Correlation corrected band structures of different homopolypeptides.

Abstract

Using the ab initio Hartree–Fock + Moller–Plesset second order method (putting the latter into the inverse Dyson equation in its diagonal approximation as self energy) the band structures for two acidic homopolypeptides and their two amides (poly(aspartic acid), poly(glutamic acid), poly(asparagine) and poly(glutamine)) as well, for a system with a ring shaped side chain (poly(histidine)), for two homopolypeptides containing a sulfur atom (poly(cysteine) and poly(methionine)) and finally for poly(proline) were calculated. The results obtained show that in these homopolypeptides with more complicated side chains (1) the band dispersion both in the HF and quasiparticle (QP) approximation are not monotonic. (2) the positions of the bands differ more from each other than in the case of simpler aliphatic side chains. Further in two cases the valence bands cross in the QP approximations and in one case the gap between VB and VB − 1 is only 0.01 eV which is smaller than the thermal energy at 310 K. The widths of the conduction bands are generally larger than those of the valence bands and in most cases the QP bands are broader than the HF ones in contrary to the results for simpler polymers. Finally the fundamental gaps are, as expected smaller by 3–4 eV in the QP case than in the HF one. However, their values of 9.5–10.5 eV are still substantially larger than the values which can be estimated on the basis of the experimental exciton spectra of polyglutamine (7–8 eV). It is outlined how this discrepancy could be decreased using a better approximation for the correlation corrections and applying better basis sets.