FLUORESCENCE AND THE STRUCTURE OF PROTEIN—XX. FLUORESCENCE OF 3-AMINOTYROSINE AND OTHER AMINOPHENOLS

Abstract— –The fluorescence of 3-NH₂ tyrosine and of simpler isomeric aminophenols was observed in aqueous solution at room temperature. Marked changes in both emission spectrum and fluorescence efficiency were observed upon change of pH and solvent composition. An explanation for the observed behavior is offered on the basis of two effects. First, peculiarities in the emission spectra were accountable in terms of proton dissociation from the cationic amino group in the excited singlet state so that emission characteristic of the free base was observed even in acidic solutions. Second, the unexpectedly low fluorescence efficiency from the free base was shown to result from quenching by the aqueous solvent. The relevance of these studies for the fluorescence of 3-NH₂ tyrosyl groups in proteins is discussed briefly.     

CHLOROPHYLL ONE-ELECTRON PHOTOCHEMISTRY: FLASH PHOTOLYSIS STUDIES OF THE CHLOROPHYLL-QUINONE REACTION IN ALCOHOL SOLVENTS*

Abstract— Flash photolysis of chlorophyll a alone in CBE (cyclohexanol-t-butanol-ethanol) yields a difference spectrum similar to those obtained upon steady illumination of chlorophyll a-quinone mixtures in this solvent. Decay kinetics in CBE and dimethylsulfoxide are faster at the Soret band than at 460–580 nm and red band regions. This difference is not obtained in other solvents (CHCI₃, CCI₄, t-butanol, ethanol), implying that two or more species are obtained in CBE and DMSO. β-Carotene in CBE increases the rate of decay of the flash-induced chlorophyll transients at 430 and 660 nm but only decreases the magnitude of the signal at 470 nm. This implies that the 470 nm absorbance is due to a product formed from the triplet state. This effect is not observed in ethanol. Adding quinone to chlorophyll solutions results in slowly decaying species being generated by flash excitation in CBE. Three components can be distinguished: the first (t_1/2? 0.2 msec) corresponds to the triplet state; the second (t_1/2= 5–10 msec) is quinone concentration and species independent; the third (t_1/2= several seconds) is dependent upon quinone concentration and species (rate is faster for higher concentrations and lower potential quinones). The ESR signal decay rate is approximately equal to the third component flash decay rate when the chlorophyll and quinone concentrations are equal. With excess quinone, the flash decay rate becomes faster, and the ESR decay rate decreases slightly. These slowly-decaying species are not produced when quinone is added to chlorophyll a in ethanol or t-butanol, or to pheophytin in CBE. One observes merely a decrease in signal height with no accompanying increase in decay rate. Mechanisms to account for all of these phenomena are presented which involve an initial chlorophyll triplet-solvent reaction with the subsequent formation of several species of chloro-phyll-quinone radical complexes.     

SINGLET OXYGEN INDUCES FRANK STRAND BREAKS AS WELL AS ALKALI- AND PIPERIDINE-LABILE SITES IN SUPERCOILED PLASMID DNA

A covalently closed, circular, supercoiled plasmid was exposed to singlet oxygen by a separated-surface sensitizer. For each exposure, the quantity of single oxygen entering the DNA target solution was estimated by its oxidation of histidine. After singlet oxygen exposure, some DNA samples were treated to disclose occult lesions. Agarose gel electrophoresis was then used to resolve the unrelaxed supercoils from the relaxed circular and linear species, and all bands were quantitated fluorometrically. Exposure of supercoiled plasmid DNA to singlet oxygen induced frank DNA strand breaks, alkali-labile sites (pH 12.5, 90 degrees C, 30 min), and piperidine-labile sites (0.4 M, 60 degrees C, 30 min), all in a dose-dependent manner. Yields of alkali-labile and piperidine-labile sites ranged from one to four times the frank strand break yield. Replacement of buffered H2O by buffered D2O as the DNA solvent for singlet oxygen exposures increased DNA lesion yields by a factor of 2.6 (averaged over lesion classes). Our data for the detection of frank strand breaks is at variance with published results from studies in which singlet oxygen was derived from a thermolabile endoperoxide dissolved in the DNA solution.     

PHOTOPRODUCTS OF BROMOURACIL-LABELED DNA AND THE STRUCTURE OF 5-BROMODEOXYURIDYLYL-(3' → 5')-THYMIDINE PHOTOPRODUCT

Abstract— –An attempt was made to identify some of the ultraviolet (u.v.) photoproducts of 5-bromouracil-labeled DNA (BrU-DNA). Two synthetic dinucleotides, 5-bromodeoxyuridylyl-(3' →5 ')-thymidine (BrdUpT) and 5-bromodeoxyuridylyl-(3' → 5')-deoxycytidine (BrdUpdC), were prepared. Each gave a single u.v. photoproduct which in turn gave a single acid hydrolysis product. 2-¹⁴C-BrU-DNA. prepared from E. coli B3, was irradiated (275–280 nm), hydrolyzed, and paper chromatographed in four systems. Comparison with the two synthetic photoproducts showed that if present at all, BrdUpT and BrdUpdC photoproducts could account for no more than 10 and 3.5 per cent respectively of the total photoproducts. At 55 per cent conversion of BrU into photoproducts, the major ¹⁴C-photoproduct was uracil (78 per cent); the remaining 22 per cent was made up of at least six products, three of which were reversed by 232 nm irradiation.
The debrominated cyclobutane structure proposed by Haug for BrdUpT photoproduct has been shown to be incorrect. It was found to contain one atom of bromine per molecule. On the basis of nuclear magnetic resonance and u.v. spectra, two possible structures are proposed for the photoproduct, each containing an eight-membered ring.     

Analysis of a few numerical integration methods for the Langevin equation

We study the position recurrence relation of several existing numerical integrators for the Langevin equation and use the modified equation approach to analyse their accuracy. We show that for the harmonic oscillator, the BBK integrator converges weakly with order 1 while the vGB82 and Langevin impulse (LÎ)? integrator converge weakly with order 2. We also study a restricted class of velocity definitions—those that lead to explicit starting procedures. We show that some recurrence relations exact for constant force, can achieve the exact virial relation by a proper definition of velocity, extending the result of Pastor et al. on the analysis of BBK integrators in 1988.     

Interpreting vibrationally resolved spectra of molecular dications (doubly positively charged molecules): HCl2+

Vibrationally resolved spectra of HCl²⁺ appear to show five vibrational levels for the X³Σ^? ground electronic state, whereas calculations of vibrational levels supported by ab initio potential energy curves have been able to locate only three vibrational levels below the barrier; this discrepancy is resolved by considering vibrational states that the potential function supports in the continuum above the barrier maximum. A low resolution spectrum produced from first principles is compared with a spectrum obtained with threshold photoelectrons in coincidence (TPEsCO) spectroscopy, with agreement sufficient to suggest that care must be taken in the inversion of vibrational spectroscopic data for molecular dications to avoid generating potential functions that are too strongly bound.     

Separation of dichloromethane-nitrogen mixtures by adsorption: experimental and molecular simulation studies

Experimental and grand canonical Monte Carlo simulation results for the separation of a CH²C1² (1.5 mol%)-N₂ binary gas mixture in molecular sieve materials are presented. AlPO₄-5 and MCM-41 molecular sieves have been used as the selective adsorbents because they consist of uniform arrays of uni-dimensional channels of micro and meso length scales, respectively. Adsorption isotherms were measured at 318 K and at pressures between 50 kPa and 130 kPa. Two MCM-41 materials have been used, one with a 33 A pore diameter and the other with a 42 Á pore diameter. For AlPO₄-5 at 110kPa the total amount adsorbed from experiment was found to be independent of equilibration time at 0.0542, 0.0538 and 0.0547 mmol per g AlPO₄-5 for 2, 24 and 48 hours, respectively. However, the selectivity for CH₂C1₂ was found to increase with time from 1.29, to 4.59, to 10.74. For MCM-41 at 110kPa the selectivity for CH₂C1₂ was found to be dependent on pore size. On increasing the pore size from 33 Å to 42 Á the selectivity for CH₂C1₂ increased considerably. Grand canonical Monte Carlo simulations agreed qualitatively with the experimental results, showing a greater selectivity for CH₂C1₂ than for N₂. The simulations indicate that MCM-41 has a lower selectivity for CH₂C1₂ than A1PO₄-5, which contradicts the experimental results. Reasons for these discrepancies are presented and discussed.     

Accurately solving the electronic Schrödinger equation of atoms and molecules using explicitly correlated (r 12-)MR-CI IV. The helium dimer (He 2)

The helium dimer interaction potential is computed using the recently proposed (explicitly correlated) r ₁₂-MR-ACPF (averaged coupled-pair functional) method and a [11s8p6d5f4g] basis set. With an MR-ACPF ansatz that contains 121 references we obtain interaction energies that are close to full CI. In a smaller reference space containing 9 functions, however, even by successively adding [3h] and [2i] functions to the basis set mentioned above, the basis set limit could not be reached. While convergence to the basis set limit is slow, it nevertheless is monotonic and therefore allows for extrapolation to the limit. We obtain basis set corrections at R = 4 a ₀ and 5.6 a ₀ which we further extend to all distances and which we apply to the potential energy curve mentioned above. From our calculations, we conclude that a very recent potential which has been calculated using the SAPT (symmetry adapted perturbation theory) method, and which previously was assumed to be the most accurate available, is insufficiently repulsive at short distances. We correct our extrapolated potential for retardation and finally calculate the expectation value of the interatomic distance (?R?) and dissociation energy (D ₀) by solving the Schrödinger equation of the vibrating ⁴He₂. Our results (?R? = 41 ± 13 Å and D ₀ = 2.2 ± 1.0 mK) are in acceptable agreement with very recent calculations in the literature, but they disagree with a recent experiment.     

MAXIMIN WITHOUT DISCOUNTING

ABSTRACT. The context of the comment by Withagen et al. is explainedto show how they use discounting of future utilities in their objective. Utilitarian andmaximin objectives have different implications for the analysis of sustainability.     

CANADA'S FOREST COVER INDICATOR: DEFINITION,METHODOLOGY AND RESULTS

ABSTRACT. The Environment and Sustainable Development Indicators (ESDI) Initiative was introduced to track Canada's overall wealth in the form of natural and human capital, in additionto familiar economic data such as the gross domestic product (GDP). One of the six ESDIs is the Forest Cover Indicator (FCI). In this paper we define FCI, outline the overall method for deriving FCI, and report results for addressing four key technical issues in carrying out this overall method. The FCI is defined as interannual variations of Canada's forest area with the middle‐summer crown closure (CC) ? 10%. Crown closure is the percentage of the ground surface covered by a downward vertical projection of the tree crowns. Theoverall monitoring method is mainly based on coarse resolution remote sensing data because of the need to cover Canada's extensive landmass during the middle‐summer months and toupdate the results annually. Medium resolution satellite data, field measurements, and modeling approaches were used for calibration, correction, validation, and down‐scaling, with a focus on the following 4 key technical issues: (1) correcting understory non‐tree vegetation effect on CC, (2) downscaling forest cover area from 1‐km to 100‐m spatial resolution as required by the FCI definition, (3) detecting the changes of CC caused by disturbances, and (4) detecting changes in CC caused by forest regrowth. Methods and results for addressing these technical issues are described in the paper. While these results indicate that the key technical issues can be solved by integrating satellite remote sensing data/products and other data, there are clear needs for further development, especially testing against field measurements.