Biomedical Engineering Reference
In-Depth Information
R
R
R
R
R
R
R
R
Fig. 1 Recognition event transduction through conformational changes in biphenyl and cyclo-
hexane derivatives
conformational changes can also give rise to marked changes in oxidation/reduction
potentials.
Moreover, the rigidity of cyclohexane derivatives can be employed to design
photophysical sensors, where the ligand's stereochemistry plays a very important
role because it can modify the signaling unit properties. Here we describe the use of
trans
-1,2-disustituted and
trans-transoid-trans
-1,2,4,5-tetrasustituted cyclohexanes
which are capable of modifying their conformation after complexation to give rise
to marked changes in their photophysical properties.
2 Biphenyl Derivatives
2.1 Recognition and Sensing of Cations
N
,
N
,
N
0
,
N
0
-tetramethylbenzidine (TMB) is a very interesting antenna for the design
of both electrochemical and fluorescent chemosensors based on conformational
changes. In fact, electrochemical oxidation of TMB systems leads to a radical
cation in which both aromatic rings are on the same plane allowing for unpaired
electron delocalization (Scheme
1
). When this moiety has bonded a binding site at
the 2,2
0
-positions, such as a crown ether or podand chains, the oxidation process can
be modified by the presence of different analytes.
Additionally, modification of TMB in its dihedral angle induced by substituents
or by the presence of different target molecules (Fig.
2
) also leads to modifications
in absorption and fluorescence emission spectra (Fig.
3
).
N
N
N
N
O
O
N
O
O
O
O
N
N
O
O
O
O
N
N
O
O
N
N
N
3
2
1
Thus, the fluorescent properties of compounds 1-3 were clearly modified in the
presence of transition metal cations [
18
]. Free ligands show a UV-Vis spectrum in
acetonitrile with bands in the UV region centered at 270 nm, which can also give
rise to an emission band at 370 nm (
l
exc
ΒΌ
270 nm). For ligands 1 and 2, the
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