Fluctuations and mechanical strength of α‐helices of polyglycine and poly(L‐alanine)

By using the static correlations of fluctuations in the dihedral angles of the α‐helices of polyglycine and poly( L ‐alanine) calculated previously, geometrical fluctuations of a section (consisting of up to 18 peptide units) of the α‐helices of infinite length are calculated. These fluctuations are found to differ in some respects (e.g., the dependence of amplitudes on the length of section) from those of a circular rod made of homogeneous continuous material. However, the moduli of the mechanical strengths (tensile Young's modulus, bending Young's modulus, and the shear modulus) of a circular rod are calculated, whose geometrical fluctuations are approximately equal to the fluctuations of a section consisting of 18 peptide units. They are of the order of 10 11 dyn/cm 2 . The tensile rigidity, flexural rigidity, and torsional rigidity are calculated to be 1.20 × 10 −3 dyn, 2.46 × 10 −19 dyn·cm 2 and 1.79 × 10 −19 dyn·cm 2 for polyglycine, and 1.96 × 10 −3 dyn, 4.05 × 10 −19 dyn·cm 2 and 3.28 × 10 −19 dyn·cm 2 for poly( L ‐alanine), respectively.