Electrospray tandem mass spectrometry of alkali-cationized BocN-carbo-alpha,beta- and -beta,alpha-peptides: Differentiation of positional isomers.
Dissociation pathways of a series of alkali-cationized hybrid peptides, viz., Boc-alpha,beta- and -beta,alpha-carbopeptides, synthesized from C-linked carbo-beta3-amino acids [Caa (S)] and alpha-alanine (L-Ala), have been investigated by electrospray ionization tandem mass spectrometry. The positional isomers (six pairs) of the cationized alpha,beta- and beta,alpha-peptides can be differentiated by the collision-induced dissociation (CID) spectra of their [M + Cat-Boc + H]+ ions which give characteristic series of alkali-cationized C- (x(n)+, y(n)+, z(n)+) and N-terminal (a(n)+, b(n)+, c(n)+) ions. Another noteworthy difference is cationized beta,alpha-peptides eliminate a molecule of ammonia whereas this pathway is absent for alpha,beta-peptides. This is useful for identifying the presence of a beta-amino acid at the N-terminus. The CID spectra of [M + Cat-Boc + H]+ ions of these peptide acids show abundant rearrangement [b(n) + 17 + Cat]+ (n = 1 to n-1) ions which is diagnostic for distinguishing between alpha- and beta-amino acid at the C-terminus. MS(n) experiments of [b(n) + Li-H]+ ions from these hybrid peptides showed the loss of CO and 72 u giving rise to [a(n) + Li-H]+ and cationized nitrile product ions which render support to earlier proposals that b(n)+ or [b(n) + Cat-H]+ ions have protonated or cationized oxazolinone structures, respectively.
Reddy
P Nagi
PN
National Center for Mass Spectrometry, Indian Institute of Chemical Technology, Hyderabad 500 007, India.
Srikanth
R
R
Srinivas
R
R
Sharma
V U M
VU
Sharma
G V M
GV
Nagendar
P
P
D000468:Alkalies / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D015394:Molecular Structure; D010455:Peptides / Q000032:analysis* / Q000737:chemistry; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D013237:Stereoisomerism; D053719:Tandem Mass Spectrometry
D000468:Alkalies; D002412:Cations; D010455:Peptides