Chemistry Reference
In-Depth Information
8.0E + 07
6.0E + 07
4.0E + 07
2.0E + 07
0.0E + 00
0
2
4
6
8
10
12
pH
Figure 4.9.
Effect of pH on the second-order rate constant for quenching of singlet
oxygen (
k
t
) by His (
) and Trp (•). The rates constants were measured in D
2
O/
acetonitrile (50 : 50 v/v) buffered with phosphate (40 mmol/dm
3
) with the addition of
NaOH or Dcl as required (adapted from Bisby et al. [159] with the permission of the
American chemical Society).
protein side chains with
1
O
2
could occur through both physical quenching and
chemical reactions [13]. It appears Trp was the only amino acid to have a
significant contribution to physical quenching with a
k
value of 10
7
/M/s, similar
to the chemical reaction rate constant (Table 4.6) [163]. The relative contribu-
tion of the two processes for Trp could be affected by several factors such as
the environment of the residue and the presence of the substituent on the
indole ring [170, 171]. The quenching of
1
O
2
with N-acetyltryptophan amide
(NATA), Trp(cH
2
)
16
, and melittin in D
2
O and in solution containing liposome
membrane using different sensitizers showed the quenching rate constants
were greatly affected by the location of the sensitizer as well as the quencher
in the liposome membrane or in the surrounding solution [165].
The products formed in the reactions of free cys, cystine, and Met with
1
O
2
have been characterized [152, 172-175]. In the case of cys, products were pH
dependent, and disulfide and oxyacids, such as cysteic acid (RSO
3
H), were
formed [152, 161]. The ratios of disulfide to oxyacids in the reactions of cys
residues in proteins with
1
O
2
are expected to vary with the protein structure
because electronic and steric barriers influence the formation of dimers. The
quantitative analysis for the products of the reaction of free cystine with
1
O
2
showed the formation of two molecules of RSS(=O)R through a zwitterionic-
peroxy intermediate (Eqs. 4.38 and 4.39). Formation of this intermediate was
suggested by conducting kinetics and trapping experiments and by carrying
out low-temperature matrix-isolated Fourier transform infrared (FTIR) spec-
troscopy of sulfur-oxygen adducts:
R S O
+
1
→
R S -OO
+
−
(4.38)
2
2
2
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