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At least 19 recordsLinked to original sources
PubMed1978

Photochemistry of cytosine derivatives. 1. Photochemistry of thymidylyl-(3' leads to 5')-deoxycytidine.

The photochemistry of thymidylyl-(3' leads to 5')- deoxycytidine (dTpdC) was studied as a model system of adjacent thymine and cytosine bases in DNA. Acetophenonesensitized irradiation causes the cytosine moiety in dTpdC to react with the thymine moiety intramolecularly. Three unstable photoproducts are formed initially which are converted into three isomeric dinucleoside phosphates of thymine-uracil cyclobutane photodimer in a ratio of 4.2:2.2:1. Under the same irradiation condition thymidylyl-(3' leads to 5')-thymidine (dTpdT) yields two products in a ratio of 6:1. The structures of these products are established by chemical and spectroscopic methods. The major product in these reactions has been identified as the stereoisomer which has the same anti,anti relationship between the pyrimidine rings and the deoxyribose group as in the parent dinucleoside phosphates. The efficiency of the intramolecular dimerization of dTpdC is about one-third that of dTpdT. The results suggest that the cytosine base in DNA may be converted to a uracil base via photodimerization with an adjacent pyrimidine base, hydrolysis, and photoreactivation.

Deoxycytidine↗
PubMed1975

Quenching of chlorophyll fluorescence and primary photochemistry in chloroplasts by dibromothymoquinone.

The quenching action of dibromothymoquinone on fluorescence and on primary photochemistry was examined in chloroplasts at minus 196 degrees C. Both the initial (F0) and final (FM) levels of fluorescence as well as the fluorescence of variable yield (FV equals FM minus FO) were quenched at minus 196 degrees C to a degree which depended on the concentration of dibromothymoquinone added prior to freezing. The initial rate of photoreduction of C-550 at minus 196 degrees C, which was assumed to be proportional to maximum yield for primary photochemistry, phipo, was also decreased in the presence of dibromothymoquinone. Simple theory predicts that the ratio FV/FM should equal phipo. Excellent agreement was found in a comparison of relative values of phipo with relative values of FV/FM at various degrees of quenching by dibromothymoquinone. These results are taken to indicate that FO and FV are the same type of fluorescence, both emanating from the bulk chlorophyll of Photosystem II. Dibromothymoquinone appears to create quenching centers in the bulk chlorophyll of Photosystem II which compete with the reaction centers for excitation energy. The rate constant for the quenching of excitation energy by dibromothymoquinone is directly proportional to the concentration of the quencher. Rate constants for the de-excitation of excited chlorophyll molecules by fluorescence, kF, by nonradiative decay processes, kD, by photochemistry, kP, and by the specific quenching of dibromothymoquinone, kQ, were calculated assuming the absolute yield of fluorescence at FO to be either 0.02 or 0.05.

Bromine↗
PubMed1975

Photochemistry of tetrachlorosalicylanilide and its relevance to the persistent light reactor.

The photochemistry of 3,5,3',4'-tetrachlorosalicylanilide has been studied in solution under carefully controlled conditions. When irradiated in a buffered solution of pH 7.4 (physiological pH), three atoms of chlorine are liberated from the molecule instead of one as suggested by earlier photochemical work. From this observation a mechanism is proposed to explain the long-term photobiological effect of this compound in skin i.e. that of the persistent light reactor.

Anilides↗
PubMed1979

[Photochemistry of visual pigments (author's transl)].

Recent studies of the photochemistry of visual pigments are reported. Results obtained on rhodopsine and model compounds, by means of techniques based upon continous or pulsed excitation are compared. The mechanique of the primary photochemical step of the bleaching of rhodopsine is discussed. The process is occuring on a time scale of picosecond and a new concept of proton transfer followed by a conformation charge is opposed to the classical cis-trans photo isomerisation of the retinal moiety. The origin of the electrical response of the photoreceptor cells is still controversial.

Animals↗
PubMedDate not supplied

The field of possible structures for the chlorophyll a dimer in photosystem I of green plants delineated by polarized photochemistry.

Photoselection experiments with immobilized photosystem I particles have been done to determine the mutual orientation of pigments in the reaction centre. When these particles are excited and interrogated with linearly polarized light, the flash-induced transient absorption changes (mainly from the chlorophyll a dimer) reveal linear dichroism, which yields information on the mutual orientation between the excited and the interrogated transition moments. The interpretation of the data, however, is ambiguous, (1) for reasons of principles inherent in the photoselection technique when applied to complex systems and (2) because of incomplete knowledge about the relative contribution of x- and of y-polarized transitions of chlorophyll a to absorption or to absorption changes at a given wavelength. We find it impossible to attribute any particular structure to the photooxidizable dimer based on photoselection data alone. Instead we present a field of possible structures, imposing constraints on proposed models for the dimer structure.

Chlorophyll↗
PubMed1978

Influence of membrane lipids on the photochemistry of bacteriorhodopsin in the purple membrane of Halobacterium halobium.

Purple membrane fragments from Halobacterium halobium were reconstituted with the native lipids replaced by dipalmitoyl phosphatidylcholine and by egg lecithin. In parallel studies the temperature dependence of bacteriorhodopsin phototransient lifetime and absorption dichroism and of in situ lipid microviscosity were determined; the former two by, respectively, conventional and polarization flash photometry, and the latter by observation of emission depolarization of an embedded fluorescent dye, 1,6-diphenyl-1,3,5-hexatriene. Discontinuities in lipid microviscosity profiles in native and egg lecithin purple membrane were reflected in both the photochemical cycle frequency and bacteriorhodopsin chromophore rotational mobility. The influence exerted by membrane-lipid viscosity appears to be a secondary effect, and points to the bacteriorhodopsin chromophoric group being situated in the protein interior.

Bacteriorhodopsins↗
PubMed1978

Competition between the 735 nm fluorescence and the photochemistry of Photosystem I in chloroplasts at low temperature.

Fluorescence emission spectra of chloroplasts, initially frozen to--196 degrees C, were measured at various temperatures as the sample was allowed to warm. The 735 nm emission band attributed to fluorescence from Photosystem I was approx. 10-fold greater at--196 degrees C than at--78 degrees C. The initial rate of photooxidation of P-700 was also measured at--196 degrees C and--78 degrees C and was found to be approximately twice as large at the higher temperature. It is proposed that the 735 nm emission band is fluorescence from a long wavelength form of chlorophyll, C-705, which acts as a trap for excitation energy in the antenna chlorophyl system of Photosystem I. Furthermore, it is proposed that C-705 only forms on cooling to low temperatures and that the temperature dependence of the 735 nm emission is the temperature dependence for the formation of C-705. C-705 and P-700 compete to trap the excitation energy in Photosystem I. It is estimated from the data that at--78 degrees C P-700 traps approx. 20 times more energy than C-705 while, at--196 degrees C, the two traps are approximately equally effective. By analogy, the 695 nm fluorescence which also appears on cooling to--196 degrees C is attributed to traps in Photosystem II which form only on cooling to temperatures near--196 degrees C.

Chlorophyll↗
PubMed1979

Photochemistry of rhodopsin and isorhodopsin investigated on a picosecond time scale.

Bovine rhodopsin and isorhodopsin were excited with a single 530-nm, 7-ps light pulse emitted by a mode-locked Nd 3+ glass laser at room temperature. Within 3 ps of excitation, absorbance changes due to formation of bathorhodopsin were observed. The difference spectra generated during and 100 ps after pulse excitation are presented. The data show that bathorhodopsin formation is completed within 3 ps for both the primary pigments and suggest that a single common bathorhodopsin is photochemically formed from both primary pigments. Our findings provide additional support for the cis-trans isomerization model of the primary event in vision. Additional absorption transients that were observed near 670 and 460 nm are discussed.

Animals↗
PubMed1992

Synthesis and photochemistry of photolabile N-glycine derivatives and effects of one on the glycine receptor.

Three photolabile precursors of glycine containing a photosensitive 2-nitrobenzyl moiety attached to the amino group have been synthesized. When exposed to ultraviolet radiation between 308 and 350 nm, the compounds photolyze to release glycine, an important inhibitory neurotransmitter in the central nervous system. The identification of glycine as a photolysis product was determined by two different methods: separation of the photolyzed sample by thin-layer chromatography followed by a reaction with ninhydrin, and recognition of derivatized glycine using the Waters pico-tag method in conjunction with high-performance liquid chromatography. The photolysis of these compounds at 22 degrees C has been investigated, and the rate of decay of a transient intermediate in the reaction, which is assumed to reflect product release, has been measured. For N-(alpha-carboxy-2-nitrobenzyl)glycine this decay rate was found to be 940 s-1 at pH 6.8 and 600 s-1 at pH 7.5. Additionally, this compound was found to exhibit biological activity upon photolysis; cultured mouse spinal cord cells containing neuronal glycine receptors were used to detect the glycine liberation. The approach adopted here is useful in demonstrating the utility of photolabile precursors of neurotransmitters that have the protecting group linked to the neurotransmitter through the amino group. The rapid photolysis of such compounds to release free neurotransmitter is valuable in gaining access to chemical kinetic studies of neurotransmitter receptors. Previously, such studies have been limited because the available methods for neurotransmitter delivery did not give a sufficiently high time resolution.

Animals↗
PubMed1978

Resonance Raman spectroscopy of squid and bovine visual pigments: the primary photochemistry in visual transduction.

Resonance Raman spectra of squid rhodopsin have been obtained under a variety of temperature and illumination conditions. The data have been characterized in terms of spectral contributions from squid rhodopsin, isorhodopsin, bathorhodopsin, lumirhodopsin, mesorhodopsin, P-465, and acid metarhodopsin. The results are compared with the spectral features obtained from bovine rhodopsin, isorhodopsin, and bathorhodopsin. The data support a proposed structure for the chromophore in bathorhodopsin which is not all trans, 11-cis, or 9-cis. This structure can be generated from either rhodopsin or isorhodopsin by a similar motion (simultaneously rotating chromophore carbon atoms 10 and 11 out-of-plane). Furthermore, we detect the same distinct bathorhodopsin vibrational modes when rhodopsin is illuminated between 4 and 100 K. This demonstrates that under steady-state illumination the light-induced chromophore structural alterations occurring at 4 K are very similar to those occurring at higher temperatures. Finally, our data indicate that bathorhodopsin is generated not only by structural transitions in the chromophore but also alterations in the opsin conformation as has recently been proposed[Lewis, A. (1978) Proc. Natl. Acad. Sci. U.S.A. 75, 549].

Animals↗
PubMed1978

Photochemistry of cytosine derivatives. 2. Photohydration of cytosine derivatives. Proton magnetic resonance study on the chemical structure and property of photohydrates.

Photohydrates of cytidine and cytidylic acids have been definitively characterized to be isomeric 6-hydroxy-5,6-dihydrocytosine derivatives. It has also been demonstrated by nuclear magnetic resonance spectroscopy that (1) the stereochemistry of photohydration is random, (2) the C5-H trans to the C6-OH undergoes a rapid selective exchange in the presence of proton acids, and (3) the dehydration of photohydrates is a trans-elimination. The mechanism of these processes is discussed.

Cytidine↗