Stereospecific assignment of the methyl 1H NMR lines of valine and leucine in polypeptides by nonrandom 13C labelling

An alternative method for the stereospecific assignment of the methyl groups of valine and leucine in the nuclear magnetic resonance (NMR) spectra of peptides and proteins is proposed, and its practical application is demonstrated with the assignment of all valine and leucine methyl groups in cyclosporin A. The method is based on the use of a mixture of fully 13 C‐labelled and unlabelled glucose as the sole carbon source for the biosynthetic production of the polypeptide studied, knowledge of the independently established stereoselectivity of the reaction pathways of valine and leucine biosynthesis, and analysis of the distribution of 13 C labels in the valyl and leucyl residues of the product by two‐dimensional heteronuclear NMR correlation experiments.