Left-handed helices for DNA: studies on poly[d(I-C)].

Abstract

For the D form (n = 8 and h = 3.03 .ANG., where n is number of nucleotide units per turn and h is height per nucleotide unit) of poly[d(A-T)], both right- and left-handed double helical models are stereochemically satisfactory and give good agreement with the observed fiber diffraction data. The conformations of the right- and left-handed D-DNA models are very similar to those of the right- and left-handed B-DNA models. This observation was consistent with the D .fwdarw. B transition in the solid phase. Similar experiments were carried out with poly[d(I-C)]. A crystalline D-form pattern (n = 8, h = 3.13 .ANG.) of the fiber was obtained at 75% relative humidity (r.h.); the hydrated (r.h. .apprxeq. 95%) form of the same fiber gave the classical B-form pattern (n = 10, h = 3.40 .ANG.). Both right- and left-handed double-helical models are consistent with the fiber diffraction data of poly[d(I-C)] in the D-form. The right- and left-handed B- and D-DNA models are probably almost equally stable. The right- and left-handed double-helical models of poly[d(I-C)] in a given form (B or D) are equally likely and the fiber diffraction data do not permit discrimination.