Direct identification of intramolecular disulfide links in peptides using negative ion electrospray mass spectra of underivatised peptides. A joint experimental and theoretical study.
[M--H]- parent anions of underivatised peptides containing an intramolecular disulfide bridge undergo characteristic loss of the elements of H2S2, a process diagnostic of the presence of the disulfide moeity. This facile process is initiated from a side-chain enolate anion. Theoretical calculations (at the HF/6-31G(d)//AM1 level of theory) indicate that the process is exothermic with a small barrier. When the disulfide link involves a C-terminal Cys, the negative ion spectrum shows an [(M--H)--(H2S2+CO2)] fragment anion which is usually the main peak of the spectrum. This process is also directed by an enolate anion: theoretical calculations suggest a stepwise sequence with loss of CO2 preceding loss of H2S2. Both [(M--H)--H2S2] and [(M--H)--(H2S2+CO2)] anions undergo backbone cleavage allowing identification of the amino acid sequence of the peptide.
Bilusich
Daniel
D
Department of Chemistry, The University of Adelaide, South Australia, 5005, Australia.
Maselli
Vita M
VM
Brinkworth
Craig S
CS
Samguina
Tatiana
T
Lebedev
Albert T
AT
Bowie
John H
JH
D000595:Amino Acid Sequence; D029845:Amphibian Proteins / Q000737:chemistry; D000818:Animals; D023181:Antimicrobial Cationic Peptides / Q000737:chemistry; D008969:Molecular Sequence Data; D010449:Peptide Mapping; D010455:Peptides / Q000737:chemistry*; D011895:Rana ridibunda; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*
D029845:Amphibian Proteins; D023181:Antimicrobial Cationic Peptides; D010455:Peptides; C078413:brevinin-1 protein, Rana