A reinterpretation of the infrared linear dichroism of oriented nucleic acid films and a calculation of some effective partial changes on the ribose phosphate backbone.

In this paper we show, based on symmetry considerations, that structural information cannot be obtained from the linear infrared dichroism of the dioxy vibrations of the phosphate group of nucleic acids. Consequently, the discrepancies between the results of x-ray structure measurements and linear dichroism measurements are not meaningful. The linear dichroism measurements are instead important for a calculation of transition dipole moments that involve both the vibrations of all the atoms of the nucleotide and their charges. Independent information on either the atomic displacements contributing to a given vibration or the atomic charges permits a refinement of the unknown quantities. Based on the molecular dynamics calculations of Prohofsky et al., atomic charges of DNA are calculated to reproduce the observed linear dichroism results. Some of the resulting charges are unexpected and may reflect the inadequacy of the molecular dynamic calculation.

A simple technique for wavelength modulation of optical spectra.

A technique for wavelength modulation of optical spectra easily applicable to any type of monochromator is described. The device consists of a blade of transparent material fastened on a piezoelectric transducer; this assembly is mounted behind the entrance slit of the monochromator. Structure in a spectrum is enhanced by the wavelength modulation technique, even if it is superimposed on an intense background. Important features of this device are that it does not alter the adjustment of any element in the monochromator, its effect on the alignment of the monochromator is negligible for most applications, and it can be very easily removed from the monochromator. Examples of its use are given and the electronic equipment is described.