Laser raman spectroscopy of a complementary base pair in the Hoogsteen configuration

Cocrystallization of m9Ade and m1Thy from water yielded crystals of te Hoogsteen base pair, m9Ade x m1Thy, as demonstrated by X-ray diffraction data. The Raman spectrum of the Hoogsteen base pair in the crystalline state was obtained and compared to the crystals of the individual monomers. Frequency changes between the heterobase dimer and both m9Ade and m1Thy are indicative of the base pairing interaction. Utilizing previously reported vibrational analyses for m9Ade, the frequency changes are clearly associated with modes involving the internal coordinates most sensitive to a Hoogsteen hydrogen bonding interaction. The spectrum of an equimolar mixture of e9Ade and ch1Ura in CHCl3 was also compared to the spectrum of the separate monomers in solution. The numerous frequency shifts and band intensity changes observed in the mixture from that of the individual monomers suggest a cyclic dimer is formed although the nature of the base pairing interaction is not clear.