Room Temperature Phosphorescence from Tryptophan and Halogenated Tryptophan Analogs in Amorphous Sucrose

Abstract

Abstract. Tryptophan phosphorescence lifetime and quantum yield are sensitive to the local environment. The phosphorescence from tryptophan analogs, however, has not been studied. We report here data on the room temperature phosphorescence of tryptophan, 4‐, 5‐ and 6‐fluoro‐DL‐tryptophan (4‐F‐trp, 5‐F‐trp and 6‐F‐trp) and 5‐bromo‐DL‐tryptophan (5‐Br‐trp) embedded in glassy powders of freeze‐dried sucrose. In aqueous solution, the absorption of the analogs was either blue‐shifted (4‐F‐trp), red‐shifted (5‐F‐trp and 5‐Br‐trp) or not shifted (6‐F‐trp) with respect to tryptophan. The phosphorescence emission spectra of all analogs were red‐shifted compared to trp (442 nm) with maxima at 446 nm (5‐F‐trp), 451 mn (6‐F‐trp), 452 nm (5‐Br‐trp) and 469 nm (4‐F‐trp). The 5‐F‐trp and 6‐F‐trp analogs had emission intensities similar to tryptophan (relative quantum yields of 0.68 and 0.91, respectively, compared to tryptophan), while the intensities of the 4‐F and 5‐Br analogs were lower (relative quantum yields of 0.039 and 0.022, respectively). All analogs exhibited complex decay behavior requiring several exponentials for an adequate fit; the average lifetimes were all lower than that of trp (1039 ms). The average lifetimes of the fluorinated analogs (5‐F, 721 ms; 6‐F, 482 ms and 4‐F, 35 ms) scaled approximately with the relative quantum yields while that of 5‐Br (0.53 ms) was significantly lower. Analysis of the individual lifetimes suggested that the fluorinated analogs differ in their sensitivity to environmental interactions, with 5‐F‐ and 6‐F‐trp quenched 1.5‐2‐fold and 4‐F‐trp about 23‐fold more efficiently than tryptophan. The red‐shifted 5‐F‐trypto‐phan analog, which has been incorporated into proteins, may provide an alternative phosphorescence probe for selective phosphorescence detection of a specific protein in a complex mixture.

Anticancer Gold(III) Peptidomimetics: From Synthesis to in vitro and ex vivo Biological Evaluations

“…In this regard, it is knownt hat the intrinsicf luorescence of BSA stems from two tryptophan residues per chain that are known to be excellent structural probes. [79][80][81] They are characterizedb y strong emission signals at room temperature and are very sensitive to the local environment. One residue is buried (Trp 213) inside the protein,w hereas the second one is located on the protein surface (Trp 134).…”

Photophysical Probes of the Amorphous Solid State of Proteins

“…Because the value of k TS1 is only ∼3% of k TS0 at low temperature in all protein matrices, the low-temperature convergence of k TS0 is known with reasonable accuracy; however, because of our estimation procedure, it is impossible to detect subtle differences in slope at low temperature. Data from other triplet probes, such as tryptophan 52 and vanillin, 53 which do not exhibit reverse intersystem crossing should provide more insight into matrix mobility in glassy proteins at low temperature.…”

“…On the other hand, the long-lived phosphorescence decays of these probes in amorphous glasses prove challenging to analyze, often requiring fits to a sum of four or more exponentials. 52,53 Furthermore, above the glass transition temperature, these probes are effectively quenched by translational motions within the mobile matrix and thus provide little to no additional information.…”

Synthesis and characterization of an unnatural boron and nitrogen-containing tryptophan analogue and its incorporation into proteins

“…In many cases, tryptophan residues serve as the target of these studies 19. Tryptophan is the main source of intrinsic fluorescence in proteins,20 participates in various π-system interactions,21 and accounts for about 1% of amino acids found in proteins 22. Tryptophan's relative rarity, role in protein stability, and utility as a spectroscopic handle mark it as a useful target for protein studies.…”