Ab initio-based vibrational analysis of α-poly(L-alanine)

Polarized ir and Raman spectra have been obtained on oriented films of α-helical poly(L-alanine) (α-PLA) and its N-deuterated derivative. These improved spectra permit a more complete assignment of observed bands to A-, E1-, and E2-species modes. A new empirical force field has been refined, based on ab initio force fields of N-methylacetamide and L-alanyl-L-alanine, which reproduces observed frequencies above 200 cm−1 to less than 5 cm−1. A new transition dipole coupling treatment avoids the weak coupling and perturbation approximations, and can now account for the newly observed and reassigned amide I (E2) mode. As a result of this improved force field, several other observed bands have also been reassigned.