APPLICATION OF A NEW MODULATION METHOD FOR LINEAR DICHROISM STUDIES OF ORIENTED BIOPOLYMERS IN THE VACUUM ULTRAVIOLET

To understand the relationship between spectral characteristics and conformational properties of biopolymers, detailed knowledge of ultraviolet spectra extended to the vacuum region is required. The classical absorption spectra, even with polarized light, are represented by broad structures. On the other hand, the small difference in absorption of parallel and perpendicular polarized light, known as linear dichroism (LD), reveals a much finer structure and also offers the advantage of having a sign associated with it. Specifically, such linear dichroism measurements are powerful for the resolution of overlapping absorption bands and thus should be particularly useful in theoretical and experimental investigation of biopolymer structure. Several devices for LD measurements have been used for studying anisotropic ;'rystals and polymer films.1-7 Recently, Jaffe, Jaffe, and Rosenheck7 proposed an ingenious device that consists of a polarizer and a multiwave plate. However, their device can give the full measurement of the LD only at the discrete, fixed wavelengths at which the plate has a half-wave retardation, and a correction has to be applied that depends on the resolution of the spectrophotometer associated with it. The actual instrument is limited to 1800 A, but, even in principle, it will not work in the region of the line spectrum of a hydrogen lamp, that is, below 1670 A. The present method, which will be sketched below and described in detail elsewhere, allows LD measurement over a continuous range of wavelengths and, furthermore, the line spectrum of hydrogen may be exploited. It has a high resolving power, limited only by the resolution of the monochromator (about 2.5 A), which is independent of wavelength. It is characterized by a high sensitivity of detection-LD 0.07 per cent (up to 1670 A) and 0.2 per cent (1670-1490 A). This is of special importance for the study of biopolymers, since their degree of orientation is often low. The method allows direct detection of the degree of polarization p = (TI Tll)/2Tm, which is zero for a random conformation or orientation, and only ordered helical macromolecules with oriented chromophores can be distinguished. In this paper we present the results of investigations that show four wellresolved polypeptide bands, and that demonstrate the presence in polynucleotides of new, far-ultraviolet transitions which can be related to the interactions of monomers in the helical array. .11Methods.-A block diagram of the apparatus is presented in Figure 1. Light from a hydrogen discharge lamp S is dispersed by a normal incidence grating monochromator Al. The light is made fairly parallel at the exit by the lens L2 and is polarized by being passed through a synthetic quartz Rochon prism. This prism is mounted inside a pulley coupled