An electropolymerized molecularly imprinted polymer for selective carnosine sensing with impedimetric capacity Page: 1,159
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Journal of Materials Chemistry B
Functional monomer Type of binding
Carboxylic acid derivative 2 Hydrogen bond
Carboxylic acid derivative 2' Hydrogen bond
Carboxylic acid derivative 2" Hydrogen bond
[18-crown-6] ether derivative 3 Inclusion interactions
Pre-polymerization complex of the carnosine template 1 and
functional monomers 2, 2', 2", and 3
MIP-carnosine film was rinsed with abundant acetonitrile to
remove excess of the supporting electrolyte. Then, the carnosine
template was removed from the film by extraction with 0.1 M
NaOH for 30 min at room temperature. Completion of this
removal was confirmed by the DPV and XPS measurements.
Moreover, a non-imprinted polymer (NIP) film was prepared
as control. It was deposited under solution conditions same as
those used for the MIP film deposition, however, in the absence
Quantum chemical calculations
The structure of the pre-polymerization complex, in vacuum, of
the carnosine template 1 and both functional monomers 2 and
3 was optimized, and the values of thermodynamic functions
were calculated using DFT at the B3LYP/6-31G(d) level using
Gaussian 09 software.52
Results and discussion
Molecular modeling of interactions between binding sites of
the carnosine template and recognition sites of functional
The structure of the pre-polymerization complex of the carno-
sine template and both functional monomers was optimized by
computational modeling (Scheme 1). In Scheme 1a, the struc-
tural formula of this complex is proposed. In this complex
(Scheme 1b), hydrogen atoms 106, 107, and 108 of the amine
group of 1 interact with the 18-crown-6 moiety of 3. Here, the
crown ether moiety of 3 plays the role of a host moiety including
the -NH3 guest moiety of 1. The hydrogen atom 103 and the
oxygen atom 101 of 1 are complementarily paired with the
oxygen atom 198 and the hydrogen atom 200 of the carboxy
group of 2', respectively, forming two distinct hydrogen bonds.
The same interactions are characteristic of atoms of the peptide
bond of 1. That is, the hydrogen atom 93 and the oxygen atom
104 of carnosine are complementarily paired with the oxygen
atom 244 and the hydrogen atom 246 of 2, respectively. The
hydrogen atom 82 of the imidazole substituent of 1 forms a
hydrogen bond with the oxygen atom 152 of the carbonyl group
of 2". Table 1 summarizes results of the calculated Gibbs free
energy change, AG, due to possible interactions of different
recognition sites of functional monomers with complementary
binding sites of the carnosine template. Calculations of the
change in thermodynamic functions corresponding to the formation
Table 1 The Gibbs free energy change (AG) corresponding to the
formation of a complex of the carnosine template with functional
AG With respect to the optimized structure of the pre-polymerization
(kJ mol'1) complex, the solution of 1, 2, and 3 with the molar ratio of 1: 3: 1,
-35.8 respectively, was prepared for electropolymerization. Unfortu-
-42.8 nately, the carnosine template is insoluble in acetonitrile.
-22.2 Therefore, 10% water was first used to dissolve all the components.
-227.4 Finally, (TBA)C104 was added to satisfy the electropolymerization
requirement of sufficiently high ionic strength. The MIP-carnosine
This journal is @ The Royal Society of Chemistry 2016
of a complex of 1, 2, and 3 of 1:3:1 stoichiometry and the
optimized structure, resulted in a substantial total negative
change in the free energy, AG = -227.4 kJ mol-1 (Table 1). This
relatively high AG gain indicated the possibility of formation of
a stable (multi host)-carnosine complex. In the optimized
structure of carnosine, there are seven possible binding sites.
Unfortunately, one site, i.e., unprotonated nitrogen of the
imidazole ring, is not accessible to any recognition site of the
functional monomers because of steric hindrance. Nevertheless,
the computational DFT modeling indicated, advantageously,
strong six-point binding of the carnosine template.
The total free energy change, AG, because of formation of the
carnosine complex was compared with the AG for the histidine-
(functional monomers) complex where histidine was chosen as
its interference because the histidine moiety is a part of the
carnosine dipeptide. For that purpose, computational modeling
was performed. The complex structure was "frozen" in such a
way that the recognition sites of the functional monomers were
left "unfrozen" .46 After that, carnosine and histidine molecules
were allowed to equilibrate, separately, with the "frozen" mole-
cular cavity. The total free energy change for the "frozen" carnosine
complex was -347.01 kJ mol-1. However, this change for the
"frozen" histidine complex was nearly half, AG = -183.85 kJ mol-1.
These results confirmed that the designed molecular cavity of the
MIP preferentially bound the carnosine dipeptide molecule.
Infrared spectroscopic characterization of pre-polymerization
complex formation in solution
Several studies confirmed that oxygen atoms of crown ethers
interact with hydrogen atoms of protonated primary and
secondary amines.61-65 In effect, inclusion complexes are formed.
We performed IR measurements to verify host-guest interactions
between the carnosine template 1 and the 18-crown-6 derivatized
bisbithiophene functional monomer 3. Accordingly, in Fig. S1
(ESIt), IR spectra of carnosine, functional monomer 3, and the
complex of carnosine-(functional monomer) in KBr are pre-
sented. Fig. Sla (ESIt) shows two peaks at 1492 and 1644 cm-1
that are assigned to bending of the N-H bond.66 These
peaks are shifted to higher wavenumbers 1607 and 1734 cm-1,
respectively, in the spectrum of the complex (Fig. S1c, ESIt)
because of interactions of oxygen atoms of the crown ether moiety
of 3 with hydrogen atoms of 1. Apparently, N-H -"-0 hydrogen
bonds were formed confirming inclusion complex formation. The
presence of mentioned peaks and their shift to higher wave-
numbers correspond to calculated theoretical IR spectra (blue
vertical lines in Fig. S1, ESIt).
Deposition of MIP-carnosine films on different working
J. Mater. Chem B, 2016, 4, 1156-1165 1 1159
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Wojnarowicz, Agnieszka; Sharma, Piyush S.; Sosnowska, Marta; Lisowski, Wojciech; Huynh, Tan-Phat; Pszona, Maria et al. An electropolymerized molecularly imprinted polymer for selective carnosine sensing with impedimetric capacity, article, Date Unknown; London, United Kingdom. (digital.library.unt.edu/ark:/67531/metadc991033/m1/4/: accessed January 18, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.