Multipurpose isothiocyanyl alanine/lysine: Use as solvatochromic IR probes and in site specific labeling/ligation of short peptides

[Display omitted] •Multifaceted use of unexplored UNAA, NCSAla = Ita and NCSLys = Itl.•The side chain –NCS does not perturb the backbone conformation of peptides.•The –NCS functionality is shown to have the solvatochromic IR sensitivity.•Site specific fluorescence labeling under physiological/mild condition.•Demonstrated the peptide ligation ability of NCSLys. The solvatochromic IR responsivity of small side chain –NCS in two unexplored unnatural amino acids, isothiocyanyl alanine (NCSAla = Ita) and lysine (NCSLys = Itl), without perturbing the conformation is demonstrated in two designed short tripeptide (BocAla-NCSAla-Ala-OMe) and hexapeptide (BocLeu-Val-Phe-Phe-NCSLys-Gly-OMe). Demonstration of site specific fluorescent labeling in both the peptides and ligation type reaction in NCSLys indicates the novelty of these two amino acids as alternative to the available canonical amino acids.