No abstract

SynopsisThe double-stranded helical complexes of poly(dG-dC) and of poly(dG-m5dC) are shown to convert from B-to Z-DNA-type conformations at moderate or low ionic strengths, lower for the 5-methyl than for the non-methyl species, in a highly cooperative temperature-dependent equilibrium. In the presence of low concentrations of divalent ion, e.g., Mg*+, the temperature at which the B -Z transition occurs is virtually independent of the salt concentration and the B-conformation is favored at lower temperature, while the 2-conformation is favored at higher temperature. Since the Debye-Huckel screening parameter changes rapidly with ionic strength in this region, electrostatic interaction with the free ions appears to be only a small factor in the forces that promote the transition; the temperature dependence must derive principally from effects on the solvent. The temperature dependence at high salt concentrations is also reported.

No abstract

The phenomenon of electro-optic orientation was discovered by John Kerr in 1875 and has been used extensively for determining the optical polarizability anisotropy of small molecules and for high-speed transmission of optical signals. Measurements on biopolymers have been made at least since 1950, but only in the last decade have these yielded definitive structural and physical information. In the course of this review, it should become obvious that among the reasons for this late development is the inherent difficulty of analysing optical data that depend simultaneously on intrinsic optical-structural properties of the molecules, and on their degree of orientation under the conditions of the experiment. The problem has been particularly difficult far biopolymers such as the nucleic acids, whose polarization in an electric field is dependent on their special polyelectrolyte properties. These unique electrostatic properties are an important feature in the interpretation of the experimental observations.

lous is used here in the customary sense that the curve of birefringence us. wavelength is not monotonic but instead has inflection points.