Biological membranes, electronic properties

Spin labels are usually molecules that contain a nitroxide moiety with an unpaired electron localized on the nitrogen and oxygen atoms. These labels are specifically incorporated in the lipid part of the biological membrane. In this way, the properties of the different regions of the membrane can be studied. ESR measures the transition... 

There are many studies of the transfer of electrons from enzymes to substrates, across biological membranes, to (or from) electrodes from (or to) substrates, between adsorbed molecular dyes and semiconductor particles, within synthetic films and nano-scale arrays, within molecular wires , and so on. Only a few, general comments will be offered on these topics here. The basic physics of molecular electron transfer does not change with the scale of the system, as long as identifiable molecular moieties are present with at least partly localized electronic configurations. The nature of the properties observed, the experimental probes available, and the level of theoretical treatment that is useful may be very different. Different approaches, different limiting models are used for extended arrays (or lattices) of very strongly coupled moieties. 

Filipin affects sterol-containing membranes at concentrations of 10-30 /Ltg/ml (<50 fiM). The use of higher concentrations for the study of sterols in biological membranes gives results which should be interpretated cautiously because filipin has detergent properties. Results obtained by Bishop (1973, 1974) on the effects of filipin on electron transfer in chloroplasts required 0.1-2 mM concentrations, and detergent effects can therefore not be discounted. Hendrix and Higinbotham (1973) applied filipin (up to 30 fiM) to a stem section ofP. sativum L. (pea). K" " and NOj uptake were inhibited, but respiration was unaffected. The effects of filipin on ion transport could be prevented by prior treatment with cholesterol. 

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