Abstract
Infrared and Raman spectra of aqueous poly(rA‐rU)·poly(rA‐rU), the double‐helical complex containing strands of alternating riboadenylate and ribouridylate residues, display significant differences from one another and from corresponding spectra of poly(rA)·poly(rU), the double‐helical complex of riboadenylate and ribouridylate homopolymers. Parallel studies on the copolymer and homopolymer complexes by cesium sulfate density gradient centrifugation, ultraviolet absorption spectroscopy, hydrogenion titration, 1‐N oxidation of adenine residues by monoperphthalic acid and X‐ray diffraction reveal, however, that the geometry of base pairing between adenine and uracil is closely similar in each complex and apparently of the Watson‐Crick type. Therefore the differences observed between vibrational spectra of poly (rA‐rU)·poly (rA‐rU) and poly(rA)·poly(rU) are not due to different base‐pairing schemes but may be attributed to differences in vibrational coupling between vertically stacked bases. Vibrational coupling may also account for the differences between infrared and Raman spectra of the same complex. Thus, the present results indicate that infrared and Raman frequencies of RNA in the region 1750–1550 cm^−1^ should be dependent on the base sequence.