The conformational behavior of polyglycine as predicted by a density functional model with periodic boundary conditions

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Abstract
Representative conformations of polyglycine are studied by means of density functional calculations, performing complete geometry optimizations under periodic boundary conditions. The calculated stability order and the equilibrium geometries are in good agreement with the available experimental results. The performance of four density functionals (LSDA, PBE, BLYP, VSXC) are compared both for the dipeptide analogue and for the infinite homopolypeptide. Our results indicate that PBE and BLYP are the models of choice for analyzing conformational equilibriums in polypeptides. While the geometry of the different conformations of polyglycine and the stability order are almost converged at the 6–31G(d) level, the relative energies are not stable until the 6–311++G(2d,2p) basis set level is reached. A comparison between the geometries of glycine dipeptide analogue and of glycine infinite homopolypeptide allows us to gain further insights on the influence of long range effects on the geometry and the stability of the different conformers. This study shows the feasibility of complete high level ab initio optimizations of infinite polypeptides, paving the route for new interesting applications of reliable quantum mechanical methods to biological systems.