Marjo Ikonen

Study of photocycle and spectral properties of bacteriorhodopsin in Langmuir-Blodgett films

Abstract Langmuir-Blodgett (LB) films of bacteriorhodopsin (BR) in soya-phosphatidylcholine (PC) matrix were prepared on quartz plates and the photocycle and spectral properties of the films were studied. The transient absorption spectrum of BR LB film was measured by the nanosecond laser flash photolysis method in a wavelength range from 380 to 650 nm. The absorption maxima were at 540 and 415 nm for the L and M states respectively. In water the rate of decay of the L state was 2.3 × 10 4 s −1 and the rates of formation and decay of the M state were 2.3 × 10 4 s −1 and 3.0 × 10 2 s −1 respectively. The photocycle reaction was not observed in nitrogen-dried BR LB films, but when the films were put back in water, even after being stored for 8 months in dry conditions, the phenomenon could be observed. The energy transfer from trytophan to BR retinal takes place in both dry and wet LB films. The films were relatively stable to changing physical conditions as well as to laser excitation and no changes were observed following repeated photocycles. Molecular surface potentials of BR-soya-PC and soya-PC Langmuir films as a function of mean molecular area decreased towards zero with increasing surface pressure.

Photophysics of rhodamine dimers in Langmuir-Blodgett films

Abstract Temperature dependent dimerization processes of octadecylrhodamine B (RB) and octadecylrhodamine 6G (R6G) in Langmuir-Blodgett (LB) films were studied by steady-state and time-resolved fluorescence methods. The geometry of the dimers in LB films is equal for both dyes, but different to the geometry of the dimers found in solutions. The sandwich-type dimers with lifetimes of 710 ps for RB and 620 ps for R6G have their fluorescence maxima at 635 and 620 nm for RB and R6G, respectively. The dimer with an oblique geometry has its fluorescence maximum at 675 nm for both dyes, and its fluorescence lifetime is 4.6 ns for RB and 4.9 ns for R6G. The proportion of fluorescent dimers increases with decreasing temperature, when the nonfluorescent H dimers reorganize to fluorescent J dimers. The activation energy for this temperature induced process is 1.4 and 2.6 kJ mol−1 for RB and R6G, respectively.

A kinetic study of monomer and excimer fluorescence of pyrene lecithin in Langmuir-Blodgett films

Abstract The fluorescence of Langmuir-Blodgett films of 1-palmitoyl-2-pyrene-decanoylphosphatidylcholine in matrices composed of dipalmitoyl-, 1-palmitoyl-2-oleyl- and dioleylphosphatidylcholine was studied quantitatively at different concentrations and temperatures. Stationary quantum yields, time-resolved life-times and spectral data were used to determine the kinetics of pyrene fluorescence. Possible schemes for monomer and excimer decays are presented and rate parameters for the individual steps are evaluated. The main paths for the monomer decay involve two kinds of non-interacting singlet species. The first is quenched by interaction with another pyrene moiety or an aggregate in the ground state and does not form excimers. The other pyrene moiety is aggregated in such a way that only a small rotational motion is required for it to attain a dimeric configuration which might be a precursor of an excimer. The formation of pyrene excimer is mainly determined by a reorganization, diffusional rotation in or out of the plane of the pyrene moieties in the lattice. Evidence for energy migration in the lattice in the excimer formation process is presented. The excimer is kinetically distinguishable from the monitored monomeric species. Relative intensities of excimer emission are considerably reduced when the temperature decreases, indicating diffusional excimer formation at higher temperatures. Activation energies for excimer formation are evaluated at different temperatures and concentrations. Evidence for the reorganization of the molecular lattice by light and temperature is presented.

The orientation of quinquethiophene in langmuir-blodgett films

Abstract Langmuir-Blodgett multilayer films consisting of 20–60 mol% α- quinquethiophene in arachid acid were deposited on quartz substrates. The orientation of α-quinquethiophene molecules in layers was determined by polarized absorption and fluorescence methods. The results showed that they stand predominantly with their long axes perpendicular to the plane of the layer.

A flash photolysis study of all-trans-retinal in langmuir-blodgett films and in liposomes

Abstract The triplet-triplet absorption decay and changes in the ground-state absorption spectra of all-trnas-retinal (ATR) has been studied in Langmuir-Blodgett films (LB films), in liposomes and in hexane solution. In addition all-trans-retinal acid (ATRA) has been studied in LB films and in hexane. In LB films the decay of the triplet absorption of ATR is bi-exponential. When the number of excitation pulses increases, the relative fraction of one decay component decreases and the decay of the triplet state changes to mono-exponential. In liposomes and hexane the decay of the triplet state is independent of the number of excitation pulses and is bi-exponential for liposomes but mono-exponential in hexane solution. The changes are due to a photopolymerization reaction via the triplet state. For ATR the polymerization can also take place by thermal processes. The triplet absorption of ATRA cannot be observed in LB films or in hexane, but the polymerization takes place in LB films under the influence of light.

Fluorescence of pyrene intramolecular excimers in Langmuir-Blodgett films

Abstract Fluorescence spectra and fluorescence decay kinetics of intramolecular excimers of di(1-pyrenylmethylene)adipate (DPA) and di(1-pyrenylmethylene)ether (DPE) were studied in Langmuir-Blodgett (LB) monolayers consisting of stearic acid (SA) and dipalmitoylphosphatidylcholine (DPPC). Two types of excimers with different lifetimes and different lifetime dependences on temperature were found for both of the compounds. No quenching of excimer fluorescence by oxygen was observed in LB films with SA as the matrix compound. When DPPC was used as a matrix only one type of excimer was quenched by oxygen. Photolysis of DPA and DPE LB films resulted in a dramatic decrease in excimer fluorescence, but negligible changes in the pyrene absorption band.

Photoinduced charge transfer in rhodamine B/anthracene mixed Langmuir-Blodgett multilayers

Abstract The photoexcited transient states of octadecylrhodamine B (R) and 2-(9-anthroyloxy) stearic acid (A) mixed in a stearic acid matrix have been studied by flash photolysis and picosecond fluorescence methods in Langmuir-Blodgett films. The two dominant excitation energy relaxation processes for the R monomer are an electron transfer from A to the excited singlet state of R and an energy transfer to the non-fluorescent R dimer. The excited R dimer relaxes via its triplet state. A charge separation process takes place in the triplet state leading to a radical pair.

Intralayer and interlayer dimerization of rhodamine 6G in Langmuir-Blodgett films

Abstract The intralayer and interlayer aggregation of octadecylrhodamine 6G mixed in a stearic acid matrix have been studied by flash photolysis and picosecond fluorescence methods in Langmuir-Blodgett films. Both intralayer and interlayer dimers relax via their triplet state. The dominant fluorescence quenching process is either excimer formation or cluster fluorescence. The activation energy of excimer formation is 3.8 kJ mol-1.

Excitation energy relaxation in chlorophyll a Langmuir-Blodgett multilayer films

Multilayer LB films of chlorophyll a in L-(alpha) -phosphatidylcholine dipalmitoyl matrix have been prepared in order to study excitation energy relaxation processes. Chlorophyll a layers were deposited on a plate previously covered with 10 layers of stearic acid and separated from each other by three layers of stearic acid, and finally covered by 8 layers of stearic acid to avoid interlayer interaction and to protect the sample. Films were deposited on quartz plates with ends cut at an angle of 45 degree(s) to use the full inner reflection of the probing beam. Fluorescence of the samples were studied in the time range from 10 ps to 10 ns and transient absorption in time range from 100 ns to 100 microsecond(s) . Results are discussed from the point of two types of chlorophyll arrangements on the films.

Time-resolved transient absorption and photoresponse studies of oriented bacteriorhodopsin in Langmuir-Blodgett films

In this work we have studied photon initiated processes of bacteriorhodopsin (BR) in Langmuir-Blodget (LB) monolayers and multilayers. As far as we know the present work is the first, where the photoresponse function and kinetics of BR is detected from a monolayer as well as the first report, where oriented monolayers have been used as a proton pump. The transient absorption spectra and the decay functions of multilayers were detected for films in water, in water vapor saturated air, and at room humidity. This had an influence on transient lifetimes. The method we have used to orient the bacteriorhodopsin molecules is based on the LB-technique and the fact that the protein orients during the coating process when a phospholipid binder is used. The photoelectric signals of the monolayers deposited by the different directions had opposite polarities. The amplitude due to five layers was five times as high as that due to a monolayer.