Biomedical Engineering Reference
In-Depth Information
30E+05
20E+05
10E+05
0
350
400
450
500
550
Wavelength (nm)
Fig. 15 Fluorescence spectra of 13 after addition of Cu
2+
at 20
C, l
exc
¼
300 nm. Initial
concentration of ligand is 5.7
10
6
MinCH
3
CN. Spectra correspond to additions of 0, 0.70,
1.40, and 2.10 equiv. of Cu
2+
, respectively
Titration of ligand 16 against Zn
2+
or Cd
2+
(Fig.
16
) in acetonitrile leads to a
pronounced and red-shifted shoulder in the UV spectra, and a well-defined
isosbestic point at 340 nm. Addition of Zn
2+
or Cd
2+
(as triflate salts) to the
acetonitrile solutions of ligand 16 gives rise to a red shift in the emission spectrum
(e.g., the emission maximum shifted by 17 nm after the addition of 2 equiv. of Zn
2+
,
and by 12 nm in the case of Cd
2+
;
340 nm). Use of the corresponding nitrate
salts in acetonitrile offers similar results. Additionally, both cations induce partial
fluorescence quenching, with Zn
2+
displaying a more marked effect (76%
quenching after 2 equiv. of metal) than Cd
2+
(38% quenching under the same
conditions) (Fig.
16
). The significant changes noted in the fluorescence spectrum
of ligand 16 upon complexation of Zn
2+
or Cd
2+
l
exc
¼
are indicative of a profound
interaction and effect on conformation.
These results are consistent with a clamp complex whose formation is
accompanied by a substantial modification of the value in the dihedral angle
between both aromatic rings. This change is responsible for the perturbations
observed in absorption and emission bands. The greater extent of quenching
observed with zinc compared to cadmium is consistent with the larger equilibrium
constant associated with the clamp-type complex formation for zinc. When com-
petitive experiments are carried out, ligand 16 selectively responds to Zn
2+
in the
presence of Cd
2+
. This ligand's greater affinity for zinc over cadmium seems to be
related to energetic factors: although clamp compound formation certainly results
in loss of entropy, it is, on the other hand, accompanied by a gain in enthalpy due to
a larger number of oxygen-metal interactions. Thus for both metal complexes, the
enthalpic factor must outweigh the entropic one when the clamp complex is formed.
Zn
2+
is a significantly harder metal ion than Cd
2+
due to its smaller size and, for this
reasons, a more marked increase in enthalpy is expected when it forms a clamp
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