SynopsisThe angle-dependent, isotropic light scattering exhibited by a diluent-swollen, ideal network is theoretically derived and compared with the light scattering exhibited experimentally by swollen real networks. I n good diluents the differepce is a measure of the spatial nonrandomness of the degree of crosslinking. A nonrandomness index (NRI) is introduced in terms of the Rayleigh ratios at zero scattering angle. A procedure is given for reliably obtaining the zero-angle Rayleigh ratio from experimental data a t finite angles (2-30"). Measurements are reported for a series of poly(2-hydroxyethyl methacrylate) (PHEMA) networks, prepared a t varying stages of dilution (in ethylene glycol, EG) and varying amounts of crosslinker (ethylene glycol dimethyacrylate, EGDMA). The theoretical Rayleigh ratios are calculated by employing the equilibrium degrees of swelling, refractive indices, and Young's moduli of the gels, measured in EG. The NRI is found to decrease upon decreasing the dilution during network formation and upon increasing the average crosslinking density. The NRI provides a probe of the network structure on a (sub)microscopic level. It is suggested that the NRI is closely correlated with the performance of elastomers under mechanical loading conditions.
The photoreactivating enzyme, PRE, monomerizes pyrimidine dimers in DNA in a light requiring reaction (lambda greater than 300 nm). However, the purified PRE from E. coli has no well-defined absorption band for lambda greater than 300 nm. Using absorption difference spectroscopy, we show that when PRE is mixed with ultraviolet-irradiated DNA, new absorption appears in the spectral region required for catalysis. There is a concomitant decrease in the absorption of the mixture for wavelength less than 300 nm. The hyperchromicity for lambda greater than 300 nm is true absorption, not an artifact due to light scattering. Both the hyperchromicity (lambda greater than 300 nm) and hypochromicity (lambda less than 300 nm) can be reversed by irradiation of 365 nm with identical first-order kinetics. We estimate the molar extinction coefficient of the new absorption to be 6900 +/- 1400 at 350 nm. We conclude that the PRE from E. coli does not possess a distinct "chromophore" which by itself is entirely responsible for the absorption of photoreactivating light. Instead, new absorption results when PRE binds its substrate, dimer-containing DNA.
SynopsisQuasi-elastic light scattering as measured by intensity fluctuation (self-beat) spect,roscopy in the time domain can be profitably used to follow both the translational diffusion D and the dominant internal flexing mode T i n t of DNA and its complexes with various histones in aqueous salt solutions. Without histones, DNA is found to have D = 1.6 X cm2/sec and 7 i n t g 5 X 10-4 sec in 0.8 M NaC1, 2 M urea a t 2OoC. Total histone as well as fraction F2A induce supercoiling (D = 2.6 X lo-" cm2/sec, 7 i n t 2.8 X sec) whereas fraction F1 induces uncoiling (D = 1.0 X loe8 cm2/sec, ~i~t 9.4 X sec). Upon increasing the salt concentration to 1.5 M the DNAhistone complex dissociates (D = 1.8 X 10-8 cmz/sec). Upon decreasing the salt concentration to far below 0.8 M , the DNA-histone complex eventually precipitates as a chromatin gel.The supercoiling of DNA in eukaryotic chromatins is believed to be induced by the binding of basic histones. lv2 The supercoiled conformation greatly reduces the template activities of DNA in chromatins from different tissues.3 We wish t o report some preliminary findings on histone-DXA complexation in various aqueous salt solutions as revealed by quasi-elastic light scattering.The histones, which are nucleoproteins of about 15,000 in molecular weight, were kindly supplied as dry powders by E. 11. Bradbury (Portsmouth Polytechnic, England). Both isolated total histones and various fractions4 were obtained. Calf thymus DNA of average molecular weight around 1.5 x 106 was purchased (Type I, D1501, Sigma Chemical, St.Louis, No.) and deproteinized by phenol treatment until a final absorbance ratio A260/&0 of 1.94 was obtained. The light-scattering instrument consisted of a helium-neon 632.8-nm laser, a temperature-controlled lightscattering cell, an RCA 7265 photomultiplier, and a pulse counting autocorrelator, interfaced with a mini-~omputer.~ The instrument operates as a self-beating spectrometer in the time, rather than frequency, domain. It produces pulse counting autocorrelation functions from which the translational diffusion coefficient and sometimes the dominant internal mode of motion of the scattering macromolecules can be calculated. Since the * The editors sorrowfully report that Willem Prins was drowned in a sailing accident in Lake Ontario, July 1974. 111
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