The elastic properties of the alpha-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 AMI 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 alpha-helix structure at the strained state. From the regression analysis of the calculated heat of formation versus cluster length, we obtained a longitudinal Young's elastic modulus of 65 GPa. The formation of a non-planar peptide bond indicates the appearance of strain localization in the a-helix structure under tensile loading.