Raman Spectra of Single Crystals of r(GCG)d(CGC) and d(CCCCGGGG) as Models for A DNA, Their Structure Transitions in Aqueous Solution, and Comparison with Double-Helical Poly(dG)•Poly(dC)

The self-complementary oligonucleotides [r(CGC)d(CGC)]₂ and [d(CCCCGGGG)]₂ in single-crystal and solution forms have been investigated by Raman spectroscopy. Comparison of the Raman spectra with results of single-crystal X-ray diffraction and with data from polynucleotides permits the identification of a number of Raman frequencies diagnostic of the A-helix structure for GC sequences. The guanine ring frequency characteristic of C3'-endo pucker and anti base orientation is assigned at 668 ± 2 cm⁻¹ for both dG and rG residues of the DNA/RNA hybrid [r(GCG)d(CGC)]₂. The A-helix backbone of crystalline [r(GCG)d(CGC)]₂ is altered slightly in the aqueous structure, consistent with the conversion of at least two residues to the C2'-endo/anti conformation. For crystalline [d(CCCCGGGG)]₂, the Raman and X-ray data indicate nucleosides of alternating 2'-endo-3'-endo pucker sandwiched between terminal and penultimate pairs of C3'-endo pucker. The A-A-B-A-B-A-A-A backbone of the crystalline octamer is converted completely to a B-DNA fragment in aqueous solution with Raman markers characteristic of C2'-endo/anti-G (682 ± 2) and the B backbone (826 ± 2 cm⁻¹). In the case of poly(dG)•poly(dC), considerable structural variability is detected. A 4% solution of the duplex is largely A DNA, but a 2% solution is predominately B DNA. On the other hand, an oriented fiber drawn at 75% relative humidity reveals Raman markers characteristic of both A DNA and a modified B DNA, not unlike the [d-(CCCCGGGG)]₂ crystal. A comparison of Raman and CD spectra of the aqueous [d(CCCCGGGG)]₂ and poly(dG)•poly(dC) structures suggests the need for caution in the interpretation of CD data from G clusters in DNA.