Electron transfer kinetics in photosynthetic reaction centers embedded in trehalose glasses: trapping of conformational substates at room temperature

We report on room temperature electron transfer in the reaction center (RC) complex purified from Rhodobacter sphaeroides. The protein was embedded in trehalose-water systems of different trehalose/water ratios. This enabled us to get new insights on the relationship between RC conformational dynamics and long-range electron transfer. In particular, we measured the kinetics of electron transfer from the primary reduced quinone acceptor (Q(A)(-)) to the primary photo oxidized donor (P(+)), by time-resolved absorption spectroscopy, as a function of the matrix composition. The composition was evaluated either by weighing (liquid samples) or by near infrared spectroscopy (highly viscous or solid glasses). Deconvolution of the observed, nonexponential kinetics required a continuous spectrum of rate constants. The average rate constant (<k> = 8.7 s(-1) in a 28% (w/w) trehalose solution) increases smoothly by increasing the trehalose/water ratio. In solid glasses, at trehalose/water ratios > or = 97%, an abrupt <k> increase is observed (<k> = 26.6 s(-1) in the driest solid sample). A dramatic broadening of the rate distribution function parallels the above sudden <k> increase. Both effects fully revert upon rehydration of the glass. We compared the kinetics observed at room temperature in extensively dried water-trehalose matrices with the ones measured in glycerol-water mixtures at cryogenic temperatures and conclude that, in solid trehalose-water glasses, the thermal fluctuations among conformational substates are inhibited. This was inferred from the large broadening of the rate constant distribution for electron transfer obtained in solid glasses, which was due to the free energy distribution barriers having become quasi static. Accordingly, the RC relaxation from dark-adapted to light-adapted conformation, which follows primary charge separation at room temperature, is progressively hindered over the time scale of P(+)Q(A)(-) charge recombination, upon decreasing the water content. In solid trehalose-water glasses the electron transfer process resulted much more affected than in RC dried in the absence of sugar. This indicated a larger hindering of the internal dynamics in trehalose-coated RC, notwithstanding the larger amount of residual water present in comparison with samples dried in the absence of sugar.

Organogel-based system for transdermal delivery of propranolol

The reverse micelle-based microemulsion was proposed as a vector for the transdermal delivery of propranolol. Soya-lecithin forms reverse micelle in iso-octane with increased solubilizing potential. The lecithin/iso-octane system was studied for the nature of solubilized water, the effect of added water on viscosity, and drug-micelle interaction. The amount and nature of solubilized water was investigated by spectroscopic methods using potassium nitrate as the polarity probe. With increasing water content the viscosity of the system was found to increase. And at a molar ratio (R) = 3, the system showed gel-like consistency (8.1 x 10(3) poise). The polarity probe study suggested the difference in environment of solubilized water as compared with the bulk water. The permeability rate of propranolol contained in the water/lecithin/iso-octane system was calculated to be 1212.0 micrograms h cm-2 while in the case of petrolatum jelly it was recorded to be 122.8 micrograms h-1 cm-2.

On the application of the Clark oxygen electrode to the study of enzyme kinetics in apolar solvents: the catalase reaction

A method for recording O2 concentrations in nonconducting organic media with the Clark oxygen electrode was developed. Spontaneous oxidation of Na2S2O4 and the enzymatic reduction of NaBO3 or H2O2 by bovine liver catalase trapped in hydrated micelles of dioctylsulfosuccinate (AOT)/toluene were used as model systems. O2 titration with the above systems showed that air-saturated 1.6 M H2O/0.2 M AOT/toluene media contain seven times more O2 (1.4 mM) than aqueous solutions (0.2 mM). The measured Km values of catalase for NaBO3 and H2O2 in organic media were Kmov = 15 and 17 mM, respectively, whereas in aqueous buffer the values were 45 and 54 mM. In the toluene media, catalase activity increased with the W0 (H2O/AOT molar ratio) of the micellar preparation, reaching maximal activity at W0 = 10-12; under this condition, the catalytic center activity (Kp) of H2O2 was 7 x 10(6) min-1, similar to that obtained in the aqueous buffer (H2O2 = 7 x 10(6) min-1). It was found that the optimal pH for catalase in toluene media (pH 8.0) was shifted 1.0 unit compared to that in the aqueous buffer (pH 7.0). On the other hand, catalase was severely inhibited by NaN3 in both media. Thus, polarography based on the Clark oxygen electrode seems to be an easy, rapid, and sensitive technique for studying enzyme reactions consuming or evolving O2 in apolar media.

Spectroscopic investigations of the water pool in lecithin reverse micelles

The nature of the water pool formed in the reverse micellar system, lecithin/nonpolar solvent/water, has been investigated by means of near infrared, ultraviolet, fluorescence emission and visible spectroscopic techniques. The three nonpolar solvents chosen in this study were benzene, carbon tetrachloride and cyclohexane. Near infrared spectroscopic studies revealed that the amount of water present in the bulk organic phase is negligible at all water concentrations studied in all three solvents. The results of the polarity probe and 8-anilinonaphthalene sulfonic acid (ANSA) fluorescence emission maxima studies indicate that the polarity of the water pool is much lower than that of bulk water. The difference in polarity between the water pool and bulk water decreases with increasing water concentration in benzene and carbon tetrachloride systems. However, in the cyclohexane system, at a water content of 6 moles of water per mole of lecithin, where the system is known to change from isotropic reverse micelle to anisotropic liquid crystalline state, the polarity of the water pool is found to decrease.

Solubilization of enzymes and nucleic acids in hydrocarbon micellar solutions

The present state of the field of biopolymers solubilized in apolar solvents via reverse micelles is reviewed. First, an extensive discussion of the physical and chemical properties of reverse micelles is presented. Particular attention is devoted to the nature of water in the water pools of reverse micelles; to the structure and shape of the micellar aggregates; and to the dynamic properties of the reverse micelles. In the second part of the paper, the mechanism of solubilization of proteins and nucleic acids in hydrocarbon reverse micelles is discussed. Spectroscopic data, mostly circular dichroism and fluorescence, are reviewed in order to clarify the conformational changes which the biopolymers undergo upon their uptake into the micellar environment and determine the location of the biopolymers inside the reverse micelles. Data from neutron scattering, light scattering, ultracentrifugation, and electron microscopy of the protein-containing micelles are reviewed and discussed with the aim of illustrating the structure of the micellar aggregates containing the biopolymer as guest molecules. The activity of enzymes and nucleic acids is discussed, with emphasis on the influence upon the chemical reactivity brought about by the micellar parameters. Finally, a brief review of the applications and potentialities of biopolymer-containing reverse micelles is presented.