Tryptophan octyl ester in detergent micelles of dodecylmaltoside: Fluorescence properties and quenching by brominated detergent analogs

The fluorescence properties of tryptophan octyl ester (TOE), a hydrophobic model of Trp in proteins, were investigated in various mixed micelles of do decylmaltoside (DM) and 7,8-dibromododecyl beta-maltoside (BrDM) or 10,11-d ibromoundecanoyl beta-maltoside (BrUM). This study focuses on the mechanism via which these brominated detergents quench the fluorescence of TOE in a micellar system. The experiments were performed at a pH at which TOE is unc harged and almost completely bound to detergent micelles. TOE binding was m onitored by its enhanced fluorescence in pure DM micelles or its quenched f luorescence in pure BrUM or BrDM micelles. In DM/BrUM and DM/BrDM mixed mic elles, the fluorescence intensity of TOE decreased, as a nonlinear function of the molar fraction of brominated detergent, to almost zero in pure brom inated detergent. The indole moiety of TOE is therefore highly accessible t o the bromine atoms located on the detergent alkyl chain because quenching by bromines occurs by direct contact with the fluorophore. TOE is simultane ously poorly accessible to iodide (I-), a water-soluble collisional quenche r. TOE time-resolved fluorescence intensity decay is heterogeneous in pure DM micelles, with four lifetimes (from 0.2 to 4.4 ns) at the maximum emissi on wavelength. Such heterogeneity may arise from dipolar relaxation process es in a motionally restricted medium, as suggested by the time-dependent (n anoseconds) red shift (11 nm) of the TOE emission spectrum, and from the ex istence of various TOE conformations. Time-resolved quenching experiments f or TOE in mixed micelles showed that the excited-state lifetime values decr eased only slightly with increases in the proportion of BrDM or BrUM. In co ntrast, the relative amplitude of the component with the longest lifetime d ecreased significantly relative to that of the short-lived species. This is consistent with a mainly static mechanism for the quenching of TOE by brom inated detergents. Molecular modeling of TOE (in vacuum and in water) sugge sted that the indole ring was stabilized by folding back upon the octyl cha in, forming a hairpin conformation. Within micelles, the presence of such f olded conformations, making it possible for the entire molecule to be locat ed in the hydrophobic part of the micelle, is consistent with the results o f fluorescence quenching experiments. TOE rotational correlation time value s, in the nanosecond range, were consistent with a hindered rotation of the indole moiety and a rotation of the complete TOE molecule in the pure DM o r mixed detergent micelles. These results, obtained with a simple micellar model system, provide a basis for the interpretation of fluorescence quench ing by brominated detergents in more complex systems such as protein- or pe ptide-detergent complexes.