Evidence for a new transition in polyribonucleotides from Raman optical activity

dVibrational Raman optical activity (ROA) measurements are reported on sing le-stranded poly(rA) and double-stranded poly(rA).poly(rU) in the temperatu re range 2-45 degrees C. The intensities of one ROA couplet in poly(rA) (ce ntred at ca 1329 cm(-1) and sensitive to sugar-base conformation) and three in poly(rh) poly(rA) (two centred at ca 1252 and 1339 cm-l and sensitive t o sugar-base conformation, and one centred at ca 1681 cm(-1) and sensitive to base stacking geometry) are plotted as a function of temperature. All fo ur plots reveal a sharp glass-like transition at ca 15-18 degrees C which a ppears to embrace the entire biopolymer, Since both single- and double-stra nded polyribonucleotides show the transition, it does not appear to be prin arily associated with base-pair opening. Differential scanning calorimetry failed to reveal any latent heat, which suggests that it is not a simple fi rst-order transition. The possibility that the transition might be associat ed with a freezing of motions between conformational substates, perhaps in conjuction with changes in water structure, is discussed. Copyright (C) 199 9 John Wiley & Sons, Ltd.