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Biomedical subjects

A Lewis

Publications and source records attributed to A Lewis.

At least 253 records · Page 14Linked to original sources
PubMed1977

Primary photophysical and photochemical processes in visual excitation.

The color of visual pigments is experimentally shown to be controlled by excited state effects. These effects which define the primary absorption of light by rhodopsin are considered together with results obtained from emission and picosecond spectroscopy. In addition, the molecular changes induced in rhodopsin when a photon is absorbed are analyzed using resonance Raman spectroscopy. The molecular changes observed are compared in bacterial and photoreceptor rhodopsins. This comparison yields a unique explanation for the biological role of the cis-trans isomerization in visual transduction.

Bacteriorhodopsins↗
PubMed1977

Kinetic resonance Raman spectroscopy: dynamics of deprotonation of the Schiff base of bacteriorhodopsin.

We have developed a kinetic technique, combining resonance Raman spectroscopy and variable-speed continuous flow methods, to study molecular dynamics of isolated sites in macromolecules. Kinetic resonance Raman spectra of the retinylidence chromophore of bacteriorhodopsin have been obtained and the dynamics of the deprotonation of the Schiff base linkage is discussed.

Bacteriorhodopsins↗
PubMed1976

A method for measuring picosecond phenomena in photolabile species: the emission lifetime of bacteriorhodopsin.

We have measured the emission lifetime of bacteriorhodopsin at physiological temperatures to be 15 +/- 3 ps using a technique which employs a mode-locked dye laser, a sum frequency light gate, and a continuous flow system. We observe no concentration dependence of the lifetime over the range of 1.1 X 10(-4) M to 1.0 X 10(-5) M. We conclude that the emission which we observe comes from bacteriorhodopsin and not one of its photochemically produced intermediates, and that the emission cannot originate from the state into which light is absorbed.

Bacteriorhodopsins↗
PubMed1976

A note on classifying phobia.

The increasing interest in the treatment of phobias underlines the need for clarifying their definition and classification. A brief historical note highlights some of the difficulties which have been, and still are, encountered.

History, 19th Century↗
PubMed1976

Modeling the resonance Raman spectrum of a metarhodopsin: implications for the color of visual pigments.

Resonance Raman spectra of an invertebrate rhodopsin are reported. The spectrum of squid acid metarhodopsin is compared with the spectra of model compounds of the retinylidene chromophore in the all-trans conformation. Correlations made between acid metarhodopsin and these crystalline model compounds with known x-ray structures indicate that the chromophore in this intermediate is an all-trans protonated Schiff base. The data suggest a mechanism for the red shift in rhodopsin.

Decapodiformes↗
PubMed1976

Tunable laser resonance Raman spectroscopic investigations of the transduction process in vertebrate rod cells.

Tunable laser resonance Raman spectroscopy has been applied to probe (in vivo) the role of rhodopsin in transducing light energy into the chemical necessary to generate a neural response. These in vivo experiments have suggested that the Schiff base linkage through which retinal is attached to opsin in rhodopsin is protonated. Furthermore, it appears that light eventually stimulates the deprotonation of the Schiff base linkage between the Meta I and Meta II steps in the intermediate sequence which is the result of light interacting with rhodopsin. Our data suggest that this deprotonation of the Schiff base occurs on the same time scale as overall proton release and uptake by the rhodopsin molecule. It is interesting to note that this series of protonations and deprotonations also occurs within the same time scale as the neural response generation in vertebrates and the generation of a proton gradient by bacteriorhodopsin, which is used by the bacterium, Halobacterium halobium, for ATP synthesis. If these data are analyzed within the context of the in vivo resonance Raman experiments (which seem to indicate that proton release is stimulated in the disc membrane during transduction) then there is a strong suggestion that the proton will assume an important role in any working hypothesis of visual transduction. In essence it appears that protons along with ATP and calcium ions must all be essential elements in the transduction process.

Adenosine Triphosphate↗
PubMed1975

Laser Raman spectroscopy as a mechanistic probe of the phosphate transfer from adenosine triphosphate in a model system.

Laser Raman spectroscopy has been used to study a phosphate transfer reaction from ATP to Pi or arsenate in dimethyl sulfoxide. The spectra support a mechanism involving Mg-2+ binding to the alpha or beta phosphates of ATP leaving the third phosphate free for the transfer reaction. The data also indicate the formation of a relatively stable intermediate which is facilitated by the presence of dimethyl sulfoxide and a dicarboxylic acid (maleate). The intermediate has a Raman spectrum with a band at 1090.5 cm- minus 1 similar to the end product ADP, but is formed much more rapidly. Since the model reaction has many features in common (e.g., activation by maleate) with the transfer reactions catalyzed by coupling factors from spinach chloroplast, Raman spectroscopy may also prove to be a useful tool in the elucidation of biological energy transfer reactions.

Adenosine Diphosphate↗