Godfrey S. Beddard

Concentration quenching of chlorophyll fluorescence in bilayer lipid vesicles and liposomes

Abstract The fluorescence yields and lifetimes of chlorophyll-a-in lipid liposomes and vesicles have been measured in an attempt to understand the light harvesting mechanism of photosynthesis. Concentration quenching of the fluorescence was observed in all systems, the extent depending on the lipid used. The system having the highest half-quenching concentration (7.0 × 10−2 molal) was 3:1 mole to mole mixture of monogalactosyl diglyceride and digalactosyl diglyceride.

Picosecond fluorescence depolarisation measured by frequency conversion

Abstract A method of using sum frequency generation in a LiIO 3 crystal has been used with picosecond pulses from a dye laser to measure fluorescence decays in solution. Fluorescence can be detected at wavelengths greater than 1 μ. Some examples of the method to measure the rotational relaxation of dyes in solution are presented. In the dye cresyl violet two sites for solvent—solute interaction are proposed; one affects mainly the electronic properti of the molecule while the other hinders molecular rotation.

Internal conversion from vibrationally excited levels

Abstract It is shown that internal conversion to the ground state can become an important pathway of radiationless decay, for molecules in excited vibrational l

The fluorescence decay kinetics of in vivo chlorophyll measured using low intensity excitation.

We report fluorescence lifetimes for in vivo chlorophyll a using a time-correlated single-photon counting technique with tunable dye laser excitation. The fluorescence decay of dark-adapted chlorella is almost exponential with a lifetime of 490 ps, which is independent of excitation from 570 nm to 640 nm. Chloroplasts show a two-component decay of 410 ps and approximately 1.4 ns, the proportion of long component depending upon the fluorescence state of the chloroplasts. The fluorescence lifetime of Photosystem I was determined to be 110 ps from measurements on fragments enriched in Photosystem I prepared from chloroplasts with digitonin.

Vibrational energy dependence of radiationless conversion in aromatic vapours

Under collision-free conditions the fluorescence lifetimes of naphthalene, 1-2 benzanthracene, chrysene and pyrene decrease with increasing excitation energy within the first excited singlet state. In anthracene the fluorescence lifetime is constant within the first excited singlet state. An explanation of these findings in terms of bond length changes between the excited singlet state and the first triplet state, and also the symmetry of the first singlet state is given. The effect of added foreign gas on fluorescence lifetimes and triplet yields is discussed in terms of a model which allows the non-radiative decay probability to vary with the degree of vibrational excitation. Excellent agreement is obtained with the experimental results on naphthalene fluorescence. The decrease of triplet yield as foreign gas pressure is reduced is also interpreted in terms of this model.

Time-Resolved Fluorescence from Biological Systems: Tryptophan and Simple Peptides

Four methods of applying mode-locked lasers to time-resolved fluorescence measurements in the subnanosecond region are compared. When time resolution below 100 ps is not required, the most precise and sensitive method is single-photon counting, and the application of this method to studies of time-resolved fluorescence of tryptophan in simple peptides is described. The dependence of lifetimes on pH and temperature are interpreted in terms of quenching by intramolecular proton and electron transfers.

The effects of cholesterol on the time-resolved emission anisotropy of 12-(9-anthroyloxy)stearic acid in dipalmitoylphosphatidylcholine bilayers

The time-resolved fluorescence emission anisotropy of 12-(9-anthroyloxy)stearic acid (12-AS) and 1,6-diphenyl-1,3,5-hexatriene (DPH) have been measured in dipalmitoylphosphatidylcholine liposomes in the presence and absence of 40 mol% cholesterol at temperatures above and below the phase transition temperature (41 degrees C). By using a synchronously-pumped mode-locked frequency-doubled dye laser and single photon counting detection with an excitation response function of 300 picosecond, rotational correlation times down to less than 1 nanosecond could be resolved. Whereas DPH showed only small changes in the limiting anisotropy on the addition of cholesterol, 12-AS showed significant increases in this parameter with the effect being potentiated at higher temperatures. This difference in behaviour has been attributed to a fluorophore-cholesterol interaction that resulted in a change in the fluorophore geometry. Not only do DPH and 12-AS sense different depolarizing rotations due to the different directions of their emission dipoles but also differ in their lipid interactions which alter their limiting anisotropies. The implication is that the comparison of steady-state anisotropy measurements between chemically identical fluorophores in different lipid environments may be complicated by molecular distortions that change the motions to which the steady-state fluorescence parameters will be sensitive.

Model Systems for Photosynthesis. V. Electron Transfer between Chlorophyll and Quinones in a Lecithin Matrix

The relative fluorescence yield, the lifetime and the triplet yield have been investigated, for solutions in lecithin, of chlorophyll α in the presence of a series of quinones. The singlet and triplet yields show the same dependence on quinone concentration; the lifetimes show a slightly weaker dependence. I t is proposed that the quenching proceeds via electron transfer from the excited singlet state of the chlorophyll α molecule to its nearest neighbour quinone. A model, based upon a random distribution of molecules, is used to derive quenching parameters. The model gives a good account of the experimental data and it is found that the quenching ability is directly related to the oxidation potential of the quinone.

Pressure effects on the fluorescence from naphthalene vapour

Abstract The fluorescence lifetimes and yields of naphthalene vapour have been measured at low pressure as a function of exciting wavelength and of the pressure of added foreign gases. The lifetimes and yields increase with pressure for short wavelength excitation ( 305 nm. The pressure dependence of the naphthalene S 1 state at low excitation energies is interpreted in terms of collisionally induced vibrational redistribution.

N-n-butyl-N,N-di(2-naphthylmethyl)amine: A compound exhibiting intramolecular excimer and exciplex formation

Abstract The dinaphthylamine (3) is unique in that it exhibits both intramolecular excimer and exciplex formation. The relative amounts of each is dependent upon the solvent polarity. Highly polar solvents, e.g. methanol, lead to the emission being entirely that of the exciplex but addition of acid to these solutions prevents exciplex formation and only excimer is observed.