HYDRATION OF SINGLE-STRANDED PHOSPHODIESTER AND PHOSPHOROTHIOATE OLIGODEOXYRIBONUCLEOTIDES

Infrared spectroscopy was used to identify hydration-sensitive structu ral differences between single-stranded phosphorothioate (PS) and phos phodiester (PO) oligodeoxyribonucleotides. Spectra were recorded in th e mid-infrared region, 500-1800 cm(-1), at relative humidities between 0 and 98%; the PS and PO spectra are substantially different. The hyd ration effects on spectral bands in these single-stranded oligodeoxyri bonucleotides is markedly different from such behavior in double- and triple-stranded oligodeoxyribonucleotides. A strong absorption occurs at 656 cm(-1) in the phosphorothioate sample which is completely absen t from the PO spectra. Gravimetric measurements were carried out on on e PS and one PO sample to monitor and confirm hydration. The calculate d BET adsorption constants [Brunauer,S., Emmett,P.H. and Teller,E. (19 38) J. Am. Chem. Soc., 60, 304-319] are 1.2 and 1.4 water molecules pe r nucleotide in the first hydration layer of PS and PO respectively. W hile the gravimetric data indicate that the single-stranded oligodeoxy ribonucleotides hydrate very similarly to duplex DNA, the mid-infrared conformational marker bands are strikingly different from those obser ved for duplex DNA. In particular, the v(as) of the phosphate group (P O2) at 1222 cm(-1) in the single-stranded PO spectra is independent of relative humidity.