Recognition mediated encapsulation and isolation of flavin-polymer conjugates using dendritic guest moieties

Article abstract of DOI:10.1039/b926746h

Diaminopyridine dendritic scaffolds encapsulate polymeric flavin via non-covalent interactions and demonstrate isolation of the redox moiety. The Royal Society of Chemistry.

Full text of DOI:10.1039/b926746h

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Recognition mediated encapsulation and isolation of flavin–polymer
conjugates using dendritic guest moietiesw
Chandramouleeswaran Subramani,a Gulen Yesilbag,b Brian J. Jordan,a Xiaoning Li,a
Abraham Khorasani,a Graeme Cooke,c Amitav Sanyal*b and Vincent M. Rotello*a
Received (in Austin, TX, USA) 18th December 2009, Accepted 4th February 2010
First published as an Advance Article on the web 17th February 2010
DOI: 10.1039/b926746h
Diaminopyridine dendritic scaffolds encapsulate polymeric flavin
via non-covalent interactions and demonstrate isolation of the
redox moiety.
the effective site isolation and encapsulation of flavin through
non-covalent interactions with dendritic scaffolds.
Herein, we report the use of dendritic scaffolds to encapsulate
flavin derivatives and tune their electrochemical behavior using
molecular recognition. Three generations of dendrons containing
diaminopyridine (DAP) unit at their focal point were utilized to
probe the supramolecular encapsulation of monomeric and
polymeric flavin units. The DAP moiety on the dendrons binds
to monomeric and polymeric flavins to produce supramolecular
structures. The resultant encapsulated supramolecular assemblies
demonstrate distinctly different redox behavior as a function
of encapsulation. Monomeric flavins exhibit reversible,
redox-modulated behavior consistent with previous flavin model
systems.10 Polymeric flavins, however, demonstrate diffusion-
limited redox behavior and site isolation when bound to increasing
DAP dendrimer generations. The effective encapsulation of
polymeric flavins via supramolecular interaction provides a
potential tool to modulate redox processes in various systems
including artificial enzymes, light harvesting systems, molecular
wires, and light-emitting diodes (Scheme 1).
Site isolation and encapsulation of redox-active cofactors
control the electron transfer process within many enzymatic
systems,1 protecting the redox center from unwanted inter-
actions with water and other interfering species. Various
methods have been used to mimic this effective biological
insulation of redox-active core moieties for use in artificial
enzymes,2 catalysts,3 light harvesting systems,4 construction of
insulated molecular wires,5 light-emitting diodes6 and fiber optics.7
Additional efforts have focused on synthetic systems, such as
inclusion complexes and dendrimer cages,8 for applications as
data storage units, sensors, catalysts, and magnets.9
Dendritic scaffolds have gained considerable interest due
to their unique molecular architecture.8,9 These dendritic
scaffolds have been used for the spatial arrangement of function-
alities and tuning the properties of redox-active moieties through
the interplay of structural subunits. However, these synthetic
systems differ from biological systems because they primarily
focus on covalent attachment of dendritic scaffolds to
control the encapsulation. Therefore, model systems where
modulation of electron transfer properties occurs via non-
covalent interactions would be more representative of typical
enzyme–cofactor systems and would also lead to supra-
molecular strategies for materials and device applications.
Among various enzyme–cofactor systems, flavoenzymes
make attractive model systems because they catalyze a wide
variety of biological processes such as redox transformations,
signal transduction, and electron transfer. They use cofactors
(FMN or FAD) that bind to the active site of the apoenzyme
through a series of non-covalent interactions. The interactions
are responsible for fine-tuning the FADH2–FAD redox cycle
and isolating the cofactor within a hydrophobic pocket.
Hence, flavin model systems serve as a useful tool to modulate
Monomeric and polymeric flavins were synthesized using a
previously reported procedure.11 Briefly, polymeric flavin was
a Department of Chemistry, University of Massachusetts Amherst,
710 North Pleasant Street, Amherst, MA 01003, USA.
E-mail: rotello@chem.umass.edu; Fax: +1 413-5452058
b Department of Chemistry, Faculty of Arts and Sciences,
Bogazici University, Bebek 34342, Istanbul, Turkey.
E-mail: amitav.sanyal@boun.edu.tr
c Glasgow Centre for Physical Organic Chemistry, WestCHEM,
Department of Chemistry, University of Glasgow, Joseph Black
Building, Glasgow, UK G12 8QQ
w Electronic supplementary information (ESI) available: Synthesis
and characterization of polymers and dendrimers, experimental details
of CV and molecular modeling. See DOI: 10.1039/b926746h
Scheme
1 Alkyne-functionalized flavin 1, control N-methylated
flavin 2, flavin polymer 3 exhibit specific three-point hydrogen bonding
interactions with complementary dendrons, control polymer 4, and a
schematic representation of encapsulation of flavin polymer.
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as a control and showed no quenching of fluorescence upon
subsequent additions of excess DAP dendrons.
In contrast to the monomeric flavin, fluorescence titrations
of flavin functionalized polymer 3 in the presence of increasing
concentrations of DAP dendrons exhibited significant quenching
of flavin fluorescence corresponding to Ka(ox) = 600, 700,
550 MÀ1, respectively, for the G1, G2, and G3 DAP dendrons
(Fig. 1). No substantial change in binding affinity was
observed for increasing DAP dendrimer generations.
However, the enhanced binding affinity recorded for polymer
3 as compared to flavin 1 was attributed to the cooperative
non-covalent interactions including hydrogen bonding and
p-stacking within the polymeric–dendritic supramolecular
complex. As expected, N-methylated flavin polymer 4 did not
show fluorescence quenching upon additions of DAP dendron.
The redox behavior for the supramolecular complexes was
determined in CH2Cl2 through cyclic voltammetry (CV).
Cyclic voltamograms were obtained for 1, 2, polymer 3,
polymer 4 using tetrabutylammonium perchlorate (TBAP) as
the supporting electrolyte (ESIw). The half wave reduction
potential was calculated for flavin
1
1 and polymer 3
(E (unbound)) along with the corresponding potentials for 1
2
1
and polymer 3 in the presence of DAP (E (bound)).
Flavin 1 showed a single reduction wave followed by two
2
characteristic return oxidation waves (ESIw) typical for an
electrochemical–chemical–electrochemical (ece) process. Upon
addition of DAP dendrons, flavin 1 became fully reversible with
the disappearance of the second oxidation wave (ESIw). A
significant positive shift in the half wave reduction potential
1
2
1
2
(DE ) was observed (DE ) to be 60 Æ 5 mV for the dendrons
(Table 1). Hence, the DAP dendrimers modulate the flavin redox
behavior which is consistent with previous flavin model systems.13
Polymer 3 demonstrated fully reversible electrochemical
behavior as well (Fig. 2). The flavin units appended to the
polymer produced isolated redox behavior, preventing fla-
vin–flavin proton transfer and the disappearance of a second
oxidation wave.14 However, upon addition of 4 equivalents of
DAP dendrons, polymer 3 exhibited distinctly different elec-
trochemical behavior from its monomeric counterpart (Fig. 2).
A drastic change in peak shape was observed for polymer 3.
The signal was significantly smaller and broader for each
corresponding dendrimer generation, indicating limited
electron diffusion as a function of dendrimer size. The peak
Fig. 1 Inverse Stern–Volmer plot showing a dramatic quenching
of flavin 1 and flavin-functionalized polymer 3 fluorescence upon
addition of DAP dendrons, whereas interactions with control
N-methylated flavin 2 and control polymer 4 show no fluorescence
quenching (50 mM, lex = 445 nm, lem = 460–650 nm).
synthesized by reacting the monomeric alkyne flavin derivative
with azide functionalized polymer via Huisgen 1,3-dipolar,
‘‘click’’ cycloadditions.
Table
(MÀ1) for monomeric and polymeric flavin systems
1
Half-wave potentiala (mV) and association constants
The association constant (Ka(ox)) of flavin 1 and flavin
polymer 3 with complementary DAP dendrons was quantified
via fluorescence titrations in CHCl3. The fluorescence intensity
of flavin was monitored upon addition of guest DAP dendrons
(Fig. 1). Fluorescence titrations of flavin 1 in the presence
of increasing concentrations of DAP dendrons showed a
dramatic quenching of fluorescence, indicative of three point
hydrogen bonding between flavin and DAP complexes.12
Flavin 1 demonstrated relatively similar binding constants
(Ka(ox) = 400 MÀ1) for increasing generations of DAP
dendrimer, suggesting that the flavin–DAP complex was
independent of dendron size. N-Methylated flavin 2, which
cannot participate in hydrogen bonding interactions, was used
c
d
1
2
1
2
1
2
Flavin
Dendron Ka(ox)b E (u)
E (b)
DE
Ka(red)
G1
G2
G3
G1
400
400
350
650
700
550
À1280 À1225 55
À1280 À1215 65
À1280 À1225 55
À1250 À1190 60
À1250 À1180 70
À1250 À1190 60
3400
5000
3000
6500
11 000
5700
Flavin 1
Polymer 3 G2
G3
a
1
2
Half-wave potential (E ) was taken from square-wave voltammetry
b
1
2
measurements (E Æ 5 mV vs. ferrocene). Neutral association
constants (Ka(ox)) determined via fluorescence spectroscopy.
c
d
Reduction potential of the unbound flavin species. Reduction
potential of the bound flavin species with DAP guest. Association
constants have the error range of r50.
ꢀc
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of DAP dendrons observed in the molecular modeling
simulations. The diameter increases (from 4.6 nm to 7.9 nm)
as the generation of dendron increased for the flavin polymer 3
whereas the control polymer 4 did not show any appreciable
change in diameter (ESIw).
In conclusion, we have demonstrated that site isolation of redox
active units via formation of specific supramolecular complexes
can be used as a tool to modulate the redox behavior. In
particular, we have shown that DAP containing dendrons showed
enhanced binding towards site isolated polymer-bound flavin
systems relative to their monomeric counterparts. At higher
equivalents of DAP dendrons, a significant insulation of redox
activity of polymeric flavin was observed due to the encapsulation
of flavin. This supramolecular based modulation of redox
behavior of flavin bound polymer motif could be potentially used
in the creation of artificial enzymes, light harvesting systems, and
molecular wires. Our future work will involve investigating these
redox active polymers and their possible applications in redox
responsive sensing systems, and photovoltaic devices.
Fig. 2 CV traces of polymeric flavin 3 exhibiting different flavin
redox behaviors in the presence of G1, G2 and G3 DAP dendrons.
to peak distances were 200, 225, 440 mV for G1, G2 and G3
respectively. The broadening of the peak to peak distance and
the dramatic decrease in the current indicate a slow rate of
electron transfer from the flavin to electrode, indicating the
effective isolation and encapsulation of appended flavins from
outside interfering species (Fig. 2).
A. S. would like to thank the financial support from Turkish
Academy of Sciences under the TUBA-GEBIP fellowship
program. V. R. acknowledges Energy Frontier Research
Center funded by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences under Award Number
DE-SC0001087.
Surprisingly, the shift in the polymeric flavin redox potential
was not significantly different than the monomeric flavin
systems. A positive shift of 60–70 mV was observed for the
reduction potential of polymer 3 for all three DAP dendrimer
generations. The shifts in reduction potential were only
slightly larger than the shifts recorded for the monomeric
flavin 1. Thus, the cooperative non-covalent interactions
appeared to modulate the flavin redox potential while the size
of the complex determined the encapsulation of the flavin unit.
Association constants in the radical anionic states (Ka(red))
were extrapolated by using the shifts in reduction potentials
Notes and references
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bound flavin in reduced state showed a 15.7-fold increase in
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flavin polymer 3 with DAP dendrons G1 and G2 have also
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not demonstrate any encapsulation of flavin units (ESIw).
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Fig. 3 Molecular dynamics simulation (Amber force field) of (a)
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(Flavin moieties are highlighted in green.)
ꢀc
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Chem. Commun., 2010, 46, 2067–2069 | 2069