Reinvestigation of an O-Salicylaldehyde Ester Functional Group in Aqueous Buffer and Discovery of a Coumarin Scaffold Probe for Selective N-Terminal Cysteine Labeling

Many intracellular proteins are metabolically unstable, and their half-life was known to be controlled by the “N-end rule,” that is, the N-terminal residue controlled protein stability. To visualize or measure the cellular stability of a protein, depending on the N-terminal residues, attention is being paid to the development of selective labeling methods for individual N-terminal amino acids. However, there are only a limited number of functional groups available for specific N-terminal amino acid labeling in a biological environment. Herein, we report a re-examination of salicylaldehyde ester for selective N-terminal residue tagging. Salicylaldehyde ester has been used for chemical ligation to N-terminal serine or threonine under pyridine/acetic acid conditions. Inspired by previous selective serine/threonine labeling, N-terminal labeling of salicylaldehyde ester in aqueous buffer has been examined by using boron-dipyrromethene (BODIPY), rhodamine, and coumarin probes. Surprisingly, the selectivity not only significantly differed, depending on the fluorophore incorporated in salicylaldehyde, but was also perturbed by the addition of a small fraction of phosphate-buffered saline. In particular, the coumarin-based salicylaldehyde ester probe showed notable selectivity against N-terminal cysteine under aqueous buffer conditions. This result reveals the serendipitous discovery of a new N-terminal cysteine labeling strategy.