Aldehyde-derivatized indoles as fluorescent probes for hydration environments

Tryptophan derivatives have long been used as site-specific fluorescent probes. 4-cyanotryptophan emits in the visible region and is the smallest blue fluorescent amino acid probe. We performed UV–Vis, steady-state and time-resolved fluorescence spectroscopy on six aldehyde-derivatized indoles dissolved in water and found that indole-4-carboxaldehyde (I4A) has the largest redshifts among all reported indole derivatives and can emit in the green region of the visible spectrum, which suggests that substitution of the indole ring of Trp using I4A may make a green fluorescent amino acid probe. It differs from tryptophan by only 3 atoms and will be the smallest green fluorescent amino acid probe that has great potential to be used in spectroscopic and microscopic measurements of proteins. We also found that I4A could be used as a fluorescent probe to detect trace water in organic solvents since its maximum emission wavelength is extremely sensitive to local hydrogen-bonding status.