Polypeptide energy-band structure

TABLE 2.8. Energy Band Structures of Periodic Polypeptides ... 

Each peptide unit lies in a plane because it consists of a delocalized system of n electrons associated with n orbitals of the C and O atoms together with the lone electron-pair orbital of the N atom. Such an electron resonance structure is sufficient to produce significant diamagnetic anisotropy in the protein. " Some two dozen amino-acid residues make up polypeptides, but only glycine and proline have a first atom in the side chain R which is not a carbon atom. Thus, many features of regularity are present which may cooperate in forming energy bands, particularly in the extended arrays of a helices and ) -pleated sheets. In fact, spectra of DNA... 

In the spectra of the SI state of anteima-bound carotenoid molecules, the frequency of the highest frequency band arising from the C=C stretching modes is lower in the peripheral than in the core antennae (Kuki et al, 1994). Since the structures of these two classes of proteins are closely related (they all derive from the aimular assembly of rather homologous polypeptides) the polarizability of the carotenoid environment is not likely to be so different in these two proteins. This frequency shift was thus rather interpreted in terms of differences in vibronic couplings between the 2 Ag and 3 Aj, electronic states, as well as a displacement of the energy level of the latter state (Kuki et al, 1994). 

The excited state of a single extended polypeptide chain is made up of a band of states with an energy spread which depends on the magnitude of the coupling constants. The transition moment to any one of these states is the sum of all the individual transition moments in the molecule, each multiplied by a phase factor (Schellman and Schellman, 1964). In the important special situation of the polypeptide chain with a twofold screw axis (sheet structures), a group of two peptide groups in sequence can be considered as a unit cell. In this situation (Davydov, 1962) the allowed motions can be described in terms of transition moments that are either in phase with one another or 180° out of phase n radians). The former case produces a parallel absorption band, the latter a perpendicular band thus, the component of the transition moments in the direction of the molecular axis contributes only to the parallel band and the component perpendicular to this axis contributes to the perpendicular band. 

---