George Strauss

Plant phenolics as cross-linkers of gelatin gels and gelatin-based coacervates for use as food ingredients

Abstract Polyphenols are known to react under oxidizing conditions with side chain amino groups of peptides, leading to formation of cross-links in proteins. Plant-derived phenolic acids and flavonoids were used to prepare cross-linked gelatin gels in bulk and cross-linked gelatin–pectin coacervates in the form of microparticles for use as food ingredients. Gels cross-linked by these materials had greater mechanical strength, reduced swelling, and fewer free amino groups. Dynamic light scattering analyses showed that such cross-linking results in denser polymeric networks and prevents extension of the peptide chains is when the pH is moved away from the isoelectric point. Coacervated gelatin–pectin microparticles when cross-linked became more lipophilic, and were stable at temperatures up to 200 °C, in contrast to un-cross-linked particles that coalesce and/or disintegrate on heating. These properties of cross-linked gelatin gels and gelatin-based coacervates have applications for the development of novel food ingredients.

ENERGY TRANSFER FROM CAROTENOIDS TO CHLOROPHYLL a IN BLACK LIPID MEMBRANES

Abstract— Black lipid membranes (BLM) were prepared from extracts of Chlorella and spinach chloroplasts. Excitation spectra of the 730 nm fluorescence of chlorophyll a in the BLM contained peaks identified as due to carotenoids and which therefore indicate sensitization of the chlorophyll fluorescence by them. The efficiency of this energy transfer was evaluated by comparison of the actual excitation spectra with those corresponding to 0 and 100 per cent transfer efficiency. Efficiencies were of the order of 40–50 per cent in BLM, but only 10 per cent in pigment solutions, when the mean distance between pigment molecules was 23 Å in both systems. The fluorescence quantum yield of chlorophyll a in such solutions was only 2 per cent of that found in BLM. Enhancement of energy transfer in BLM is considered to be mainly due to suppression of competing deactivation processes of excited carotenoid states, such as diffusional quenching by ground‐state molecules and internal conversion. Favorable orientation of pigment molecules in the BLM constitutes a further enhancement factor.

Reaction characteristics and mechanisms of lipid bilayer vesicle fusion

Abstract Small unilamellar lipid bilayer vesicles were prepared from brain phosphatidylserine, egg phosphatidylcholine, and synthetic dipalmitoylphosphatidylcholine, and were fused into larger structures by freezing and thawing, addition of calcium chloride, and passage through the lipid phase transition temperature. Fusion reactions were studied by electron microscopy, light scattering, and use of fluorescent probes. Fusion was accompanied by leakage of lipid vesicle constituents and of water-soluble solutes in the inner vesicle compartments, and by uptake of these types of components from the external solution. Such leakage was greater during fusion by freezing than by Ca 2+ . Passage through the transition temperature produced a moderate degree of fusion, without loss of membrane components. It is concluded that each fusion method gives rise to a characteristic size or narrow range of sizes of fusion products. The fraction of small vesicles fused into larger structure depends on the method of vesicle preparation, composition of the lipid bilayer, and composition of the external solution. Fusion is induced by creation of a discontinuity in the bilayer or by removal of water associated with the bilayer. The amount of water removed controls the extent of fusion. This is maximized in bilayers when in the liquid-crystal phase, as against the gel phase, in vesicles made by ethanol injection, as against sonication, and in charged bilayers, as against neutral ones.

Optical spectroscopy of bilayer membranes.

Abstract— The absorption and fluorescence spectroscopy of natural and model bilayer lipid membranes is reviewed. Basic structural features of biological membranes and the relative advantages of black lipid membranes (BLM) and of liposomes are discussed.

ENERGY TRANSFER AND ENERGY LOSSES IN BILAYER MEMBRANE VESICLES (LIPOSOMES)

Abstract. The efficiency of singlet‐singlet energy transfer was studied in bilayer lipid membrane vesicles (liposomes) for the following donor‐acceptor systems: (1) p‐terphenyl (TP) and diphenyloctatetraene (DPO); (2) DPO and chlorophyll a (Chl a); and (3) β‐carotene and Chl a. The energy transfer efficiency φDA was measured by sensitized fluorescence of the acceptor. Fractional quenching of the donor φQ was found from the donor fluorescence in absence and presence of the acceptor. For TP‐DPO and for DPO‐Chl a, the transfer efficiency increased with increasing acceptor concentration but was essentially independent of the donor concentration. No energy transfer from β‐carotene to Chl a could be detected. In liposomes, φDA differed only slightly from φQ at all donor and acceptor concentrations, thus demonstrating the absence of any appreciable energy losses. For solutions of the same donor‐acceptor pairs in cyclohexane φQ was considerably larger than φDA. The difference represents energy lost, principally by internal conversion, due to collisional quenching. The principal function of the lipid membrane appears to be the suppression of such losses. In addition, the rate of energy transfer in lipid membranes is about double that in solutions (at the same intermolecular distance) due to more favorable orientation.