The A-B transition: temperature and base composition effects on hydration of DNA

Natural DNAs and some polynucleotides organised in fiber present the A-B form transition at a relative humidity (r.h.) which depends on the temperature. A shift of the midpoint of that helix-helix transition to higher r.h. values is observed when the temperature is risen. It is shown that the average number of water molecules associated to a nucleotide pair is the relevant parameter for the A-B transition and that this parameter can be given a precise value by a combination of different r.h. and temperature values. The minimum number of water molecules necessary to get the B form depends on the base composition of the DNA. It is observed that AT base pairs have a higher affinity toward water molecules than GC base pairs. In the B form there are 27 water molecules per CC nucleotide pair and 44 per AT pair. Moreover, we noted that the fraction of nucleotides in the B form as a function of the average number of water molecules associated per base pair does not depend on the temperature. The A helical form is obtained with about II water molecules per nucleotide pair and this number is not very sensitive to the base composition of DNA. (C) 2001 Elsevier Science B.V. All rights reserved.