Specific features of photoisomerization of provitamin D 3 in a nematic liquid crystal

Photoisomerization of provitamin D ₃ (7-dehydrocholesterol) in a nematic liquid crystal (ZLI-1695, Merck) is investigated in detail by UV absorption spectroscopy. It is found that dissolution of chiral molecules of provitamin D ₃ induces the cholesteric phase in a nematic. The spectral kinetics of photoisomerization in this phase changes significantly from that in an ethanol solution. A sharp nonmonotonic dependence of the increase in accumulation of trans isomer tachysterol in a liquid crystal matrix with a decrease in the induced cholesteric pitch from 2200 to 25 μm is revealed.     

Investigating the mechanisms of diamond polishing using Raman spectroscopy

Recent research has shown that a phase transformation of diamond to a different form of carbon is involved when diamonds are polished in the traditional fashion. The question as to how this phase transformation is activated and maintained to produce high wear rates is of great technological interest since it may radically change the way we view the processing of diamond. This paper describes the use of Raman spectroscopy to examine debris produced on the diamond polishing wheel, both during its preparation and during polishing. In addition, polished diamond surfaces were examined for the possible existence of non-diamond surface layers in an attempt to identify material removal mechanisms. Raman spectroscopy proves ideal for these analyses because its relatively high spatial resolution is well suited to the analysis of small wear features and debris particles, and because of the wealth of information it reveals about chemical structure. This level of structural information has been lacking in previous analyses of diamond polishing debris. In addition to the non-diamond carbon found in the wear debris, significant quantities of two iron oxides, magnetite (Fe₃O₄) and haematite (α-Fe₂O₃), were also found. An interesting observation was that a transformation from magnetite to haematite could be induced either by using high power laser excitation or by frictional heating during polishing. It is suggested that some of the Raman peaks previously attributed to lonsdaleite might better be explained by the presence of these oxides.     

Time dynamics of the nematic liquid crystal director in the field of a sequence of rectangular pulses

We study theoretically and experimentally the Freedericksz transition in a sequence of rectangular pulses. We show the dependence of the accumulation effect on the pulse filling coefficient. We demonstrate that at a constant energy value in a pulse, the accumulation phenomenon substantially depends on the pulse repetition frequency. An analytical consideration of the initial stage of development of orientational instability fully confirms the conclusions of numerical analysis. The results of experiment agree well with the conclusions of theoretical analysis.     

Testing heteroscedasticity by wavelets in a nonparametric regression model

In the nonparametric regression models, a homoscedastic structure is usually assumed. However, the homoscedasticity cannot be guaranteed a priori. Hence, testing the heteroscedasticity is needed. In this paper we propose a consistent nonparametric test for heteroscedasticity, based on wavelets. The empirical wavelet coefficients of the conditional variance in a regression model are defined first. Then they are shown to be asymptotically normal, based on which a test statistic for the heteroscedasticity is constructed by using Fan's wavelet thresholding idea. Simulations show that our test is superior to the traditional nonparametric test.     

Development of Personal UV Biodosimeter Based on Vitamin D Photosynthesis

Abstract

A new approach to monitoring of vitamin D synthetic capacity of UV solar/artificial radiation is described. Nematic liquid crystal (LC) was converted into cholesteric phase by chiral dopant of 7-dehydrocholesterol (provitamin D₃), and the effects of UV irradiation were studied using spectral and polarized observations. Significant changes in optical characteristics of the LC films depending on UV exposure were observed as a result of UV initiated photoisomerization that changed helical twisting power of dopant molecules.     

Theoretical basis for the possibility of conformational control of the products of photochemical transformations of provitamin D

A theoretical basis has been provided for the possibility of effecting conformational control of the products of the photochemical transformations of provitamin D by acting expressly on the conformational equilibrium of the previtamin molecule by causing an interaction with an appropriate reaction medium. The results of the analysis that was carried out are valid for any extended photoreaction with a conformationally fluxional intermediate.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 4, pp. 326–332, July–August, 1993.The present work was carried out in the framework of project No. 4/664 financed by the State Committee of the Ukraine on Problems of Science and Technology from a fundamental research fund.     

Testing coeffcients of AR and bilinear time series models by a graphical approach

AR and bilinear time series models are expressed as time series chain graphical models, based on which, it is shown that the coefficients of AR and bilinear models are the conditional correlation coefficients conditioned on the other components of the time series. Then a graphically based procedure is proposed to test the significance of the coeffcients of AR and bilinear time series. Simulations show that our procedure performs well both in sizes and powers.     

Conformational control in previtamin d photochemistry by heterogeneous reaction media

The possibility of conformational control realization in the reaction of previtamin D photosynthesis in vitro was examined. The sharp changes in the kinetics of provitamin D photoisomerization in some heterogeneous media were attributed to the removal of previtamin D conformational equilibrium by the restriction of its intramolecular rotation. It was found that the quantitative evaluation of the reaction medium effect on the specific previtamin D reaction channels requires consideration of the irreversible photoconversions.     

Steroid motor: dynamics of cholesteric helix induction in the nematic droplet

A nematic liquid crystal can be converted into a cholesteric phase by a chiral dopant, and the cholesteric pitch can be changed by its photochemical transformations [1]. For the first time we investigate the dynamics of the cholesteric phase induction using seven steroids (vitamin D isomers and related compounds) as chiral dopants. Here we report the new effect of rotation of a rod-like steroid crystal (0.1-1 mm length) when it is placed at the surface of a nematic drop and its dissolution course was followed with a polarizing microscope. For all the compounds univocal correspondence was noticed between the crystal rotation direction, the helicity of the molecular steroid ring system [2] and the sign of the cholesteric macrohelix determined by the Cano-Grandjean method [3]. No rotation was observed in the isotropic phase.