Efficient Reagent-Saving Method for the N‑Terminal Labeling of Bioactive Peptides with Organometallic Carboxylic Acids by Solid-Phase Synthesis

Labeling of biomolecules with organometallic moieties holds great promise as a tool for chemical biology and for the investigation of biochemical signaling pathways. Herein, we report a robust and reproducible synthetic strategy for the synthesis of ruthenocenecarboxylic acid, giving the acid in 53% overall yield. This organometallic label was conjugated via solid-phase peptide synthesis in near-quantitative yield to a number of different biologically active peptides, using only 1 equiv of the acid and coupling reagents, thereby avoiding wasting the precious organometallic acid. This optimized method of stoichiometric N-terminal acylation was then also successfully applied to conjugating ferrocenecarboxylic acid and a novel organometallic Re^I(CO)₃ complex, showing the generality of the synthetic procedure.