Prediction of a fluctuational local mode inB-form poly(dA)-poly(dT)

The reduction of the interbase hydrogen-bond force constants in a unit cell of B form poly(dA)-poly(dT) creates a local mode in the gap between 63.9 and 66.7 cm^-1. This mode emerges from the band with a zone-center frequency of 66.7 cm^-1. This band has a large h-bond stretch amplitude. Our calculations suggest that this local mode can be created by thermal fluctuations in the perfect homopolymer. We track the behavior of this mode with softening of the h bonds. As expected, we find that the mode shifts to lower frequencies as the bonds soften. The h-bond stretch amplitudes are found to increase with h-bond softening, up to a point, beyond which they decrease with further softening.