Synthesis of photoactive analogues of a cystine knot trypsin inhibitor protein.

We describe the preparation of a recently described photoactive amino acid analogue (photoMethionine) by two novel synthetic routes, one of which is flexible and enantiospecific, and the site-specific chemical incorporation of photoMethionine into a defined and functionally active protein using a combination of solid-phase peptide synthesis and modern chemical ligation methodology. Site-specific labeling of proteins with this amino acid analogue through chemical synthesis provides valuable probes for photoaffinity cross-linking studies.