Investigation of DNA structural changes by infrared spectroscopy

DNA‐oriented samples of various origins were studied under different conditions of humidiity and sodium chloride content by means of infrared spectroscopy. ( 1 ) Oriented DNA ( M. Lysodeikticus , E. coli , calf thymus and salmon sperm) films at 3–4% sodium chloride yield polarized spectra which show drastic changes at relative humidities (r.h.) between 94% and 0% indicative of conformational changes:B form → a form → disordered formThe measurements of the infrared dichroism at frequencies of about 1230 cm −1 and at about 1090 cm −1 allow one to determine the orientation of the phosphate group, whereas the measurements at 1710 cm −1 characterize the base orientation. At humidities higher than 90% r.h. (B form) the bisector of OPO forms an angle of 70° relative to the helix axis, whereas at lower humidities, between 75% and 50% r.h. (A form) a rotation to about 45° is observed. Simultaneously, the 0—0 line of phosphate group changes its orientation from 55° to 65° to the helix when B → A transition takes place. The results are in general agreement with that of X‐ray diffraction and allow one to determine the orientation of the phosphate group with greater precision. ( 2 ) The B–A conformational change is not observed for satellite DNA, isolated from Cancer pagurus , of which the guanine + cytosine content is below 5%. As a function of decreasing humidities, one observes the transition:B form → disordered formA diagram of conformational changes of DNA's as a function of base composition and of r.h., suggests that B–A transition will occur for DNA of relatively higher G + C content, whereas for high (A + T) content, base sequence may be of importance. The B–A transition is prevented in DNA at a relatively high or very low sodium chloride content.