, 1 H), 1.40 (s, 9 H), 1.37 (s, 9 H).
13C NMR (100 MHz, DMSO): δ = 174.3, 171.8, 170.1, 169.1,
169.1, 156.2, 81.0, 78.5, 52.0, 43.7, 42.3, 41.8, 30.4, 28.6, 28.1,
27.8.
952.3785; found: 952.3802.
2-[(2-({2-[(2-Aminoacetyl)amino]acetyl}amino)-5-{bis[2-(di-
phenylphosphoryl)ethyl]amino}-5-oxopentanoyl)amino]acetic
Acid (13)
TFA (4 mL) was added to a soln of 12 (311 mg, 0.334 mmol) in
CH2Cl2 (15 mL) and the mixture was stirred for 3 h at 23 °C. Tolu-
ene (20 mL) was added and the mixture was evaporated under re-
duced pressure; this gave 13 (partial TFA salt) as a semi-solid gum.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C20H34N4O9: 497.2218;
found: 497.2236.
Yield: 282 mg (0.318 mmol, 95%).
31P NMR (162 MHz, CDCl3): δ = 35.51, 33.86.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C39H45N5O8P2:
tert-Butyl 12-(3-{Bis[2-(diphenylphosphino)ethyl]amino}-3-
oxopropyl)-2,2-dimethyl-4,7,10,13-tetraoxo-3-oxa-5,8,11,14-
tetraazahexadecan-16-oate (11)
796.2636; found: 796.2607.
HOBt (0.213 g, 1.58 mmol) and EDC·HCl (0.302 g, 1.58 mmol)
were added to a soln of 9 (0.750 g, 1.58 mmol) in CH2Cl2 (30 mL)
and the mixture was stirred at 23 °C for 30 min. A soln of 3 (0.696
g, 1.58 mmol) in CH2Cl2 (5 mL) was added and the mixture was
stirred for a further 16 h at 23 °C. Sat. aq NaCl (100 mL), H2O (100
mL), and EtOAc (200 mL) were added and the aqueous phase was
extracted with EtOAc (100 mL). The combined organic layers were
washed with sat. aq NaCl (6 × 50 mL) and dried (Na2SO4), and all
solvents were removed under reduced pressure. The residue was pu-
rified by flash column chromatography (silica gel, EtOAc–acetone,
8:2); this gave 11 as a semi-solid gum.
Solubilization of Quantum Dots
CdSe/ZnS QDs (613 nm-emitting) in TBP/TOPO were precipitated
with MeOH and air-dried (~5–10 mg). Phosphine-containing pep-
tide 1 (~200 mg) was dissolved in CHCl3 (1.5 mL) and this soln was
added to the precipitated QDs for 2 h. Then 18.2 MΩ H2O (4.5 mL)
was added to the CHCl3 soln, and the mixture was stirred at 60 °C
for >3 h. KOH (~100 mg) was added to the soln to deprotonate the
carboxylic acid of phosphine-containing peptide. The CHCl3 layer
was extracted from the soln and discarded. The H2O layer contain-
ing the phosphine peptide-QDs was centrifuged to separate the QDs
from the solid white precipitate, and the QDs were then syringe-fil-
tered (0.2 μm filter). The phosphine peptide-QDs were then run
through a NAP column (Amersham Biosciences) for purification.
The hydrodynamic radius of the QDs was measured by fluorescence
correlation spectroscopy (FCS) as stated in the manuscript. These
measurements showed definitively that the phosphine-coated QDs
had a smaller hydrodynamic radius (8 nm) than the commercially
available product QD605 (Invitrogen QDs), the hydrodynamic radi-
us of which is 30.3±3.2 nm. Since FCS is a sensitive tool for mea-
suring hydrodynamic radius, TEM and DLS after the phosphine
coating were not performed. FCS does not rely on the fluorescence
spectrum to give values for size; rather it relies on the residence/dif-
fusion of the fluorescent species within a defined excitation volume.
Details of FCS and the hydrodynamic radius of peptide-coated QDs
and QDC quantum dots have been described elsewhere.14
Yield: 1.09 g (1.21 mmol, 77%).
1H NMR (400 MHz, CDCl3): δ = 8.19 (br d, J = 6.6 Hz, 1 H), 7.41–
7.28 (m, 21 H), 7.04 (br s, 1 H), 7.96 (br s, 1 H), 5.66 (br s, 1 H),
4.39–4.34 (m, 1 H), 3.95–3.76 (m, 6 H), 3.52–3.32 (m, 2 H), 3.27–
3.15 (m, 2 H), 2.40–2.06 (m, 6 H), 1.99–1.90 (m, 2 H), 1.42 (s, 9 H),
1.39 (s, 9 H).
13C NMR (100 MHz, CDCl3): δ = 173.0, 171.3, 170.5, 169.0, 168.6,
155.9, 137.0 (d, J = 11.9 Hz), 136.8 (d, J = 11.9 Hz), 132.6 (d,
J = 18.6 Hz), 132.5 (d, J = 18.6 Hz), 129.0 (d, J = 5.1 Hz), 128.8 (d,
J = 5.1 Hz), 128.6 (d, J = 6.8 Hz), 128.5 (d, J = 6.8 Hz), 81.7, 80.0,
53.0, 45.4 (d, J = 25.4 Hz), 44.2, 43.6 (d, J = 22.9 Hz), 43.0, 41.8,
29.3, 28.3, 27.9, 27.5 (d, J = 15.3 Hz), 26.8, 26.3 (d, J = 13.6 Hz).
31P NMR (162 MHz, CDCl3): δ = –20.52, –21.64.
2-[(2-({2-[(2-Aminoacetyl)amino]acetyl}amino)-5-{bis[2-(di-
phenylphosphino)ethyl]amino}-5-oxopentanoyl)amino]acetic
Acid (1)
TFA (5 mL) was added to a soln of 11 (500 mg, 0.56 mmol) in
CH2Cl2 (20 mL) and the mixture was stirred for 3 h at 23 °C. Tolu-
ene (20 mL) was added and the mixture was evaporated under re-
duced pressure; this gave 1 (partial TFA salt) as a semi-solid gum.
Acknowledgment
We thank the National Science Foundation (CHE 0614591) and a
Jonsson Comprehensive Cancer Center multidisciplinary grant for
generous support of this work.
Yield: 460 mg (0.54 mmol, 96%).
31P NMR (162 MHz, CDCl3): δ = –18.99, –21.69.
HRMS (ESI-TOF): m/z [M + H]+ calcd for C39H45N5O6P2:
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.SnoIufproi
m
tgioSrantnugIifoop
r
itmnatr
742.2918; found: 742.2937.
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2013, 45, 2426–2430

2430
M. E. Jung et al.
PAPER
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Synthesis 2013, 45, 2426–2430
© Georg Thieme Verlag Stuttgart · New York