CHLOROPHYLL ONE-ELECTRON PHOTOCHEMISTRY: LIGHT-INDUCED ABSORBANCE CHANGES AND ESR SIGNALS FOR VARIOUS PORPHYRIN-QUINONE AND HYDROQUINONE SYSTEMS IN ALCOHOL SOLVENTS*

Abstract— Light-dark optical difference spectra of degassed ethanol or pyridine solutions of chlorophyll and benzoquinone or hydroquinone at temperatures above — 50°C show only the semiquinone absorbance band. Decay of the signals is second order, with a rate constant in agreement with earlier ESR results. Light-induced optical changes due to chlorophyll can be elicited by lowering the temperature of ethanol solutions of chlorophyll and benzoquinone to a region of high viscosity. Hydroquinone is not effective in producing these optical changes. Similar results are achieved at room temperature by using as solvent a degassed mixture of the alcohols: cyclohexanol, tert-butanol, and ethanol (CBE). Difference spectra show bleaching of the chlorophyll bands and increased absorbance in the intermediate wavelength region (460–580 nm). Decay kinetics are first order, while the rise is complicated (probably biphasic). ESR signals have no hyperfine structure and also decay by first order kinetics, at a rate which is faster than that of the optical changes. The ESR signals reach a steady state more rapidly than the optical signals, without biphasic kinetics. These results demonstrate that at least two species are generated. Addition of acid increases the amount of bleaching in CBE, while small amounts of base decrease it. Larger amounts of base cause chlorophyll bleaching to completely disappear and only the semiquinone anion is observed. Activation energies for the chlorophyll a-benzoquinone photoreaction in CBE are 10–14 kcal/mole. Lower potential quinones give lower activation energies. The rate constant for quenching of the triplet state of chlorophyll a by β-carotene in CBE is 7.5±0.5×10⁸ (M set)^-1. β-carotene also quenches photoproduct formation. The bimolecular rate constant for formation of the photoproduct with benzoquinone was calculated to be 7×10⁸ (Msec)^-1. The redox potential of the quinone affects both the magnitude of the chlorophyll absorbance changes and the rate of decay. The higher the potential, the larger the changes and the slower the decay. Other porphyrin systems show similar photoreactions only if they are chelated with a group II metal, such as Mg²⁺, Cd⁺², or Zn⁺². The results are interpreted in terms of the formation, by a triplet-sensitized one-electron transfer from solvent to quinone, of a chlorophyll-semiquinone complex which is stabilized via coordination with the chelated metal.     

KINETIC PROPERTIES OF THE TRIPLET STATES OF METHYLENE BLUE AND OTHER PHOTOSENSITIZING DYES

Abstract— The long-lived (> 1 μsec) transients formed in the flash excitation of the representative photosensitizers methylene blue, eosin Y and pyrene have been investigated and various criteria have been used to distinguish between triplet state intermediates and chemical intermediates. Previous assignments of the triplet transients of methylene blue appeared less secure in view of the photochemical reactivity of this dye and its lack of phosphorescence. Earlier assignments of monomeric and dimeric triplet transients of methylene blue are substantiated, however, by the observations that the rate constant for quenching by oxygen is approximately 1/9^th diffusion controlled and the formation rates are commensurate with singlet decay rates and by the observation of triplet-triplet annihilation. Additional evidence in support of monomer triplet assignments for methylene blue and eosin Y is provided by the effect of heavy atom quenchers Cs⁺, Hg²⁺ and T1⁺ on decay rates. Due to chemical reactivity, quenching by I⁻appears less suitable as a diagnostic test for triplet state intermediates. The effect of N₃⁻, which is known to quench singlet oxygen molecules and to alter the course of photosensitized oxidations, on the triplet decay of methylene blue, eosin Y and pyrene is also investigated.     

RESONANCE RAMAN SPECTRA OF BACTERIORHODOPSIN MUTANTS WITH SUBSTITUTIONS AT ASP-85, ASP-96, AND ARG-82

Detergent solubilized bacteriorhodopsin (BR) proteins which contain alterations made by site-directed mutagenesis (Asp-96----Asn, D96N; Asp-85----Asn, D85N; and Arg-82----Gln, R82Q) have been studied with resonance Raman spectroscopy. Raman spectra of the light-adapted (BRLA) and M species in D96N are identical to those of native BR, indicating that this residue is not located near the chromophore. The BRLA states of D85N and especially R82Q contain more of the 13-cis, C = N syn (BR555) species under ambient illumination compared to solubilized native BR. Replacement of Asp-85 with Asn causes a 25 nm red-shift of the absorption maximum and a frequency decrease in both the ethylenic (-7 cm-1) and the Schiff base C = NH+ (-3 cm-1) stretching modes of BRLA. These changes indicate that Asp-85 is located close to the protonated retinal Schiff base. The BRLA spectrum of R82Q exhibits a slight perturbation of the C = NH+ band, but its M spectrum is unperturbed. The Raman spectra and the absorption properties of D85N and R82Q suggest that the protein counterion environment involves the residues Asp-85-, Arg-82+ and presumably Asp-212-. These data are consistent with a model where the strength of the protein-chromophore interaction and hence the absorption maximum depends on the overall charge of the Schiff base counterion environment.     

On the Completeness of Sets of Solutions to the Helmholtz Equation

Completeness in L²(D) is established for sets of functions formedfrom solutions to the two-dimensional Helmholtz equation ina domain D. Each function is a linear combination of a solution(found by separation of variables) and its normal derivativeon D, so the sets may be used to solve impedance-type boundaryvalue problems. Sets that contain either regular Bessel functionsor singular Hankel functions are considered. Methods of proofare employed that provide alternatives to the conventional potential-theoreticapproaches. In the majority of cases, the domain of interestis bounded and simply connected. One completeness result fora bounded, doubly-connected domain is proved. In some circumstances,one of the methods leads to a mild but inessential eigenvaluerestriction.     

SINGLET AND TRIPLET REACTIVITY IN THE PHOTOREDUCTION OF OXONINE(3,7-DIAMINOPHENOXAZIN-5-IUM CHLORIDE) BY IRON (II)

The oxazine dye, oxonine (3,7-diaminophenoxazin-5-ium chloride), 1, is photoreduced by Fe (II) sulfate in dilute sulfuric acid. The reaction mechanism is analogous to that for the photo-reduction of thiazine dyes by Fe (II), the most important difference being that reduction of oxonine occurs predominantly from its excited singlet state, S1, rather than from the triplet state, T1. The latter is formed with an intersystem crossing (isc) quantum yield of ca 1.7 x 10(-3). The quenching of S1 by Fe (II) has a rate constant kSQ = 2.2 +/- 0.1 x 10(9) M-1 s-1 and affords the one electron reduced product, semioxonine (R), with a limiting quantum yield, phi SR, of 0.26 +/- 0.02. In contrast, quenching of T1, generated by bromide ion quenching of S1 or by diacetyl sensitization, occurs with KTQ approximately 1.2 x 10(6) M-1 s-1, extrapolated to zero ionic strength, and affords R with a limiting probability, phi TR = 1.1 +/- 0.2. Three possible reasons for the lower quantum yield of the more exothermic S1 reduction are discussed. These are energy transfer from S1 to Fe (II), different rates of escape of R from the encounter complex as a consequence of the different states of protonation of R as initially formed from S1 and T1, and spin allowed back electron transfer in an exciplex formed between S1 and Fe (II). Evidence is also presented for a very low probability (ca 1%) induced isc from the encounter of S1 with paramagnetic Fe (II). Rate parameters for other processes important to the overall reduction mechanism such as disproportionation of R to leucooxonine L and oxonine, k(R)DIS = 1.7 +/- 0.2 x 10(9) M-1 s-1, oxidation of R by Fe (III), k(R)OX = 1.5 +/- 0.1 x 10(5) M-1 s-1, and oxidation of L by Fe (III), kLOX = 1.1 +/- 0.1 x 10(3) M-1 s-1, have also been measured. These results are contrasted with those for the closely related thionine/Fe(II) photoredox reaction, the most well understood system for photogalvanic energy conversion.     

Diffraction of Progressing Waves by a Screen

Problems of diffraction of a time-dependent wave by a planescreen of arbitrary shape are discussed. The method is basedon the progressing wave formalism which is a time-dependentversion of the geometrical theory of diffraction and yieldsapproximate solutions of diffraction problems. The diffractionproblem is formulated as an initial-boundary value problem forthe scalar wave equation. The solution is obtained by generalizinga "uniform" asymptotic solution recently found for the time-harmoniccase. These solutions are free of the anomalous singularitiesat the shadow boundaries and at the edge of the screen whichare present in the solutions constructed by the geometricaltheory of diffraction.     

TAILORING POINT COUNTS FOR INFERENCE ABOUT AVIAN DENSITY: DEALING WITH NONDETECTION AND AVAILABILITY

Point counts are commonly used for bird surveys, but interpretation is ambiguous unless there is an accounting for the imperfect detection of individuals. We show how repeated point counts, supplemented by observation distances, can account for two aspects of the counting process: (1) detection of birds conditional on being available for observation and (2) the availability of birds for detection given presence. We propose a hierarchical model that permits the radius in which birds are available for detection to vary with forest stand age (or other relevant habitat features), so that the number of birds available at each location is described by a Poisson‐gamma mixture. Conditional on availability, the number of birds detected at each location is modeled by a beta‐binomial distribution. We fit this model to repeated point count data of Florida scrub‐jays and found evidence that the area in which birds were available for detection decreased with increasing stand age. Estimated density was 0.083 (95%CI: 0.060–0.113) scrub‐jays/ha. Point counts of birds have a number of appealing features. Based on our findings, however, an accounting for both components of the counting process may be necessary to ensure that abundance estimates are comparable across time and space. Our approach could easily be adapted to other species and habitats.