Changes in Raman vibrational bands of calf thymus DNA during the B‐to‐A transition

Abstract The B ‐to‐ A conformational transition of calf thymus DNA fibers was followed employing Raman spectroscopy. The transition was induced by soaking DNA fibers in water/ethanol mixtures increasing from 60 to 85% ethanol (v/v). Intensity changes of 17 Raman vibrational bands were quantified in the region from 400 to 860 cm −1 . Two bands at 500 and 784 cm −1 were employed as internal standards. These bands do not appear to change in intensity with ethanol concentration. Large intensity changes relative to these two bands are observed between 70 and 74% ethanol for backbone vibrations at 708, 808, and 835 cm −1 , and base vibrations at 682, 730, and 750 cm −1 . These results indicate that a highly cooperative conformational change takes place between different portions of DNA in the B ‐to‐ A transition. Relative intensity changes preceding the onset of the major transition are observed in only two bands; at 835 cm −1 , assigned to a ribose–phosphate vibration, and at 750 cm −1 , assigned to thymine. The implications of these pretransition changes are discussed.