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

Earlier, we showed that, for the D form (n = 8 and h = 3.03 A, 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. It was also noted that 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 leads to B transition in the solid phase. As a continuation of our earlier studies, we have carried out similar experiments with poly[d(I-C)]. We could obtain a crystalline D-form pattern (n = 8, h = 3.13 A) of the fiber at 75% relative humidity (r.h.); the hydrated (r.h. approximately equal to 95%) form of the same fiber gave the classical B-form pattern (n = 10, h = 3.40 A). In the present report, we show that both right- and left-handed double-helical models are consistent with the fiber diffraction data of poly[d(I-C)] in the D-form. Theoretical energy calculations also suggest that the right- and left-handed B- and D-DNA models are almost equally stable. Hence, we conclude that the right- and left-handed double-helical models of poly[d(I-C)] in a given form (B or D) are equally likely and that the fiber diffraction data do not permit discrimination.

Unusual Structural Isomerization Of Monopropiophenone Thiocarbonohydrazone

Monopropiophenone thiocarbonohydrazone has been isolated in both linear and cyclic isomeric forms. Each form has been shown to isomerize and exist in equilibrium with the other in DMSO-d6 solution by 1H and 13C NMR spectroscopy. The kinetics of this transformation show attainment of equilibrium in approximately 6 h, with a linear to cyclic configuration ratio of 40:60.

Chemical bond approach to determining conductivity band gaps in amorphous chalcogenides and pnictides

The minimum energy required for the formation of conjugate pair of charged defects is found to be approximately equal to the experimental activation energy for d.c. conductivity in a number of amorphous chalcoganides and pnictides. This observation implies that the defect pair formation energy represents an intrinsic gap for transport in amorphous chalcogenides.