Mixed monolayers association

Scheme 12 Surface reconstitution of apo-glucose oxidase on a mixed monolayer associated with an electrode consisting of an FAD cofactor and photoisomerizable nitrospiropyran units, and reversible photoswitching of the bioelectrocatalytic functions of the enzyme electrode.

Recently Bourges, Small, and Dervichian (5) reported that a para-crystalline lamellar structure of egg lecithin can solubilize cholesterol up to a maximum of one molecule of cholesterol per molecule of lecithin. However, they conclude that this should not be considered as a molecular association but rather the consequence of the relative arrangement of the molecules in the lamellar structure which is a mutual (solid) solution of lecithin and cholesterol. They also reported that the state of compression in the lamellar structure corresponds to that of a highly compressed mixed monolayer of lecithin-cholesterol. The NMR results of Chapman and Penkett (8) also appear to indicate that solubilization of cholesterol in egg lecithin dispersions results in a highly packed structure in which fatty acyl chains possess little molecular motion. Our results from lecithin-cholesterol monolayers also suggest that these mixed mono-layers are two-dimensional solutions with no specific interaction and that the apparent condensation in some instances is caused by the steric factors of the fatty acyl chains and not by the interaction or association between lecithin and cholesterol. 

In another example, a mixed monolayer composed of a photoisomerizabie component and an electrochemical catalyst was applied to switch the electrocatalytic properties of a modified eleetrode between ON and OFF states. A Au-electrode surface functionalized with a nitrospiropyran mono-layer and PQQ moieties incorporated into the monolayer was applied to control the electrocatalytic oxidation of 1,4-dihydri-P-nicotinamide adenine dinucleotide (NADH) by light. The positively charged nitromerocyanine-state interface resulted in the repulsion of Ca cations, which are promoters for the NADH oxidation by the PQQ, thus resulting in the inhibition of the electrocatalytic process. In the nitrospiropyran state, the monolayer does not prevent the association of the PQQ catalyst and promoter thus it provides efficient electrocatalytic oxidation of NADH. Similar outcomes have been achieved using a combination of the photo- and thermal effects resulting... 

Three QCM experiments based on vancomycin-peptide interactions were conducted. First, vancomycin was used as the sample to inject to the A C mixed monolayer surface and PBS was used as the running buffer (pH 7.3). The second experiment was carried out under a condition similar to the first experiment, but the value of the pH was changed to 2.1. The third experiment used the B C surface and PBS running buffer (pH 7.3) for vancomycin-peptide binding study. Since second and third QCM experiments were employed as the control for the first measurement, as expected, vancomycin could only be correctly associated with the o-Ala-o-Ala containing peptide on the A C surface at the physiological pH (Figure 14.8). 

The association of the acidic phospholipid PI with the cationic linear polysoap PVPC6 is cooperative in both mixed spread monolayers and mixed swollen multibilayers. This association has a condensation effect on the spread mixed monolayers on one hand and affects the properties of the interbilayer structural water of swollen mixed multibilayers on the other hand. Namely the increase in the degree of... 

In mixed monolayer, cholesterol and phosphatide molecules are in close contact, and therefore, should interact at least part of the time. The question whether such interactions lead to complexes, i.e., associations of appreciably longer half-life than associations between like molecules, would seem to depend on whether the van der Waals interactions generated by complex formation are appreciably higher than those generated between like molecules. Hence the half-life of complexes may be altered by the nature of acyl chains (length, position, unsaturation). 

The implications for films cast from mixtures of enantiomers is that diagrams similar to those obtained for phase changes (i.e., melting point, etc.) versus composition for the bulk surfactant may be obtained if a film property is plotted as a function of composition. In the case of enantiomeric mixtures, these monolayer properties should be symmetric about the racemic mixture, and may help to determine whether the associations in the racemic film are homochiral, heterochiral, or ideal. Monolayers cast from non-enantiomeric chiral surfactant mixtures normally will not exhibit this feature. In addition, a systematic study of binary films cast from a mixture of chiral and achiral surfactants may help to determine the limits for chiral discrimination in monolayers doped with an achiral diluent. However, to our knowledge, there has never been any other systematic investigation of the thermodynamic, rheological and mixing properties of chiral monolayers than those reported below from this laboratory. 

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