A climatology of solar erythema dose

Abstract —Semi-empirical formulas for the ultraviolet erythema dose derived in an earlier paper are used to deduce an ultraviolet photoclimatology. We calculate the climatology of daily erythema radiation doses for the northern hemisphere at 5d? latitude intervals. Similar dose calculations are also performed specifically for ten metropolitan areas. Effects of seasonal and geographic variations of ozone, turbidity, and cloudiness on the local erythema doses are also investigated. We present a simple approximate analytic formula for the annual erythema dose as a function of latitude, cloud cover, and ground albedo for use in connection with studies of the epidemiology of skin cancer. The implications of possible ozone depletion due to a future fleet of supersonic aircraft in the stratosphere are discussed. These calculations are made for a normal ozone thickness of 0.32 cm and for a 5, 10, 20, and 50 per cent ozone reduction.     

High-resolution photoelectron spectroscopy through deconvolution

A new iterative deconvolution algorithm for the development of very high resolution Hc(I)-excited photoelectron spectra of gases is presented. The algorithm accepts as input a medium-resolution spectrum and an instrument function obtained by scanning intrinsically narrow line (e.g. the Ar²P_3/2 line) under conditions identical to those used to acquire the medium resolution spectrum. The deconvolved partial spectrum of an O₂H₂O mixture is presented as a test case prior to presentation of results for three nitrogen lines. For comparison purposes directly obtained high-resolution spectra of the nitrogen lines are included. The shapes of the nitrogen lines are discussed within the framework of a one-center expansion theory of photoionization. The conditions under which deconvolution can be profitably applied are briefly discussed and an attempt is made to establish the deconvolution (contrary to still popular belief) is not in any way equivalent to curve fitting.     

PHOTODYNAMIC THERAPY INDUCED ULTRASTRUCTURAL ALTERATIONS IN MICROVASCULATURE OF THE RAT CREMASTER MUSCLE

Abstract— Photodynamic therapy disrupts blood flow to tumors and produces tumor necrosis. These effects may be due to a localized generation of singlet oxygen. The current studies used direct observations of the rat cremaster microvasculature to examine the vascular effects of PDT. The objective of the morphological examination was to delineate the structural basis for the altered blood flow in photodynamic therapy. Dihematoporphyrin ether given 30 min or 48 h prior to the experiment was activated with green light (wavelength530–560 nm, 120 J/cm²). After the in vivo activation the tissues were prepared for electron microscopy. Light alone induced little or no change in the luminal content or vessel wall. On exposure to activating light both acute (30 min) and long term (48 h) dihematoporphyrin ether pretreated samples displayed formation of luminal aggregates, granulocyte margination and migration, and endothelial cell and smooth muscle cell damage. The latter was more pronounced in the arterioles than the venules. Perivascular changes included interstitial edema and damage to striated myocytes. Some of the alterations such as interstitial edema may be transient; however, smooth and skeletal muscle cell injury are important in normal and tumor tissue necrosis after photodynamic therapy.     

ULTRAVIOLET SOLAR RADIATION IN THE HIGH LATITUDES OF SOUTH AMERICA

Abstract Measurements of the UV solar irradiance are available from Ushuaia, Tierra del Fuego during the spring and summer seasons of 4 consecutive years beginning in 1989. In addition, column ozone amounts derived from satellite-based measurements exist for this location over the entire period from 1980 through 1991. Monthly mean column ozone over Ushuaia shows a general decline over the observing period, and a large day-to-day variability exists within a given month. Ozone amounts for the years 1980 through 1986 combined with a model of radiative transfer provide a climatological baseline against which to interpret the more recent ground-based irradiance data. We focus on monthly mean noontime irradiances integrated over 5 nm wide spectral bands near 305 nm and 340 nm, respectively. Measurements in the 340 nm band show that cloudiness has a large influence on both the absolute monthly mean irradiances and their interannual variability. For example, during December the 340 nm band irradiance varied from approximately 50% of the clear-sky value in 1992 to 65% in 1991. When the influence of cloudiness is removed, most of the months show irradiances in the 305 nm band that are larger than predicted from the climatological ozone amounts. The largest percentage enhancement occurred in October 1991 when the irradiance exceeded the baseline by 56%. The largest absolute irradiances occur in December, where the measurements range from 5.8% below the baseline in 1991 to 31% above in 1990.     

Stereochemical assignment of the C1-C6 fragment of psymberin by synthesis and natural product degradation

[reaction: see text] Psymberin is a sponge-derived natural product that shows striking selectivity as a cytotoxic agent. Conformational mobility has precluded stereochemical assignment for the acyl fragment of this molecule (psymberic acid) by NMR. Herein we report stereoselective syntheses of all four stereoisomers of psymberic acid. A comparison of the acid-mediated cyclization products of these compounds to the product of psymberin's acidic methanolysis showed the stereochemical configuration of this fragment to be 4S,5S.     

Amperometric biosensor for the rapid determination of salicylate in whole blood

A method for the determination of salicylate in whole blood is described. The assay uses salicylate hydroxylase to convert salicylate to catechol in the presence of NADH and molecular oxygen. The formation of catechol is monitored amperometrically by oxidation at +300 mV vs.Ag/AgCl and the size of the oxidation current is related to the concentration of salicylate in the sample. The reagents are incorporated into the working electrode of a disposable strip, allowing measurements to be made on a drop of blood within 1 min. The functional range of the assay can be extended to the equivalent of 7.2 mM plasma salicylate by incorporating benzoate as a component of the reaction system. The method has the advantages of simplicity and speed compared with standard procedures, and should prove especially useful in suspected overdose situations.     

The synthesis and thermal curing parameters of a novel quinoxaline monomer with terminal ethynyl thermoset and phenylethynyl intramolecular cycloaddition postcure capability

The Synthesis of 3,3′-bis(4-[3-ethynylphenoxy]phenyl)-7,7′-bis(phenylethynyl)-2,2′-diphenyl-6,6′-biquinoxaline (I) was accomplished by the reaction of 2,2′-bis(phenylethynyl)-5,5′-diaminobenzidine (II) and 4-(3-ethynylphenoxy)benzil. Thermal analysis of I indicated a softening temperature of 107°C, followed by an exotherm above 150°C that corresponded to a independent crosslinking reaction of the terminal acetylene groups and an intramolecular cycloaddition (IMC) reaction of the 2,2′-bis(phenylethynyl)biphenyl moieties. In the synthetic work substantial improvements were made in the synthesis of II. The sample of I was cured at 200°C and the maximum partially cured transition temperature attained was 280°C. A sample of 3,3′-bis(4,[3-ethynylphenoxy]phenyl)-2,2′-diphenyl-6,6′-biquinoxaline (IV) was similarly tested as a model without IMC capability and its corresponding value was 250°C. The difference between these two values is discussed briefly.     

Chiral conflict. The effect of temperature on the helical sense of a polymer controlled by the competition between structurally different enantiomers: from dilute solution to the lyotropic liquid crystal state

Helical polymers appended with paired structurally different enantiomers, which have opposing helical sense preferences, yield a new kind of relationship between optical activity and temperature, and also reveal unusual details of the nature of chiral interactions. Consistent with a statistical physical theory developed for these experiments, the proportion of the competing chiral groups, determined by synthesis, fixes the compensation temperature at which the helical senses are equally populated. The lyotropic liquid crystal state formed by these polymers yields therefore a nematic state at any chosen temperature over a very wide range, with a cholesteric state arising with tightening pitch as temperature deviates from this point. Far from the nematic temperature, the pitch reaches the nanometer scale and therefore the reflection of visible light. Before crossing zero at the nematic temperature, the optical activity becomes so large that it may be observed with the unaided eye through crossed polarizers.     

Accurate and efficient method for predicting thermochemistry of polycyclic aromatic hydrocarbons - bond-centered group additivity

A self-consistent estimation method for the thermochemical properties of polycyclic aromatic hydrocarbons (PAH) is presented. This method is based on enthalpies of formation (DeltaHf(degrees), entropies (S(degrees)298, and heat capacities (C(degrees)p obtained from B3LYP/6-31G(d) calculations of the total energies and frequencies for 139 PAHs, including C(60) and C(70) fullerenes. The enthalpies of formation were calculated using an optimized set of homodesmic reactions given the available experimental DeltaHf(degrees) of PAHs. The theoretical entropies were compared with the existing experimental entropies, and some inconsistencies in the experimental data were identified. The estimation method presented here is a systematic extension of the widely employed atom-centered group additivity method originally proposed by Benson. This new method is based on bond-centered groups that define bonds linking two atom-centered groups and specify the size of the rings to which they belong. In addition, a term to describe the resonance energy is included. The thermochemical properties of PAHs up to C(70) fullerene are estimated with a mean average deviation of 2.8 kcal mol(-1) in DeltaHf(degrees), 0.7 cal K(-1) mol(-1) in S(degrees)298, and about 0.5 cal K(-1) mol(-1) in the C(degrees)p. This bond-centered group additivity method for the thermochemical properties of PAHs significantly expands both the range of systems that can be estimated and the accuracy of the estimations. The results of this work also allow us to assess the quality of available experimental data. For example, there are strong indications that the literature DeltaHf(degrees)of benzo[k]fluoranthene is about 10 kcal mol(-1) too low.     

Isolation and structure of [HC[CH(SiMe(3))(SnMe(3))](2)](+): a remarkably stable sec-alkyl cation

The reaction of the tin-substituted propene Me3Sn(R)CHCH=CHR (R = SiMe3) with MCl4 in dichloromethane in the presence of Me3SnCl gives the first examples of isolable sec-alkyl carbocation salts, [HC{CH(R)SnMe3}2]+M2Cl9- (M = Zr, Hf). The compounds are thermally stable and, unlike previously isolated trialkyl carbocations, do not require superacidic media or weakly coordinating anions for stability. The crystal structure and DFT calculations suggest polarization of the Sn substituents and hyperconjugation as the reason for the unexpected stability. The stabilizing effect of tin is significantly stronger than that of Si. The carbocations are effective initiators for the polymerization of isobutene, isoprene, and alpha-methylstyrene.