Tryptophan Octyl Ester in Detergent Micelles of Dodecylmaltoside: Fluorescence Properties and Quenching by Brominated Detergent Analogs

Abstract

The fluorescence properties of tryptophan octyl ester (TOE), a hydrophobic model of Trp in proteins, were investigated in various mixed micelles of dodecylmaltoside (DM) and 7,8-dibromododecyl beta-maltoside (BrDM) or 10,11-dibromoundecanoyl 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 uncharged and almost completely bound to detergent micelles. TOE binding was monitored by its enhanced fluorescence in pure DM micelles or its quenched fluorescence in pure BrUM or BrDM micelles. In DM/BrUM and DM/BrDM mixed micelles, the fluorescence intensity of TOE decreased, as a nonlinear function of the molar fraction of brominated detergent, to almost zero in pure brominated detergent. The indole moiety of TOE is therefore highly accessible to the bromine atoms located on the detergent alkyl chain because quenching by bromines occurs by direct contact with the fluorophore. TOE is simultaneously poorly accessible to iodide (I(-)), a water-soluble collisional quencher. 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 emission wavelength. Such heterogeneity may arise from dipolar relaxation processes in a motionally restricted medium, as suggested by the time-dependent (nanoseconds) red shift (11 nm) of the TOE emission spectrum, and from the existence of various TOE conformations. Time-resolved quenching experiments for TOE in mixed micelles showed that the excited-state lifetime values decreased only slightly with increases in the proportion of BrDM or BrUM. In contrast, the relative amplitude of the component with the longest lifetime decreased significantly relative to that of the short-lived species. This is consistent with a mainly static mechanism for the quenching of TOE by brominated detergents. Molecular modeling of TOE (in vacuum and in water) suggested that the indole ring was stabilized by folding back upon the octyl chain, forming a hairpin conformation. Within micelles, the presence of such folded conformations, making it possible for the entire molecule to be located in the hydrophobic part of the micelle, is consistent with the results of fluorescence quenching experiments. TOE rotational correlation time values, 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 or mixed detergent micelles. These results, obtained with a simple micellar model system, provide a basis for the interpretation of fluorescence quenching by brominated detergents in more complex systems such as protein- or peptide-detergent complexes.

Influence of Reverse Micellar Environments on the Fluorescence Emission Properties of Tryptophan Octyl Ester

“…However, it is being increasingly recognized that the hydrophobic fluorophore tryptophan octyl ester (TOE, structure shown in Fig. 1 inset) is a more suitable model for tryptophan, in case of membrane proteins (12)(13)(14)(15)(16)(17). Thus it is of considerable importance to carry out detailed characterization of the fluorescence emission properties of TOE in appropriate environments relevant to biomembranes.…”

“…In particular, they investigated its partitioning and motional characteristics in such membranes as a function of its ionization state. More recently, Foresta et al (12) investigated the fluorescence properties of TOE in various mixed micellar systems of dodecylmaltoside (DM) and 7,8-dibromododecyl ␤-maltoside (BrDM) or 10,11-dibromoundecanoyl ␤-maltoside (BrUM). They found that the indole moiety of TOE is highly accessible to the bromine atoms located on the detergent alkyl chain because quenching by bromine occurs by direct contact with the fluorophore.…”

Water Gradient in the Membrane–Water Interface: A Time-Resolved Study of the Series of n-(9-Anthroyloxy) Stearic Acids Incorporated in AOT/Water/iso-octane Reverse Micelles

“…84 This is a common observation made using this technique 9,13,26,45,85,86 as well as in MD simulations. 75,87−89 This slow relaxation has been proposed to be due to the motion of interfacial water in exchange with the bulk, 9,84 to the local dynamics of polar moieties belonging to the macromolecules or their assemblies, 90 or to the mobility of the probes themselves.…”

Novel Insights Into Protein Structure and Dynamics Utilizing the Red Edge Excitation Shift Approach

“…[133][134][135][136][137][138][139] Tryptophan octyl ester (TOE) has been recognized as an important model for membrane-bound tryptophan residues. 62,[140][141][142] The fluorescence characteristics of TOE incorporated into model membranes and membrane-mimetic systems have been shown to be similar to that of membrane-bound tryptophans. Consistent with the preferred interfacial location of the tryptophan residue and the solvent-restricted environment of the interface, TOE has been found to exhibit REES.…”