Peptides, electronic transitions

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. 

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. 

These studies show that Cu(III)-peptide complexes have relatively low electrode potentials and suggest that Cu(III) may be a far more common oxidation state than had previously been thought possible. Furthermore, the decomposition reactions of Cu(III)-peptides indicate that two-electron transitions to give Cu(I) species are possible. Two-electron redox reactions in biological systems are intriguing because high energy, free radical intermediates are avoided. However, as yet we know very little about possible Cu(I) complexes. This oxidation state is poorly characterized in aqueous solution, and studies with various model complexes are needed. 

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]. 

Moreover, to model proteins and peptides, MARCH-INSIDE biodescriptors were derived from the kth powers of an electron-transition stochastic matrix based on the —> Electronic Charge Index used in place of the electronegativity [Ramos de Armas, Gonzalez Diaz et al., 2005]. 

Now electronic transitions in the polypeptide are associated with the delocalized carbonyl electron. If the carbonyl group alone is considered in C2v symmetry, the n-tt (symmetry forbidden transition) is a2 and tt-tt is b in Cs symmetry one has a" and a allowed transitions. Neither is precisely correct, but for present purposes we can treat the ti-tt transition as polarized perpendicular to the carbonyl bond and in the plane of the peptide group, and tt-tt as in the plane of the peptide group and nearly parallel to... 

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. 

A new peptidic Au(i)-metalloamphiphile able to self-assemble into micellar nanostructures of 14 nm in diameter shows interesting photophysical properties. In particular, the luminescence of these species can be assigned to electronic transitions from triplet-excited states with excited state life times of 1.5 ps. ... 

Comparison of the quantum chemical calculations for electronic transitions for the structure modeling the interaction of LiCI(I)-l2-a-dextrin-peptide complex with the nucleotide triplet indicates that the DNA nucleotides can displace polypeptide and form stable complexes with molecular iodine and lithium halogenides. Interestingly, in such structures, molecular iodine binds both nucleotide triplet and lithium halogenides. 

In protein molecules the main source of optical activity is the peptide bonds. Consequently CD spectra are taken at wavelengths shorter than 240 nm where the predominant absorption comes from peptide bonds. This arises from three electronic transitions, namely 0) sn n- Jt transition at -210-220 nm, in which an electron in the non-bonding molecular orbital of the carbonyl oxygen is promoted to an antibonding n molecular orbital, (ii) a it r transition at -190 nm, and (iii) a probable transi-... 

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