ASSOCIATION OF THE 5-METHYLPHENAZINIUM CATION AND THE 5-METHYL-10-HYDRO-PHENAZINIUM RADICAL CATION WITH MICELLES

Abstract— 5‐Methylphenazinium (MP+) cation salts are known to stimulate photophosphorylation in photosynthetic systems. The mechanism is thought to involve the translocation of protons across the thylakoid membrane although few details are known concerning the mechanism and the identity of the proton carrier. As there is some indication that the semireduced species, the 5‐methyl‐10‐hydro‐phenazinium (MPH+) cation radical, may be the proton carrier, we have studied the interaction of MPH+ with micelles as a model system for aqueouslipid interactions in a membrane. In this study we have used the techniques of NMR and EPR. In the presence of sodium dodecyl sulfate (NaLS) micelles the EPR spectrum of MPH + is broadened indicating specific binding of MPH+ to the micelle. A binding constant of ‐105M‐1 was obtained. The 220 MHz NMR spectrum of a micellar solution with added MPH! shows specific broadening of the alkyl protons. The use of model paramagnetic adducts has allowed us to infer from the NMR results that the MPH+ ion penetrates the hydrocarbon part of the micelle to a considerable extent. This would indicate to us that MPH+ can indeed function as a proton carrier as it should be able to penetrate deeply into the lipid layer of the membrane. From these results, we conclude that: Both MP+ and MPH+ interact strongly with NaLS micelles. MPH+ interacts more weakly with neutral Triton X‐100 micelles while in this case, no evidence of interaction is seen for MP+. No binding of either MP+ or MPH+ is observed with cationic micelles from CTAB. MP+ is adsorbed to NaLS micelles in a very hydrophilic region, probably In the Stern layer. MPH+, however, is much less hydrophilic than MP+ and appears to reside in the Palisade layer a few angstroms below the head groups. On the basis of these results, we make the following proposals: MPH+ is sufficiently hydrophobic to be a candidate for the reduced MP+ species which actually carries the proton across the thylakoid membrane. The site at which MP+ is reduced by X must be anionic in nature in order for MP+ to be bound strongly. Copyright © 1979, Wiley Blackwell. All rights reserved