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

Abstract

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-, E 1 -, and E 2 -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 (E 2 ) mode. As a result of this improved force field, several other observed bands have also been reassigned. © 1998 John Wiley & Sons, Inc. Biopoly 46: 283–317, 1998