羧甲基纤维素钠水凝胶的制备及其生物降解性研究

用羧甲基纤维素钠(CMC—Na)制得了含水量高达98％的水凝胶，考察了防腐剂、交联剂、无机态氮素、有机态氮素、碳水化合物的加入量以及环境中pH值等因素对生物降解性的影响。结果表明：制备条件不同，水凝胶的生物降解性不同；环境中一定量铵根离子的存在有利于水凝胶的生物降解；在pH=5．2的环境中纤维素酶活性最高，降解程度最大。     

Chemically induced dynamic electron spin polarization-detected energy transfer. Substrate size effects and solvent dependence

Time-resolved electron paramagnetic resonance spectroscopy is used to probe energy transfer from aromatic photoexcited triplet states to azo compounds in liquid solution. The observation of chemically induced dynamic electron spin polarization in the spectra gives precise information regarding the spin physics and mechanism of the energy transfer process. The substrate size is varied by altering the chain length of alkyl chains covalently attached to the azo compounds via ester or amide linkages. The solvent dependence of the energy transfer process is also investigated. The results are discussed in terms of Dexter and F?rster mechanisms for energy transfer, the properties of the excited states, and the diffusive properties of the molecules in the solvents of interest. Decomposition rate studies and fluorescence measurements are also reported.     

Wave-mechanical theory of field ionization and field-ion energy distributions

Although field-ion energy distributions have been measured for many years, no theoretical treatment has ever been published that is strictly compatible with the axioms of quantum mechanics. This article shows how a fully wave-mechanical theory of field ionization and field-ion energy distributions can be constructed, based on the idea that surface field ionization is the fermion analogy for Dirac's treatment of photon emission. The theory is formal and suggests, as expected, that existing quasiclassical treatments are essentially satisfactory in normal circumstances.The discussion comprises of three main steps. The first is to clarify the various definitions of energy and potential used in field-ion theory, and to codify the corresponding energy diagrams. The second is to derive and expression for the rate-constant for transfer from one vibrational state to another. The third is to sum over initial states to derive a formal expression for the ion energy distribution. The article concludes by discussing the possible development of this theory.     

Unusually low barrier to carbamate C-N rotation

The barrier for rotation about an N-alkylcarbamate C(carbonyl)-N bond is around 16 kcal/mol. In the case of an N-phenylcarbamate, the rotational barrier is lowered to 12.5 kcal/mol, but with N-(2-pyrimidyl)carbamates the barriers are so low (<9 kcal/mol) that the syn and anti rotamers cannot be observed as separate signals by 500 MHz NMR spectroscopy at 183 K. X-ray and computational data show that the N-(2-pyrimidyl) carbamates have C(carbonyl)-N bonds that are on average 0.03 A longer than for related N-phenylcarbamates. The computational results trace the origin of the effect to increased single bond character for the C(carbonyl)-N bond due to the increased electron-withdrawing ability of the pyrimidyl ring.     

EFFECT OF UV RADIATION ON SURVIVAL OF NON-HAIRED MICE

Abstract— Patterns of mortality in a series of photocarcinogenesis experiments were examined. All experiments involved chronic irradiation of genetically non-haired mice with simulated sunlight from a Xe lamp. Experimental variables included genetic origin of the test animals, incident dose of radiation, and the spectral quality of the radiation. In 16 experiments involving 10 genetic origins of mice the following patterns were detected: (1) survival was inversely proportional to the delivered radiation dose; (2) within origins the life-shortening efficacy of radiation was closely correlated with its carcinogenic efficacy; (3) between genetic origins the carcinogenicity and lethality of a radiation dose were qualitatively correlated, but relative efficacy for the two effects varied; (4) altering the source spectrum by modifying the short-wave (UVB) cutoff produced similar changes in carcinogenicity and lethality, suggesting that UVB was a significant contributor to lethal efficacy; (5) nature and relative timing of the carcinogenic response were such that carcinogenesis was not likely to have caused the observed mortality. It is speculated that systemic effects, possibly immunologic or toxic, are produced by chronic UV irradiation, and that these apparently cumulative, dose-dependent effects result in premature death of non-haired mice.     

Thermally induced defects in a polycrystalline diamond layer on a tungsten carbide substrate

In this study we make use of laser heating of HTHP industrial diamond, to study temperature induced changes to the diamond structure, both chemically and mechanically, in the absence of mechanical forces. This has relevance to the efficacy of diamond as a hard material in such applications as rock drilling and material processing. We report on the induced defects when the diamond is irradiated with high power CO₂ and Nd:YAG lasers respectively, and show that the thermal induced stresses in the diamond are sufficient to radically alter its physical properties, resulting in critical fracture. Raman spectroscopy, X-ray diffraction and scanning electron microscopy indicate that the heating does not result in graphitisation of the diamond, but rather diffusion from the non-diamond base results in cobalt and tungsten oxides forming on the diamond surface. This has a deleterious effect on the diamond performance.     

Trapped electrons in amorphous solids: A method for estimating the concentration of molecular vacancies

The densities for n-propanol and n-butanol in their glassy and crystalline forms have been measured at 77 K. The density difference of ca. 0.11 cm³ g^?1 is comparable with that normally found between liquids and solids at their melting points. The maximum yield of trapped electrons in the γ-irradiated glassy alcohols has been estimated and, if it is assumed that they are exclusively trapped at molecular-sized vacancies present only in the glasses, then such vacancies account for no more than ca. 1.0% of the volume difference between the crystalline and glassy materials. Much of the remaining difference can be accounted for in terms of long and bent hydrogen bonds in the glasses.     

Solvent- and Wavelength-Dependent Photochemistry of 1,3-Dihydroxyacetone Studied by TREPR: Implications for Commercial Sunless Tanning Lotions

1,3-Dihydroxyacetone (DHA) is the active ingredient in commercial personal care products for the production of a “sunless” tan on human skin. The DHA molecule is the simplest ketose sugar, and it reacts with amino acids on the surface of the skin in a biochemical pathway known as the Maillard reaction. However, DHA is known to be photochemically active and will produce free radicals with high quantum yield under ultraviolet (UV) excitation. We report here a detailed study of the free radicals produced by DHA photolysis as a function of wavelength and solvent. In contrary to the previous optical and steady-state electron paramagnetic resonance (EPR) studies, X-band time-resolved (TR)-EPR spectra reveal a complex reactivity pattern: Norrish I α-cleavage is observed in almost all cases, but when good H-atom donors are present, H-atom abstraction by the solvent is observed. Changing the wavelength of excitation from 308 to 248 nm can reverse this observation. Comparison of TREPR spectra obtained from commercially available sunless tanning lotions shows that many free radicals, including those from DHA, can be detected using direct detection TREPR upon photolysis of the lotions themselves. The results suggest that caution should be observed in the use of these products in conjunction with UV exposure.     

Glutathione-mediated delivery and release using monolayer protected nanoparticle carriers

We demonstrate here the effective delivery of a dye payload into cells using 2-nm core gold nanoparticles, with release occurring via place exchange of glutathione onto the particle surface. In vitro experiments demonstrate effective release of drug analogues upon addition of glutathione. Cell culture experiments show rapid uptake of nanoparticle and effective release of payload. The role of glutathione in the release process was demonstrated through improved payload release upon transient increase in glutathione levels achieved via introduction of glutathione ethyl ester into the cell.