Synthesis, characterization, X-ray crystallography, and cytotoxicity of a cymantrene keto carboxylic acid for IR labelling of bioactive peptides on a solid support

Cym-CO-CH2-CH2-COOH was prepared in good yield by Friedel-Crafts reaction of cymantrene (Cym, CpMn(CO)(3)) with succinic anhydride for the IR labelling of peptides and fully characterized, including an X-ray structure analysis (monoclinic space group P2(1)/n, a=5.727(3) angstrom, b=19.865(9) angstrom, c = 10.518(5) angstrom, beta = 91.211(9)degrees). The compound was isolated in pure form without the need for chromatographic work-up and subsequently used for solution-phase synthesis of a bioconjugate with phenylalanine methyl ester to allow a complete spectroscopic characterization of this model system. The cymantrene keto carboxylic acid also turned out to be a very robust marker in automated microwave-assisted solid phase peptide synthesis (SPPS). [Leu(5)]-enkephalin (Tyr-Gly-Gly-Phe-Leu) was prepared on a Wang resin and labelled with the cymantrene derivative on the solid support under microwave irradiation in all steps. The metal-carbonyl marker stayed intact during cleavage from the resin with concentrated trifluoroacetic acid. After simple precipitation and lyophilization, the cymantrene-enkephalin bioconjugate could be obtained in analytically pure form without the need of HPLC purification. As required, the compound is non-cytotoxic against MCF-7 cells at up to 100 mu M. This protocol thus allows one to introduce organometallic IR spectroscopic labels to peptides in a very straightforward way. (C) 2008 Elsevier Inc. All rights reserved.