Electronic transition peptide chromophore

Although the n-n and tz-tz electronic transitions of the urea chromophore have not been studied as extensively as amides, the contribution of the backbone is expected to dominate the far UV spectra of oligoureas in a fashion similar to that which is observed for peptides. The CD spectra recorded in MeOH of oligoureas 177 and 178 show an intense maximum near 204 nm (Figure 2.48). This is in contrast to helical y" -peptides that do not exhibit any characteristic CD signature. 

The electronic transitions of the bound chromophore are coupled to the transitions of the protein, e.g., aromatic amino acid side chains and/or peptide bonds by the coupled oscillator mechanism [68-70]. 

Exciton-coupled circular dichroism (ECCD) spectroscopy was performed on the Aib-Stb-D peptide to fiirther substantiate the interaction between the methyl stilbene side-chains, which was observed via PL experiments. The interaction between the excited state of chromophores in a chiral environment causes split Cotton effects upon absorption of circularly polarized light by the chromophores (29, 30). It can be observed from Figure 7 that Aib-Stb-D exhibits a split CD Cotton effect presumably because of the chiral presentation of the methyl stilbene molecules on the same side of the a-helical peptide the asymmetric nature of the split observed may be due to some other electronic transitions or of additional background ellipticity, as has been observed in other systems (29, 31). The ECCD results confirm the close proximity of the methyl stilbene side chains mediated by the peptide backbone, which permits interaction between the side-chains in the excited state. 

Resonance Raman Spectroscopy. If the excitation wavelength is chosen to correspond to an absorption maximum of the species being studied, a 10 —10 enhancement of the Raman scatter of the chromophore is observed. This effect is called resonance enhancement or resonance Raman (RR) spectroscopy. There are several mechanisms to explain this phenomenon, the most common of which is Franck-Condon enhancement. In this case, a band intensity is enhanced if some component of the vibrational motion is along one of the directions in which the molecule expands in the electronic excited state. The intensity is roughly proportional to the distortion of the molecule along this axis. RR spectroscopy has been an important biochemical tool, and it may have industrial uses in some areas of pigment chemistry. Two biological appHcations include the deterrnination of helix transitions of deoxyribonucleic acid (DNA) (18), and the elucidation of several peptide stmctures (19). A review of topics in this area has been pubHshed (20). 

Suppose that a compound contains two chromophoric groups that exhibit electronic bands at va and vb as shown in Fig. 1-29. Then, vibrations of chromophore A are resonance-enhanced when v0 is chosen near vA, and those of chromophore B are resonance-enhanced when vo is chosen near vB. For example, heme proteins such as hemoglobin and cytochromes (Chapter 6, Section 6.1) exhibit porphyrin core n-n transitions in the 400-600 nm region and peptide chain transitions below 250 nm. Thus, the porphyrin core and peptide chain vibrations can be selectively enhanced by choosing exciting lines, in the regions of these electronic absorptions. 

The extension of the spectral range to the vacuum uv region (below 180 nm) would make possible the study of additional n-S and ti-ti transitions of amino acids and peptides not yet accessible with existing commercial instruments. On the longer-wavelength side, it has become technically possible to measure vibrational optical activity via ir and Raman techniques, and one may hope that commercial equipment for such measurements will eventually become available. Present ORD/CD techniques that are used to measure electronic optical activity require the presence of a chromophoric group in the molecule. In contrast, any vibrationally excitable bond in an assymmetric molecule will give rise to vibrational optical activity. 

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