Infrared linear dichroism investigations of deoxyribonucleic complexes with histones H2B and H3

Abstract
Complexes between DNA [solmon sperm] and histones H2B and H3 were studied by means of IR linear dichroism in a wide range of histone to DNA ratios and of different relative humidities. The measurement of the dichroic ratios allows determination of the secondary structure of DNA in the complexes. The progressive addition of histone H2B or H3 to DNA probably inhibits the structural B .fwdarw. A transition and DNA remains in a B-type form at low relative humidity. A new simple method is proposed to evaluate the amount of A or B forms of DNA when both structures are present. The B .fwdarw. A transition is probably fully inhibited when only 1 molecule of H2B or H3 histone is bound per about 3 or 4 turns of DNA helix, respectively. About 4-3 turns of DNA helix represent the critical length of DNA (minimum cooperative unit) for the B .fwdarw. A transition.