Local Raman Tensors of Double-Helical DNA in the Crystal: A Basis for Determining DNA Residue Orientations

We report the first experimental determination of Raman scattering tensors for localized base and sugar-phosphate vibrations of double-helical DNA. Argon-laser excitation was employed in combination with a multichannel Raman microscope system to measure polarized Raman scattering intensities from oriented single crystals of d(CGCGCG) in the left-handed Z conformation. The Raman measurements were made on crystals of dimensions abc = 30 × 50 × 70 μm in space group P2₁2₁2₁, for which the three-dimensional structure has been solved by X-ray methods.^1,2 For each intense band in the 300–1700-cm⁻¹ interval of the Raman spectrum, we determined the relative scattering intensities, I_aa, I_bb, and I_cc, corresponding to the aa, bb, and cc components of the crystal Raman tensors. The tensor quotients from the crystal were augmented with measured depolarization ratios of analogous Raman bands in the solution isotropic form of Z-DN A and its nucleotide constituents. From these data, we have calculated the shapes and orientations of localized, vibration-specific, Raman scattering tensors applicable to normal modes of the bases (625 (dG), 670 (dG), 784 (dC), 1264 (dC), 1318 (dG), 1486 (dG), and 1579 (dG) cm⁻¹), phosphate-ester moieties (749, 796, 868, and 1095 cm⁻¹), and furanose substituents (1426 and 1433 cm⁻¹). The results differentiate normal modes with polarizability changes in the planes of the bases (dC or dG), from those perpendicular to the base planes, and along specific bond-angle bisectors (OPO, HCH). These findings provide a basis for future applications of Raman microscopy as a probe of DNA orientation and anisotropy in biological complexes.