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Many phenolic compounds are known to enhance the chemiluminescence associated with the horseradish peroxidase-catalyzed oxidation of luminol, but the mechanism of enhancement is still unproved. Using stoppedflow spectrophotometry, we have found that a series of luminescence enhancers react rapidly with the peroxidase reactive intermediates (compound I and compound II) supporting the hypothesis that the enhancement is due to the acceleration of the enzyme turnover. In addition, pulse radiolysis experiments have shown that the enhancers' phenoxyl radicals oxidize luminol, consistent with a redox mediator role for the enhancers. The latter reaction was found to be reversible, showing that enhancers of low reduction potential, which are efficient in accelerating the enzyme turnover, are also scavengers of luminol radicals and therefore luminescence quenchers. Using these data, a simple model is proposed which correctly predicts that the efficiency of a phenolic compound as luminescence enhancer depends on the reduction potential of the respective phenoxyl radical according to a bell-shaped function with a maximum at ∼0.8 V.

Cholesterol hemisuccinate has been shown to equilibrate readily with liposomes and with the (Ca2+-Mg2+)-ATPase from sarcoplasmic reticulum and has been used to modify the sterol content of these membranes. Cholesterol hemisuccinate incorporates into dioleoylphosphatidylcholine (DOPC) up to a molar ratio of 3:1 sterol to DOPC. Effects on lipid order as detected by electron spin resonance and fluorescence polarization are comparable to those of cholesterol. Binding constants have been determined, and the uncharged form of the sterol binds more strongly than the anionic form. Binding to DOPC and to the lipid component of the ATPase system is comparable. From use of the fluorescence quenching properties of 1,2-bis(9,10- dibromooleoyl )phosphatidylcholine and dibromocholesterol hemisuccinate, two classes of binding sites on the ATPase have been deduced. At the lipid/protein interface, the binding constant for cholesterol hemisuccinate is considerably less than that for DOPC. At the second set of sites ( nonannular sites), binding occurs with Kd = 0.55 in molar ratio units. The effect of cholesterol hemisuccinate on the activity of the ATPase depends on the phospholipid present in the system: ATPase reconstituted with DOPC is inhibited whereas ATPase reconstituted with dimyristoleoylphosphatidylcholine is activated. We conclude that changes in membrane fluidity are not important in determining ATPase activity in these systems.

The effect of various compositional substitutions on the colour of long-lived F-centres in the sodalite structure has been investigated by the synthesis and characterisation of several phases of the general composition A 8 [M 0 M 00 O 4 ] 6 [X,S] 2 . The visible-region absorption spectrum of the excited state, formed by 256 nm irradiation of the as-synthesised materials, shows a broad absorption whose position correlates with the dimensions of the sodalite cage occupied by the excited electron.

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