Ester-Derivatized indoles as fluorescent and infrared probes for hydration environments

Abstract

Tryptophan derivatives have long been used as site-specific biological probes. 4-Cyanotryptophan emits in the visible region and is the smallest blue fluorescent amino acid probe for biological applications. Other indole or tryptophan analogs may emit at even longer wavelengths than 4-cyanotryptophan. We performed FTIR, UV-Vis, and steady-state and time-resolved fluorescence spectroscopy on six ester-derivatized indoles in different solvents. Methyl indole-4-carboxylate emits at 450 nm with a long fluorescence lifetime, and is a promising candidate for a fluorescent probe. The ester-derivatized indoles could be used as spectroscopic probes to study local protein environments. Our measurements provide a guide for choosing esterderivatized indoles to use in practice and data for computational modeling of the effect of substitution on the electronic transitions of indole.

Tryptophan as a Template for Development of Visible Fluorescent Amino Acids

“…8.8 ns in water. 38 Taken together, these findings suggest that the amino acid counterparts of these indole derivatives could find useful applications in biological spectroscopy and microscopy. For example, the fluorescence property of 4CO 2 H-Trp may exhibit a strong dependence on pH, due to its carboxyl group.…”

“…Other Derivatives. The studies of Huang et al 38 and of Micikas et al 46 showed that the absorption and emission spectra of 4CO 2 H-indole and 4CO 2 Me-indole are further red-shifted from those of 4CN-indole. Moreover, both indole derivatives have a fairly large fluorescence QY (0.4−0.5) in protic solvents, 38,46 and 4CO 2 Me-indole has a fluorescence lifetime of ca.…”

“…However, some of them, such as 4-azatryptophan, 7-azatryptophan, and 7-cyanotryptophan, − exhibit an absorption/emission spectrum that is red-shifted from that of Trp, suggesting that the underlying electronic transitions are tunable via substitution on the indole ring. Indeed, several indole derivatives were found to emit visible fluorescence . Even more encouragingly, several studies − have shown that when Trp is subjected to certain photoreactions, some of the resultant photoproducts have color and emit visible fluorescence.…”

“…Moreover, both indole derivatives have a fairly large fluorescence QY (0.4–0.5) in protic solvents, , and 4CO 2 Me-indole has a fluorescence lifetime of ca. 8.8 ns in water . Taken together, these findings suggest that the amino acid counterparts of these indole derivatives could find useful applications in biological spectroscopy and microscopy.…”

“…Indeed, several indole derivatives were found to emit visible fluorescence. 38 Even more encouragingly, several studies 39−41 have shown that when Trp is subjected to certain photoreactions, some of the resultant photoproducts have color and emit visible fluorescence. Taken together, those previous studies motivated the notion that it is possible to discover simple Trp derivatives that have "better" photophysical property and hence broader utility in biological spectroscopy and/or microscopy.…”