REACTIVE SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 17

x 2 y 2 z 2 x 1 y 1 z 1

R 2a

R 1

R 3

W 2a

H 2 H 2

R 2b

W 2b

H

CH 2 CH CH 2

H H

H

V 2

H 2 N CH CO NH CH CO NH CH COOH

H 2 H 2

d 2b d 2a

a 1 b 1 c 1 a 2 b 2 c 2

Figure 1.6. Fragment ions of peptide (adapted from Soltwisch and Dreisewerd [167]
with the permission of the American Chemical Society).

an enhancement of fragmentation along the peptide chain [161, 173-176]. The
interpretation of spectra was simplified because only y-ions were detected.
The amino group of the lysine residue in peptides was selectively modified
by reactions with peroxycarbonate [167]. The lysine peroxycarbamates formed
undergo hemolytic fragmentation under low-energy CID in ESI and MALDI-
MS. The fragmentation of deuterated analogues of modified lysine resulted in
the formation of a-, c-, or z-types of ions with MS, which suggested the involve-
ment of a free radical mechanism in the fragmentation. The results in MALDI-
MS are shown in Table 1.6 [157]. Mostly, the fragments of a- and z-ions were
observed at the Lys site with one Lys or multiple Lys in the sequence of pep-
tides. Entry 4 showed the disulfide bond remains intact; however, the peptide
bond near the Lys was cleaved. Entry 1 in Table 1.6 showed the loss of a side
chain of Val at the N-terminus in the fragment with m / z 961.45. Side-chain
losses of Lys, Tyr, and Ser were also observed (entries 2, 3, and 5 in Table 1.6).
Other dissociation and ionization techniques besides CID have also been
applied in the MS-based structural determination of peptides and proteins.
These include ultraviolet photodissocation, nanoelectrospray ionization
(nanoESI), easy ambient sonic spray ionization (EASI), easy ambient sonic
spray ionization-membrane interface mass spectrometry (EASI-MIMS), laser
ablation electrospray ionization (LAESI), desorption electrospray ionization
(DESI), electrospray-assisted laser desorption/ionization (ELDI), electron
capture dissociation (ECD), electron transfer dissociation (ETD), infrared
multiphoton dissociation (IRMPD), atmospheric pressure chemical ionization
(APCI), and ambient liquid mass spectrometry (ALMS) [162, 163, 165-167,
177-189]. CID and IRMPD normally produce heterolytic cleavage of the
amide bonds in the polypeptide chain and resulting b and y fragments having
N- and C-termini, respectively [190]. ECD cleaves the N-C α bonds giving
mainly fragmented ions of the N-terminus c' and C-terminus z • . A recent study
compared top-down CID, CID, and IRMPD of nitrated proteins: hen egg-
white lysozyme (HEWL), myoglobin, and cytochrome c [191]. The total
number of fragments in ECD was greater than those in IRMPD or CID, but