POLARIZED RAMAN-SPECTRA OF ORIENTED FIBERS OF A-DNA AND B-DNA - ANISOTROPIC AND ISOTROPIC LOCAL RAMAN TENSORS OF BASE AND BACKBONE VIBRATIONS

Polarized Raman spectra of oriented fibers of calf thymus DNA in the A and B conformations have been obtained by use of a Raman microscope o perating in the 180 degrees back-scattering geometry. The following po larized Raman intensities in the spectral interval 200-1800 cm(-1) wer e measured with both 514.5 and 488.0 nm laser excitations: (1) I-cc, i n which the incident and scattered light are polarized parallel to the DNA helical axis (c axis); (2) I-bb, in which the incident and scatte red light are polarized perpendicular to c; and (3) I-bc and I-cb, in which the incident and scattered light are polarized in mutually perpe ndicular directions. High degrees of structural homogeneity and unidir ectional orientation were confirmed for both the A and B form fibers, as judged by comparison of the observed Raman markers and intensity an isotropies with measurements reported previously for oligonucleotide s ingle crystals of known three-dimensional structures. The fiber Raman anisotropies have been combined with solution Raman depolarization rat ios to evaluate the local tensors corresponding to key conformation-se nsitive Raman bands of the DNA bases and sugar-phosphate backbone. The present study yields novel vibrational assignments for both A DNA and B DNA conformers and also confirms many previously proposed Raman vib rational assignments. Among the significant new findings are the demon stration of complex patterns of A form and B form indicator bands in t he spectral intervals 750-900 and 1050-1100 cm(-1), the identification of highly anisotropic tensors corresponding to vibrations of base, de oxyribose, and phosphate moieties, and the determination of relatively isotropic Raman tensors for the symmetrical stretching mode of phosph odioxy groups in A and B DNA. The present fiber results provide a basi s for exploitation of polarized Raman spectroscopy to determine DNA he lix orientation as well as to probe specific nucleotide residue orient ations in nucleoproteins, viruses, and other complex biological assemb lies.