Use of the Sonogashira coupling reaction for the "two-step" labeling of phenylalanine peptide side chains with organometallic compounds.

The Pd-catalyzed Sonogashira coupling of ferrocene alkyne derivatives as metal probes to iodophenylalanine containing peptides is described. 4-Iodophenylalanine was incorporated into dipeptides and the neuropeptide [Leu5]-enkephalin (Enk) by solid phase peptide synthesis, thereby creating a functional group suitable for the Sonogashira coupling. The reaction with two different ferrocene alkynes resulted in the corresponding ferrocene-labeled derivatives, which were obtained in good yield and purity. All new compounds were comprehensively characterized, including elemental analysis, 1D and 2D NMR, EI-, FAB- or ESI-MS, IR and UV-vis spectroscopy, and electrochemistry of the ferrocene label. Unlike well-established conjugation methods for peptide side chains such as lysine and cystein, the phenyl group in Phe is not readily available for derivatization. This work presents a versatile procedure for the regioselective introduction of an organometallic label into biologically relevant peptides as exemplified for enkephalin.