| Abstract: | Calculations of the dependence of the conformational energy and the rotational strength of the amide n–π* electronic transition (in a series of α-helical polyhel-α- amino acids with different side chains) on conformation have been carried out. The conformational energies were computed by procedures developed in this laboratory; the computation of rotational strengths was carried out by the method of Schellman and Oriel, with a slight modification. Polyamino acids with both nonpolar and polar side chains were considered; in the latter case, it was assumed that the only influence of the polar side chain was on the backbone conformation and on the electrostatic field which perturbs the amide chromophore of the backbone. Only conformations in the range of backbone dihedral angles of the right- and left-handed a-helices were considered, and the assumption of regularity (i.e., uniformity of dihedral angles in every residue) was made. The rotational strength per residue was found to vary markedly with chain length (in oligomers of up to 40 residues long); both the conformational energy per residue and the rotational strength per residue were found to vary significantly with the backbone conformation, which in turn depends on the nature of the side chain. The geometry of the hydrogen bond in the α-helical backbone is the most important factor which influences the dependence of the rotational strength on conformation. The implications of these results, for the interpretation of experimental circular dichroism and optical rotatory dispersion data, are discussed. |
