LOW-FREQUENCY VIBRATIONAL MODES OF AN RNA - POLY(I)-POLY(C)

A recent calculation of the normal modes of double-helical poly(A).cntdot.poly(U) in the A conformation (11-fold helices) predicts large amplitude breathing and rocking modes having frequencies < 450 cm-1. (The breathing mode consists of in-phase stretching of the Watson-Crick base-pair H-bonds while the rocking mode is an out-of-phase stretching of the same bonds.) Nucleic acid films have a large water content [at 51% relative humidity poly(I).cntdot.poly(C) has 0.28 mg H2O/mg dry RNA.] The large absorption coefficients of water in the far IR mask any absorption due to the nucleic acid except when .delta. < 100 cm-1. Here the water absorption decreases and what is believed to be the 1st description of the low-frequency modes of a nucleic acid is reported.