The polar headgroup of the detergent governs the accessibility to water of tryptophan octyl ester in host micelles

AbstractMany attempts have been made to rationalize the use of detergents for membrane protein studies [J. Biol. Chem. 264 (1989) 4907]. The barrier properties of the detergent headgroup may be one parameter critically involved in protein protection. In this paper, we analyzed these properties using a model system, by comparing the accessibility of tryptophan octyl ester (TOE) to water-soluble collisional quenchers (iodide and acrylamide) in three detergent micelles. The detergents used differed only in the chemical nature of their polar headgroups, zwitterionic for dodecylphosphocholine (DPC) and nonionic for octa(ethylene glycol) dodecyl monoether (C12E8) and dodecylmaltoside (DM). In all cases, in phosphate buffer at pH 7.5, the binding of 5 μM TOE was complete in the presence of a slight excess of detergent micelles over TOE molecules, resulting in a significant blue shift and greater intensity of TOE fluorescence emission. The resulting quantum yield of bound TOE was between 0.08 (in DPC) and 0.12 (in DM) with an emission maximum (λmax) of ∼335 nm whatever the detergent micelle. Time-resolved fluorescence intensity decays of TOE at λmax were heterogeneous in all micelles (3–4 lifetime populations), with mean lifetimes of 1.7 ns in DPC, and 2 ns in both C12E8 and DM. TOE fluorescence quenching by iodide, in detergent micelles, yielded linear Stern–Volmer plots characteristic of a dynamic quenching process. The accessibility of TOE to this ion was the greatest with C12E8, followed by DPC and finally DM (Stern–Volmer quenching constants Ksv of 2 to 5.5 M−1). In contrast, the accessibility of TOE to acrylamide was greatest with DPC, followed by C12E8 and finally DM (Ksv=2.7–7.1 M−1). TOE also presents less rotational mobility in DM than in the other two detergents, as shown from anisotropy decay measurements. These results, together with previous TOE quenching measurements with brominated detergents [Biophys. J. 77 (1999) 3071] provide reference data for analyzing Trp characteristics in peptide (and more indirectly protein)–detergent complexes. The main finding of this study was that TOE was less accessible (to soluble quenchers) in DM than in DPC and C12E8, the cohesion of DM headgroup region being suggested to play a role in the ability of this detergent to protect function and stability of solubilized membrane proteins