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Resonance Raman spectra have been obtained at several uv wavelengths (200–266 nm) for poly(rA)-poly(rU), poly(dA-dU), poly(dA)-poly(dT), and poly(dA-dT), representing nucleic acid duplexes containing A-U and A-T base pairs with different stacking interactions and different backbone conformations. Frequency shifts are seen in the exocyclic modes corresponding to coupled C4O and C5C6 stretching of U and T, although the NH2 scissors frequency of A is unshifted relative to that of the mononucleotide. These frequency patterns are interpreted as the superposition of H bonding and dipole–dipole coupling effects. Strong hypochromism is seen for most of the ring modes, resulting from the absorption hypochromism and from shifts in the electronic transitions due to stacking. The effects are larger for A bands in the homopolymers than in the heteropolymer duplexes, reflecting the larger influence of A/A than A/U(T) stacking. Poly(rA)-poly(rU) stands out among these polymers in showing 10 cm−1 downshifts in one U and several A modes. These shifts may be related to the A form structural parameters of this duplex, but the physical mechanism is not obvious.

Carboxylate and guanidinium groups of amino acids have been studied by ultraviolet resonance Raman (UVRR) spectroscopy. On near-resonant excitation (192 nm) into the allowed π–π* transition of the carboxylate chromophore, the vibrational spectrum of aspartate displays only one band originating from the COO− symmetric stretching mode, consistent with an A-term enhancement mechanism. The UVRR spectra of the guanidinium ion reveals a complex vibrational pattern, and the bands have been assigned with the aid of a normal coordinate analysis. UV enhancement is observed for both asymmetric (E′) and totally symmetric (A1′) modes, and also for the overtone of the out-of-plane CN3 bending mode, Γ(A2″). The E′ enhancement is suggested to reflect Jahn—Teller splitting in the first allowed electronic transition A2″ E″, whereas the 2Γ(A2″) enhancement requires a lowering of the force constant for out-of-plane bending in the excited state, consistent with its expected tendency toward pyramidalization. Similar enhancement patterns are seen in the UVRR spectra of the guanidinium chromophore in methylguanidinium and arginine, which show, in addition, splittings and intensity redistribution due to the substituents. The excitation profiles of guanidinium and arginine reveal a complex structure that may arise from an excited-state Jahn—Teller effect. The Raman enhancements are too low to permit detection of guanidinium or carboxylate in the UVRR spectra of proteins, even at wavelengths as low as 192 nm.