A Structural Investigation of Some Terminally Cyano-Substituted Side Chain Liquid-Crystalline Polysiloxanes

A series of terminally cyano substituted side chain polysiloxanes have been characterized using optical microscopy, D.S.C., and X-ray diffraction. Values for the lamellar spacings, d, of the interdigitated smectic A phases were obtained, and the variations in d with temperature and length of the flexible aliphatic spacer are discussed. The spacing for m = 4, 5, and 6 increased regularly with m whereas the short m = 3 spacer gave an anomalously high value.     

Group actions on C*-algebras, 3-cocycles and quantum field theory

We study group extensions , where acts on a C^*-algebraA. Given a twisted covariant representation ,V of the pairA, we construct 3-cocycles on with values in the centre of the group generated byV(). These 3-cocycles are obstructions to the existence of an extension of byV() which acts onA compatibly with . The main theorems of the paper introduce a subsidiary invariant which classifies actions of onV() and in terms of which a necessary and sufficient condition for the the cohomology class of the 3-cocycle to be non-trivial may be formulated. Examples are provided which show how non-trivial 3-cocycles may be realised. The framework we choose to exhibit these essentially mathematical results is influenced by anomalous gauge field theories. We show how to interpret our results in that setting in two ways, one motivated by an algebraic approach to constrained dynamics and the other by the descent equation approach to constructing cocycles on gauge groups. In order to make comparisons with the usual approach to cohomology in gauge theory we conclude with a Lie algebra version of the invariant and the 3-cocycle.     

Ion channels from linear and branched bola-amphiphiles

The synthesis and characterization of the ion channel activity of three new bola-amphiphiles is described. These compounds are conceptually derived from a previously reported bis-cyclophane bola-amphiphile through opening of the cyclophanes to acyclic structures and were found to readily form ion channels in planar bilayer membranes as assessed by bilayer clamp single-channel analysis. All three compounds behaved very similarly: the dominant channels formed by all three are Ohmic with specific conductance of 10 +/- 1 pS (NaCl electrolyte) and 39 +/- 1 pS (CsCl electrolyte). Single-ion permeability ratios, determined from dissymmetric electrolyte experiments, showed the selectivity P(Cs(+)) > P(Na(+)) > P(Cl(-)). Less frequently, lower conductance channels were also observed to act independently of the dominant channels. The lifetimes of the dominant channels range from 70 to 280 ms for the three compounds with some very long-lived openings (20-40 s) observed for two of the three. The lower conductance states have shorter lifetimes. This study demonstrates that bis-macrocyclic compounds are not essential for channel formation by bola-amphiphiles, and opens a new class of channel-forming compounds for structure-activity optimization.     

Biological effects of polychromatic light

Predicting the effects of polychromatic light on biological systems is a central goal of environmental photobiology. If the dose-response function for a process is a linear function of the light incident on a system at each wavelength within the spectrum, the effect of a polychromatic spectrum is obtained by integrating the product of the cross section for the reaction at each wavelength and the spectral irradiance at that wavelength over both wavelength and time. This procedure cannot be used, however, if the dose-response functions for an effect are not linear functions of photon dose. Although many photochemical reactions are linear within the biologically relevant range of doses, many biological end points are not. I describe procedures for calculating the effects of polychromatic irradiations on systems that exhibit certain classes of dose-response functions, including power law responses typical of mutation induction and exponential dose-responses typical of cell survival. I also present an approach to predict the effects of polychromatic spectra on systems in which the ultraviolet components form pyrimidine dimers, and the longer-wavelength ultraviolet and visible components remove them by photoreactivation, thus generating complex dose-response functions for these coupled light-driven reactions.     

RESEARCH NOTE

A previous report [Freeman et al. (1986) Photochem. Photobiol. 43S, 93S] indicated that irradiation of human skin in situ with 385 or 405 nm radiation produced detectable levels of pyrimidine dimers in DNA. Since these wavelengths are absorbed poorly by DNA, these results suggested that DNA damage was sensitized by other absorbing molecules present in skin. Examination of two experimental aspects of the previous work indicates that (1) the static gel electrophoresis method for DNA dispersion used in lesion determination gave accurate values of the levels of induced dimers, and (2) the DNA damage apparently induced by 385 nm was actually induced by shorter wavelength UV present in the 20 nm bandpass beam of the monochromator. The current results indicate that monochromatic 385 and 405 nm radiation are ineffective in dimer production in human skin in situ.