Vesicle Interface Research Articles

An electron magnetic resonance study on the photoionization of N-alkylphenothiazines in dioctadecyldimethylammonium chloride frozen vesicles: the effect of urea, 1,3-dimethylurea, 1,3-diethylurea, and 1,1',3,3'-tetramethylurea

Photoionization of N-alkylphenothiazines was carried out in frozen dioctadecyldimethylammonium chloride (DODAC) vesicle suspensions to which various concentrations of urea, 1,3-dimethylurea (DMU), 1,3-diethylurea (DEU), 1,1{prime},3,3{prime}-tetramethylurea (TMU) had been added. The photoproduced cation radical yields were investigated by electron spin resonance (ESR). Electron spin echo modulation (ESEM) and electron nuclear double resonance (ENDOR) spectroscopies were used to probe the microenvironment of the photoproduced cation. The concentration of each of the urea derivatives was systematically varied. Urea, DMU, and DEU showed a similar trend in the photoyield versus concentration, with the highest yield obtained at 1 M additive. For TMU the photoyield of phenothiazine increased as a function of additive concentration. The different photoyield trends are explained in terms of the effect of urea on the DODAC vesicule interface. This interpretation is supported by the ESEM and ENDOR studies. Urea and its derivatives appear to interact with water through the {open_quotes}direct mechanism{close_quotes}. The ESR data for these species suggest that impregnation for the vesicle interface with urea decreases the strength of the photoproduced cation-water interactions which leads to a decrease in the photoyield. The ESE and ENDOR results for TMU suggest that this bulky additive opens up the vesicle interface to water penetration.more » The steady increase in the photoyield with TMU concentration together with the ESEM and ENDOR results support this interpretation of the ESR data. 49 refs., 10 figs.« less

Electron spin echo modulation studies of doxylstearic acid spin probes in frozen dihexadecyl phosphate and dioctadecyldimethylammonium chloride vesicles: interaction of the spin probe with deuterated water and effects of cholesterol addition

Electron spin echo studies have been carried out for a series of x-doxylstearic acid (x=5, 7, 10, 12, and 16) and 3-doxyl-5α-cholestane spin probes in frozen deuterated aqueous solutions of anionic dihexadecyl phosphate (DHP) vesicles and cationic dioctadecyldimethylammonium chloride (DODAC) vesicles. Modulation effects due to interactions of the spin probe nitroxide group with D 2 O at the vesicle interface give distance-based information which can be used to draw conclusions about the ordering and chain packing of the vesicles

Location and photoionization studies of a series of alkylporphyrin derivatives solubilized in dioctadecyldimethylammonium chloride and dihexadecyl phosphate vesicles

Light-induced electron transfer reactions in micelles and vesicles solutions have been extensively investigated with the aim of optimizing the net conversion of light energy into chemical energy. Three alkylporphyrins were synthesized, 5-(1-nonylpyridinium-4-yl)-10,15,20-triphenylporphyrin bromide (H[sub 2]PC[sub 9]), 5-(1-dodecylpyridinium-4-yl)-10,15,20-triphenylporphyrin bromide (H[sub 2]PC[sub 12]), and 5-(1-hexadecylpyridinium-4-yl)-10,15,20-triphenylporphyrin bromide (H[sub 2]PC[sub 16]). The location of these 5-(1-alkylpyridinium-4-yl)-10,15,20-triphenylporphyrin bromides (H[sub 2]PC[sub n]) solubilized in cationic dioctadecyldimethylammonium chloride (DODAC) and anionic dihexadecyl phosphate (DHP) vesicles solutions was investigated using optical absorption spectroscopy. Electron spin resonance (ESR) was applied to observe the yields of photoionization of H[sub 2]PC[sub n] in rapidly frozen DODAC and DHP vesicle solutions versus alkyl chain length. A decrease in photoyield correlated with a increase in the porphyrin-to-vesicle interface distance. The photoionization of H[sub 2]PC[sub n] in rapidly frozen DODAC and DHP vesicle solutions containing electron acceptors was also studied by ESR. Two different electron acceptors were used in this study, tetrabromobenzoquinone (TBBQ) and potassium ferricyanide (K[sub 3]Fe(CN)[sub 6]). Electron transfer within the vesicle itself was examined with TBBQ, which solubilizes inside the lipophilic region of the vesicle near the interface, whereas K[sub 3]Fe(CN)[sub 6] involved electron transfer across the vesicle interface. The yields of photoionization of H[sub 2]PC[sub n] have generally been foundmore » to decrease upon addition of either acceptor. The results are discussed in terms of alkyl chain length, position of the porphyrin group relative to the vesicle interface, and vesicle surface charge.« less

Electron spin resonance, electron spin echo, and electron nuclear double resonance investigations of the photoreduction yield of a series of alkylmethylviologens in dihexadecyl phosphate vesicles: effect of added cholesterol

The initial event in photosynthesis, photoinduced charge separation, takes place in a membrane bound proton/chlorophyll complex - the photosynthetic reaction center. An electron spin resonance (ESR), electron spin echo (ESE), and electron nuclear double resonance (ENDOR) investigation was performed on a series of alkylmethylviolgens which were solubilized in dihexadecyl phosphate (DHP) vesicles and rapidly frozen at 77 K. Photoreduction yields were measured by ESR, while ESE and matrix ENDOR were used to determine the location of the viologen radical in the lipid bilayer with respect to the water interface and the hydrocarbon interior, respectively. The effect of various concentrations on cholesterol on both the photoreduction yield and the location of the viologen probe within the DHP vesicle was measured. The ESE and ENDOR results indicate that the increased lipophilicity of the longer chained viologens effects deeper solubilization into the bilayer. This increased lipophilicity correlates with an enhancement of the photoreduction yield due to a reduced back electron transfer rate. Addition of cholesterol results in a disruption of the integrity of the vesicle interface and some increase in the photoreduction yield. This increase is mostly probably due to a decrease in effective interface charge density of the vesicle as a resultmore » of the intercalation of cholesterol molecules into the bilayer.« less

Temperature dependence of the photoionization yield of chlorophyll-a in rapidly frozen vesicles

The photoionization yield of chlorophyll-a (Chla) in rapidly frozen dipalmitoylphosphatidylcholine (DPPC) vesicles containing lipophilic and lipophobic electron scavengers has been studied as a function of the temperature by electron spin resonance (ESR) measurements. Samples irradiated at 77 K prepared with lipophilic tetrachloroparabenzoquinone (TCBQ) show an ESR signal intensity which varies as 1/ T when measured at various temperatures. Samples prepared with TCBQ or lipophobic potassium ferricyanide (FC) were irradiated at various temperatures. The photoionization yield increases with increasing irradiation temperature. It increases monotonically with temperature with TCBQ while it increases suddenly near 190 K with FC. The results are interpreted as indicating that softening of the matrix enables the reactants to reach an optimal configuration for electron transfer. The different locations of TCBQ inside the vesicle and FC at the vesicle interface can account for their different effects.

Effects of the addition of alcohols, cryoprotective agents, and salts on the photoionization yield of chlorophyll a in frozen vesicle solutions with and without electron acceptors

The photoionization yield of chlorophyll a (Chla) in rapidly frozen vesicles with and without potassium ferricyanide (FC) or tetrachloro-p-benzoquinone (TCBQ) has been studied versus several structural variations of phospholipid vesicles, including the addition of medium chain length alcohols, the effect of added salts (metal chlorides), the presence of a double bond in the alkyl tail of the surfactant, and the addition of dimethyl sulfoxide or glycerol which tend to enhance vesicular structure retention upon freezing. Variations in the photoionization yield versus these structural parameters are discussed in terms of distance variations between Chla and electron acceptors, loss of integrity of the vesicle structure, and differences in the degree of hydration of the headgroups of the surfactant molecules. Electron spin echo (ESE) deuterium modulation associated with a 5-doxylstearic acid spin probe interacting with deuterated water probes the degree of water interaction at the vesicle interface. The ESE data support a correlation between the degree of interface hydration and the photoionization yield for vesicles containing Chla and FC as an electron acceptor. Parallel ESE studies of 5-doxylstearic acid spin probes in anionic and cationic surfactant vesicles reveal changes in the interface hydration if the surfactant counterion is changed; this can be roughlymore » correlated to the Chla photoionization yields.« less

Phase fluorometric studies of spectral relaxation at the lipid-water interface of phospholipid vesicles

We examined the dynamic properties of the lipid-water interface region of model membranes using the probe 2- p-toluidinylnaphthalene-6-sulfonic acid (TNS). For comparison we also examined the temperature-dependent spectral properties of TNS in the viscous solvent glycerol. Fluorescence phase shift and demodulation measurements were used to prove that the membranes relax around the excited state of TNS on the ns time scale. The rate of spectral relaxation is thought to reflect the mobility of the polar interface region of the membranes on this same time scale. The spectral relaxation times were estimated by the use of phase-sensitive detection of fluorescence. Using this method one may directly record, in an approximate fashion, the emission spectra of the relaxed and the initially excited states of TNS. The relative intensities of these phase-sensitive spectra, in combination with the measured phase and modulation values on the short and long wavelength sides of the emission, yield the spectral relaxation times. For saturated and unsaturated phosphatidylcholines, at temperatures ranging from 5 to 50°C, the relaxation times ranged from 5 to 1 ns. The activation energies for spectral relaxation were near 4 kcal/mol. Surprisingly, the relaxation times decreased smoothly with increasing temperature, and did not change abruptly at the phase transition temperatures. These results indicate that the small molecular motions of the interface region of membranes, which are responsible for spectral relaxation, are not dramatically influenced by the phase state of the acyl side chain region of the membranes.