Raman tensor of adenine residue for the 1580 cm−1 vibration and its orientation in a biopolymer

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Abstract
The adenine residues of nucleic acids give a medium‐intensity Raman band around 1580 cm−1 and protonated adenine residue around 1560 cm−1. Each of them shows a peculiarly high degree of depolarization, α = 0.65, in aqueous solutions. A corresponding band, at 1562 cm−1, shows a high anisotropy in the single crystal of uridylyl‐(3′‐5′)‐adenosine (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 shape and orientation of the Raman tensor associated with this 1562 cm−1 vibration have been determined: one of its principal axes (y‐axis) is directed along the long axis of the adenine residue and r1 = αxx/αzz = − 3 and r2 = αyy/αzz = 4. A general relationship has also been derived between the orientation angles (θ and χ) of the adenine residues with a uniaxial arrangement in a biopolymer and the anisotropy of the 1580 cm−1 Raman scattering intensity. This relationship has been used in a discussion of the orientation of the adenine residue in the poly(rA)·poly(rU) duplex fibre.