Sh. Lin et al., Sequencing cyclic peptide inhibitors of mammalian ribonucleotide reductaseby electrospray ionization mass spectrometry, J MASS SPEC, 36(6), 2001, pp. 658-663
Mammalian ribonucleotide reductase (mRR) is a potential target for cancer i
ntervention. A series of lactam-bridged cyclic peptide inhibitors (1-9) of
mRR have been synthesized and tested in previous work. These inhibitors con
sist of cyclic and linear regions, causing their mass spectral characteriza
tion to be a challenge. We determined the fragmentation mechanism of cyclic
peptides 1-9 using an ion-trap mass spectrometer equipped with an ESI sour
ce. Low-energy collision-induced dissociation of sodiated cyclic peptides c
ontaining linear branches follows a general pathway. Fragmentation of the l
inear peptide region produced mainly a and b ions. The ring peptide region
was more stable and ring opening required higher collision energy, mainly o
ccurring at the amide bond adjacent to the lactam bridge. The sodium ion, w
hich bound to the carbonyl oxygen of the lactam bridge, acted as a fixed ch
arge 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-remo
te fragmentation. This methodology will permit unambiguous identification o
f high-affinity ligands within cyclic peptide libraries. Copyright (C) 2001
John Wiley & Sons, Ltd.