Chemistry Reference
In-Depth Information
in less efficient nitrogen nucleophile in the ring-closing Michael reaction
(Scheme 4.3). The c1 hydroperoxide (species
6
) was the major product in the
reaction of the peptide-bound Tyr residue with
1
O
2
. Species
5
and
7
in Schemes
4.2 and 4.3 were formed via species
4
and
6
, respectively.
Oxidation of free His by
1
O
2
was postulated to happen through the initial
formation of one or more endoperoxides, similar to species
8
(Scheme 4.4, Fig.
4.11). One mole of His was consumed by 1 mol of
1
O
2
[182]. At a very low
temperature, peroxides such as species
8
have been seen in the reactions of
1
O
2
with substituted His derivatives [183]. Such species rapidly decompose to
poorly characterized intermediates, which ultimately yield a complex mixture
of products including aspartic acid and aspartic derivatives and urea (Scheme
4.4). A dipeptide carosine, which contains His and β-amino alanine, showed
damage to the imidazole ring when reacted with
1
O
2
[160]. The stretching
vibration of the c=c bond and the bending vibration of the NH bond in the
imidazole ring were changed significantly in the Raman spectroscopy charac-
terization of the products [160].
Exposure of
1
O
2
to a wide range of proteins showed the formation of per-
oxide species regardless of the composition of proteins [13]. Peroxides were
generated on Trp, Tyr, or His residues within the proteins, and the yields of
peroxides were enhanced with D
2
O as the solvent. In the case of tyrosine
phosphotase-1B, inactivation of the proteins by
1
O
2
resulted in oxidation of
the active-site cys 215 [168]. The capsid protein in the bacteriophage MS2 was
also modified by
1
O
2
[167]. The oxidized product detected was of the trypsin
peptide (Ser84-Lys106). High yields of peroxides were formed on exposure of
visible light to bulk cellular proteins, extracted from the cells, in the presence
of sensitizers [13].
In a recent study, a detailed mass spectral analysis of cytochrome
c
and two
model peptides P824 and tryptophan cage (Trp-cage) by
1
O
2
, generated by
irradiation at 670- to 675-nm light in the presence of phthalocyanine Pc
4-malate salt, was conducted to see how
1
O
2
oxidizes amino acid side chains
of proteins and inactivates enzymes [169]. cyt-
c
contains two Met (Met65 and
Met80), three His (His 18, His 26, and His 33), one Trp, four Tyr, and four Phe
(Phe10, Phe36, Phe46, and Phe82) residues. P824 is an eight-residue peptide
(ASHLgLAR) with a single His residue. Trp-cage is a designed 20-residue
miniprotein (NLyIQWLKDggPSSgRPPPS) in which the Trp side chain is
buried and more than 95% of it is folded in water at physiological pH [184].
The oxidized modification of cyt-
c
was studied using matrix-assisted laser
desorption/ionization-time of flight (MALDI-TOF) and tandem MS methods
and is provided in Table 4.7 [169]. Modification of only one peptide, HKTg-
PNLHgLFg, was observed in H
2
O, while several modifications occurred in
the presence of D
2
O (Table 4.7). The predominance of M + 16 and M + 32
modifications in D
2
O suggests a single modification.
Modification as M + 16 in the case of Met65 and Met80 indicates sulfoxide
as the oxidized product of this amino acid. Met80 showed an oxidized product
of both +16 and +32. The product of the +32 adduct may be related to either
Search WWH ::
Custom Search