THE INVESTIGATION OF THE ELECTRONIC-STRUCTURE OF PERIODIC AND NONPERIODIC COLLAGEN MODELS

Collagen is a protein which is not periodic, but its sequence still ha s some well defined regularities. We have performed model investigatio ns for the electronic structure of this protein which is one of the fe w native polypeptides which does not possess a nearly random sequence (intermediate case between periodic polypeptides and polypeptides with random sequences). The total electron density of states has been dete rmined for model polypeptide chains of the type I collagen triple heli x, with periodic and aperiodic amino acid sequences. Calculations have been performed using the negative factor counting technique in its ab initio Hartree-Fock matrix block form. The well-known sequence regula rities of collagen have been taken into account, in order to simplify the model calculations. Beside glycine, proline, hydroxyproline and al anine, only the next two most frequently occurring amino acid residues (glutamine and arginine) have been incorporated in the sequences. The conformation of the polypeptide chain was that occurring in the colla gen triple helix. It has been taken from previous empirical potential energy calculations of collagen-like model triple helices by Nemethy a nd Scheraga. The density of states histograms for the periodic polypep tide chains show only very narrow peaks in both the valence and conduc tion bands attributable to the sequence regularities, while the aperio dic chains exhibit both narrow peaks and broader allowed regions in bo th bands. In other words, the electronic structure of the aperiodic mo del chains investigated falls between those for periodic and completel y random polypeptides. The results are discussed in terms of the gener al regularities of the collagen sequence.