Elastic properties of the poly-l-glycine α-helix from periodic SCF-LMO calculations

The elastic properties of the α-helix of poly-l-glycine have been investigated by means of periodic self-consistent field (SCF) calculations and localized molecular orbitals (LMO), within the framework of the semi-empirical AM1 method. A cluster containing eighteen l-glycine residues was progressively stretched along the main axis and its geometry was re-optimized by fixing only the total length, in order to maintain the α-helix structure at the strained state. From the regression analysis of the calculated heat of formation versus cluster length, we obtained a longitudinal Youngʹs elastic modulus of 65 GPa. The formation of a non-planar peptide bond indicates the appearance of strain localization in the α-helix structure under tensile loading.