Use of N,O‐bis‐Fmoc‐d‐Tyr‐ONSu for introduction of an oxidative iodination site into cholecystokinin family peptides

We report the synthesis of a new reagent for the introduction of an oxidative iodination site into the amino terminus of acid‐labile peptides, and the use of this reagent to synthesize a novel affinity‐labeling probe for the cholecystokinin (CCK) receptor. The acylation reagent, N,O‐bis‐fluorenylmethyloxycarbonyl‐d‐tyrosine hydroxysuccinimide ester, utilizes base‐labile protection of both the alpha amino group and the aromatic ring hydroxyl. This can be safely removed to expose a cross‐linkable free amino group on the aminopeptidase‐resistant d‐enantiomer of tyrosine. The synthetic probe, d‐Tyr‐Gly‐Asp‐Tyr(OSO₃H)‐Nle‐Gly‐Trp‐Nle‐Asp‐Phe‐NH₂, was fully biologically active, could be radioiodinated to high‐specific radioactivity (2000 Ci/mmol), bound with high affinity to the pancreatic CCK receptor, and covalently labeled the hormone‐binding site. This reagent should be useful for the synthesis of a wide variety of analogues of CCK and other acid‐labile peptides.