Characterization of the A ⇄ B transition of DNA in fibers and gels by laser Raman spectroscopy

Both Raman spectra and X‐ray diffraction patterns have been obtained from oriented fibers of sodium deoxyribonucleic acid (Na‐DNA) as a function of salt content and relative humidity. We have confirmed the previously reported X‐ray results that, for oriented fibers, the A‐form always exists between 75 and 92% relative humidity and that the conformation will change to the B‐form at 92% relative humidity only if an excess (3–5%) of added salt is present. Oriented fibers containing low amounts of added salt remain in the A‐type conformation at 92% relative humidity and higher. An exact correlation has been found between the familiar A‐ and B‐type X‐ray diffraction patterns of DNA fibers and the Raman spectra previously reported without X‐ray verification from this laboratory for the A‐ and B‐forms. In particular, a band at 807 cm −1 was always present when a fiber showed the A‐type diffraction pattern, and this band shifts to 790 cm −1 in the B‐form. Using the Raman spectrum to determine the specific conformation of DNA in samples less amenable to X‐ray analysis, we have studied the A ⇄ Btransformation in unoriented fibrous masses of DNA and in concentrated, oriented gels. We find that in unoriented fibrous masses, the A ⇄ B transition always occurs at 92% relative humidity even at very low salt concentration (0–4%). However, in oriented DNA gels at low salt, the A‐form can persist as a metastable state to concentration as low as 20% DNA. The origin of the bands at 807 and 790 cm −1 and the possible biological implications of these findings are discussed.