Quasi-Static Self-Quenching of Trp-X and X-Trp Dipeptides in Water: Ultrafast Fluorescence Decay

Abstract

Time-resolved fluorescence decay profiles of N-acetyl-L-tryptophan-amide (NATA) and tryptophan (Trp) dipeptides of the form Trp-X and X-Trp, where X is another aminoacyl residue, have been investigated using an ultraviolet upconversion spectrophoto fluorometer with time resolution better than 350 fs, together with a time correlated single photon counting apparatus on the 100ps to 20ns time scale. We analyzed the set of fluorescence decay profiles at multiple wavelengths using the global analysis technique. Nanosecond fluorescence transients for Trp dipeptides all show multiexponential decay, while NATA exhibits a monoexponential decay near 3 ns independent of pH. In the first 100 ps, a time constant for the water “bulk relaxation” around Trp, NATA and Trp dipeptides is seen near 1-2 ps, with an associated preexponential amplitude that is positive or negative depending on emission wavelength, as expected for a population-conserving spectral shift. The initial brightness (sub-ps) we measure for all these dipeptides is less than that of NATA, implying even faster (<200fs) intra-molecular (quasi) static quenching occurs within them. A new, third, ultrafast decay, bearing an exponential time constant of 20-30 ps with positive amplitude, has been found in many of these dipeptides. We believe it verifies our previous predictions of dipeptide QSSQ (“quasi static self quenching”) –the loss of quantum yield to sub-100ps decay process (Chen et al., Biochemistry, 1991, 30, 5184). Most important, this term is found in proteins as well (J.A.C.S., 2006, 128, 1214; Biophysical Journal 2008, 94, 546; 2009, 96, 46a), suggesting an ultrafast quenching mechanism must be common to both.

Hydration dynamics at fluorinated protein surfaces

“…Every protein except A1m-H displayed a short-lifetime component of a few hundred ps, present at all wavelengths. These types of quenching processes have been attributed to Trp interactions with nearby charged residues (37)(38)(39), and the absence of such a feature in A1m-H again indicates that this protein has a local structure different from those of the other eight proteins. The perturbation of local structure and, thus, local solvent exposure can result in different hydration dynamics, making it unreliable to compare the dynamics of A1m-H to those of the other A1m proteins.…”

Tryptophan Fluorescence Yields and Lifetimes as a Probe of Conformational Changes in Human Glucokinase

“…The existence of dark excited state species [16], or extremely short excited species [17] are well documented. We do not know any literature example in a protein where the lifetime does not significantly change while the intensity changes so much (>200 %).…”

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.…”