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A vibrational spectroscopic study of the self‐association of polyinosinic acid and polyguanylic acid in aqueous solution

We have studied by Raman and ir spectroscopy the structure of self‐associated polyinosinic acid and polyguanylic acid in aqueous solution. The results are consistent with the formation of a four‐stranded complex, which melts cooperatively near 60°C in the case of poly (I) in the presence of K+ ions. The conformation of the ribose in both systems is mixed C2′‐endo/C3′‐endo, giving a structure that is intermediate between the extremes proposed previously from x‐ray diffraction studies. Characteristic Raman bands for the C2′‐endo ribose conformation in polyribonucleotides are identified. The four‐stranded structure of poly (I) appears to be very flexible, with ≈15% of the tetrameric segments being disrupted and ≈30% of the ribose units adopting a disordered conformation prior to melting. This disordering process increases to ≈75% above the melting transition, with the remaining ≈25% of the ribose units keeping an ordered C2′‐endo or C3′‐endo conformation. © 1994 John Wiley & Sons, Inc.