Ester-derivatized indoles as sensitive infrared probes for local environment

[Display omitted] •The CO stretching frequencies of methyl indole-4-carboxaldehyde is sensitive to local hydration environments.•The CO stretching frequencies of methyl indole-4-carboxaldehyde can probe the solvent polarizabilities.•The changing of the CO stretching frequencies in aprotic solvents can be described by the Onsager field model.•Methyl indole-4-carboxylate could be used as a site-specific IR probe. Methyl indole-4-carboxylate emits around 450 nm with fairly long fluorescence lifetime, which makes it a promising fluorescent probe for protein local structure and dynamics. Plus, it can also be used as an IR probe to sense the local hydration environments. However, there have been no systematic studies performed on the methyl indole-4-carboxylate as an infrared probe. Here, we systematically study the carbonyl stretching mode of methyl indole-4-carboxylate using Fourier-transform infrared spectroscopy. Our results suggested that the methyl indole-4-carboxylate could be used as a site-specific IR probe both for the electric and local environments.