Fluorescence of tryptophan dipeptides: correlations with the rotamer model

Abstract

The multiexponential decay of tryptophan derivatives has previously been explained by the presence of rotamers having different fluorescence lifetimes, but it has been difficult to correlate rotamer structure and physical properties. New time-resolved and static data on dipeptides of the type Trp-X and X-Trp, where X is another aminoacyl residue, are consistent with the rotamer model and allow some correlations. That a dominant rotamer of Trp-X zwitterion has the -NH3+ group near the indole ring was inferred from absorption and fluorescence spectra, titrimetric determination of pKa values, photochemical hydrogen-deuterium-exchange experiments, decay-associated spectra, quantum yields, and decay kinetics. Analysis of the lifetime and quantum yield data for Trp dipeptides, especially X-Trp, suggests that static self-quenching is not uncommon. Highly quenched and weak components of the fluorescence do not contribute to the calculated mean lifetime, thus resulting in apparent static quenching. We propose the term quasi-static self-quenching (QSSQ) to distinguish this phenomenon from quenching due to ground-state formation of a dark complex. Mechanisms of quenching and the structure of statically quenched rotamers are discussed. The occurrence of QSSQ supports the idea that rotamers interconvert slowly. A major perceived deficiency of the rotamer model, namely, the apparent inability to predict reasonable rotamer populations from fluorescence decay data, may result from the presence of statically quenched species, which do not contribute to the fluorescence.

Ultrafast Polarized Fluorescence Measurements on Tryptophan and a Tryptophan-Containing Peptide

“…The three isotropic times were also interpreted as originating from different rotameric states from the tryptophan. 26 In our data, only one ns time was found. However, it is difficult to extract two ns isotropic times from our measurements, considering the restricted time range of the streak camera synchroscan unit (2.2 ns, although the "backsweep" enables us to estimate longer times).…”

Kinetics of the Association/Dissociation Cycle of an ATP-binding Cassette Nucleotide-binding Domain

“…Because the distance between the Trp rings is 3.0 -3.5 Å, energy transfer is expected to make a significant contribution to the fluorescence quenching because this distance is shorter than the Förster distance for Trp-Trp FRET homotransfer (R 0 estimated at 4 -16 Å) (29). However, other mechanisms such as static quenching (30) are also likely to play a role.…”

Fluorescence quenching of tryptophan and tryptophanyl dipeptides in solution

“…Work on tryptophanyl dipeptides has been used very elegantly by Knutson and coworkers to investigate the multiexponential decay of tryptophan in light of the rotamer model [22][23]. In these time resolved studies, the authors studied the multiexponential fluorescence decay in the picosecond regime and concluded that it is associated to ground state heterogeneity that may arise from different conformational rotamers of the peptide side chains.…”