B-DNA Helix Stability in a Solvent-Free Environment

B-DNA is the most common DNA helix conformation under physiological conditions. However, when the amount of water in a DNA solution is decreased, B-to-A helix transitions have been observed. To understand what type of helix conformations exist in a solvent-free environment, a series of poly d(CG)(n) and mixed sequence DNA duplexes from 18 to 30 bp were examined with circular dichroism (CD), ESI-MS, ion mobility, and molecular dynamics. From the CD spectra, it was observed that all sequences had B-form helices in solution. However, the solvent-free results were more complex. For the poly d(CG)(n) series, the 18 bp duplex had an A-form helix conformation, both A- and B-helices were present for the 22 bp duplex, and only B-helices were observed for the 26 and 30 bp duplexes. Since these sequences were all present as B-DNA in solution, the observed solvent-free structures illustrate that smaller helices with fewer base pairs convert to A-DNA more easily than larger helices in the absence of solvent. A similar trend was observed for the mixed sequence duplexes where both an A- and B-helix were present for the 18 bp duplex, while only B-helices occur for the larger 22, 26, and 30 bp duplexes. Since the solvent-free B-helices appear at smaller sizes for the mixed sequences than for the pure d(CG)(n) duplexes, the pure d(CG)(n) duplexes have a greater A-philicity.