Crystal and Solution Structures of the B-DNA Dodecamer d(CGCAAATTTGCG) Probed by Raman Spectroscopy: Heterogeneity in the Crystal Structure Does Not Persist in the Solution Structure

The self-complementary dodecamer d(CGCAAATTTGCG) crystallizes as a double helix of the B form and manifests a Raman spectrum with features not observed in Raman spectra of either DNA solutions or wet DNA fibers. A number of Raman bands are assigned to specific nucleoside sugar and phosphodiester conformations associated with this model B-DNA crystal structure. The Raman bands proposed as markers of the crystalline B-DNA structure are compared and contrasted with previously proposed markers of Z-DNA and A-DNA crystals. The results indicate that the three canonical forms of DNA can be readily distinguished by Raman spectroscopy. However, unlike Z-DNA and A-DNA, which retain their characteristic Raman fingerprints in aqueous solution, the B-DNA Raman spectrum is not completely conserved between crystal and solution states. The Raman spectra reveal greater heterogeneity of nucleoside conformations (sugar puckers) in the DNA molecules of the crystal structure than in those of the solution structure. The results are consistent with conversion of one-third of the dG residues from the C2′-endo/anti conformation in the solution structure to another conformation, deduced to be Cl′-exo/anti, in the crystal. The dodecamer crystal also exhibits unusually broad Raman bands at 790 and 820 cm^-1, associated with the geometry of the phosphodiester backbone and indicating a wider range of (α, ζ) backbone torsion angles in the crystal than in the solution structure. The results suggest that backbone torsion angles in the CGC and GCG sequences, which flank the central AAATTT sequence, are significantly different for crystal and solution structures, the former containing the greater diversity. The Raman data also suggest no significant change in the geometry of the central AAATTT domain between crystal and solution structures.