Sequencing cyclic peptide inhibitors of mammalian ribonucleotide reductase by electrospray ionization mass spectrometry

• Published in: Journal of mass spectrometry. Vol. 36; no. 6; pp. 658 - 663
• Main Authors: Lin, Shanhua, Liehr, Sebastian, Cooperman, Barry S., Cotter, Robert J.
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.06.2001; Wiley
• Subjects: Analytical, structural and metabolic biochemistry; Biological and medical sciences; charge-remote fragmentation; collision-induced dissociation; cyclic peptide; electrospray ionization mass spectrometry; Enzyme Inhibitors - chemistry; Enzyme Inhibitors - pharmacology; Enzymes and enzyme inhibitors; Fundamental and applied biological sciences. Psychology; Gas Chromatography-Mass Spectrometry; mammalian ribonucleotide reductase; Oxidoreductases; Peptides, Cyclic - chemistry; Peptides, Cyclic - pharmacology; Ribonucleotide Reductases - antagonists & inhibitors; Spectrometry, Mass, Electrospray Ionization; Collisional activation; Enzyme; Cyclic peptides; Lactam; Enzyme inhibitor; Fragmentation pattern; Primary structure; Electrospray; Mass spectrometry MS/MS; Oxidoreductases; Ribonucleoside-diphosphate reductase; Aminoacid sequence; Structural analysis
• ISSN: 1076-5174; 1096-9888
• DOI: 10.1002/jms.167

Mammalian ribonucleotide reductase (mRR) is a potential target for cancer intervention. A series of lactam‐bridged cyclic peptide inhibitors (1–9) of mRR have been synthesized and tested in previous work. These inhibitors consist of cyclic and linear regions, causing their mass spectral characterization to be a challenge. We determined the fragmentation mechanism of cyclic peptides 1–9 using an ion‐trap mass spectrometer equipped with an ESI source. Low‐energy collision‐induced dissociation of sodiated cyclic peptides containing linear branches follows a general pathway. Fragmentation of the linear peptide region produced mainly a and b ions. The ring peptide region was more stable and ring opening required higher collision energy, mainly occurring at the amide bond adjacent to the lactam bridge. The sodium ion, which bound to the carbonyl oxygen of the lactam bridge, acted as a fixed charge site and directed a charge‐remote, sequence‐specific fragmentation of the ring‐opened peptide. Amino acid residues were cleaved sequentially from the C‐terminus to the N‐terminus. Our findings have established a new way to sequence cyclic peptides containing a lactam bridge based on charge‐remote fragmentation. This methodology will permit unambiguous identification of high‐affinity ligands within cyclic peptide libraries. Copyright © 2001 John Wiley & Sons, Ltd.