RAMAN TENSOR OF ADENINE RESIDUE FOR THE 1580 CM(-1) VIBRATION AND ITSORIENTATION IN A BIOPOLYMER

The adenine residues of nucleic acids give a medium-intensity Raman ba nd around 1580 cm(-1) and protonated adenine residue around 1560 cm(-1 ). Each of them shows a peculiarly high degree of depolarization, rho = 0.65, in aqueous solutions. A corresponding band, at 1562 cm(-1), sh ows a high anisotropy in the single crystal of uridylyl-(3'-5')-adenos ine (UpA). On the basis of the known structure of this crystal, and on the basis of the polarized Raman data (488.0 nm excitation), the shap e and orientation of the Raman tensor associated with this 1562 cm(-1) vibration have been determined: one of its principal axes Cy-axis) is directed along the long axis of the adenine residue and r(1) = alpha( xx)/alpha(zz), = -3 and r(2) = alpha>(yy)/alpha(zz) = 4. A general rel ationship has also been derived between the orientation angles (theta and chi) of the adenine residues with a uniaxial arrangement in a biop olymer and the anisotropy of the 1588 cm(-1) Raman scattering intensit y. This relationship has been used in a discussion of the orientation of the adenine residue in the poly(rA). poly(rU) duplex fibre.