Cysteine Sulfenic Acid
A R T I C L E S
Scheme 1. Nomenclature and Oxidation States of Common
Derivatives of Cysteine
(CySO2H), and cysteine sulfonic acid (CySO3H) is observed
by H NMR. However, if less vigorous methods are employed
1
to initiate the reaction between CySH and HOCl, such as the
use of pipettes, a 1:1 mixture CySSCy and N,N′-dichlorocystine
NDC ) [-SCH2CH(NHCl)(CO2H)]2) is produced.3
7,38
While
(
CySSCy was the main stable product that was observed for the
reaction of HOX (X ) Cl or Br) with excess CySH between
pH 10 and 14, at very high pH, under some circumstances,
CySO2H was a minor product of the reaction of CySH and HOX
via a mechanism that cannot be attributed to the hydrolysis of
CySSCy (because the latter reaction is not kinetically compe-
39
tent). We will later attribute the production of CySO2H during
the oxidation of CySH by HOX to overoxidation (the reaction
of multiple equivalents of HOX).
Kinetics of the Reaction of CySH with HOX (X ) Cl or
Br). The kinetics of the reaction of CySH with HOCl have been
4
0
of a few derivatives that are stabilized through steric hindrance,
H-bonding, or conjugation,19-24 sulfenic acids are generally
considered to be transient species.25 We describe herein the
facile generation of CySOH under alkaline conditions (wherein
it exists principally as the sulfenate anion)26 by the reaction of
CySH with HOX (X ) Cl or Br). Of note, HOX is produced
during inflammatory response by defensive human peroxidases,
and one of its principal targets in ViVo is CySH.27-31 Thus, the
redox cascade that begins with the oxidation of CySH by HOX
is also of physiological relevance.6
previously investigated by Armesto et al. We have repeated
some of these measurements (data not shown), and we have
9
-1 -1
confirmed their rate constants: k(HOCl) ) 1.2 × 10 M
s
-
5
-1 -1
and k(OCl ) ) 1.9 × 10 M s . However, we disagree with
the conclusion that cysteine sulfenyl chloride (CySCl) has
transitory stability (Vide infra). We have also carried out
preliminary measurements for the reaction of CySH with HOBr.
Even under very alkaline conditions, the reaction tests the limits
of the stopped-flow method. For most of the stopped-flow
experiments that are described herein, the oxidation of CySH
by HOX (X ) Cl or Br) occurred within the time of mixing.
Following these oxidation reactions, two subsequent reactions
are observed when the pH is between 10 and 12 (Figure 1).
One of the reactions is essentially pH independent between pH
10 and 12 (although it becomes pH dependent below pH 10;
,32-36
Results
Products of the Reaction of CySH with HOX (X ) Cl or
Br). When HOCl is reacted with excess CySH, the only
1
cysteine-derived product that is observed by H NMR (within
3
9
data not shown). The other reaction exhibits a complex pH
dependency, whereby nonlinear [H ] dependency was observed
between pH 10 and 12 (Figure 6), and linear [H ] dependency
was observed between pH 12 and 14 (Figures 2 and 3). The
effective second-order rate constants of the pH-dependent
reaction between pH 12 and 14 (which were computed by
dividing the observed pseudo-first-order rate constants by the
concentration of the reactant in excess, i.e., keff ) kobs/[CySH]0)
5
min of mixing) is CySSCy. Furthermore, when turbulent
+
(
(
(
stopped-flow) mixing is employed, lower ratios of CySH:HOCl
+
down to a ratio of 2:1) also produce essentially only CySSCy
-
and sometimes a small amount of CySO2 ) at pH 11.3. We
have observed that when turbulent conditions are employed to
mix a 1:1 ratio of CySH with HOCl at pH 11.3 (stopped-flow
or a hand-mixer consisting of two Hamilton syringes and a
T-mixer), a 3.3:1:1.1 mixture of CySSCy, cysteine sulfinic acid
-
can be modeled with a linear relationship of keff versus 1/[OH ]
(
(
(
(
(
19) Goto, K.; Shimada, K.; Furukawa, S.; Miyasaka, S.; Takahashi, Y.;
that passes through the origin (Figure 2 and squares and solid
line of Figure 3). Above pH 10, the pH-dependent reaction also
exhibits a linear dependence on [CySH]0 that is the same
regardless of whether HOCl or HOBr is employed as the oxidant
(e.g., Figure 4). We note that the reaction that exhibits pH
dependency cannot be monitored below pH 10 (Vide supra).
Thus, the pH-dependent reaction exhibits the same rate for X
Kawashima, T. Chem. Lett. 2006, 35, 862-863.
20) Goto, K.; Holler, M.; Okazaki, R. J. Am. Chem. Soc. 1997, 119, 1460-
1
461.
21) Ishii, A.; Komiya, K.; Nakayama, J. J. Am. Chem. Soc. 1996, 118, 12836-
2837.
22) Tripolt, R.; Belaj, F.; Nachbaur, E. Z. Naturforsch., B: Chem. Sci. 1993,
1
4
8, 1212-1222.
23) Yoshimura, T.; Tsukurimichi, E.; Yamazaki, S.; Soga, S.; Shimasaki, C.;
Hasegawa, K. Chem. Commun. 1992, 1337-1338.
(
24) Nakamura, N. J. Am. Chem. Soc. 1983, 105, 7172-7173.
25) Patai, S. The Chemistry of Sulfenic Acids and Their DeriVatiVes; John
Wiley: New York, 1990.
)
Cl and X ) Br and overall third-order kinetics above pH 12:
(
first-order each in [oxidized cysteine intermediate], [CySH], and
[H ]. The reaction, that is essentially pH independent, also
(
26) O’Donnell, J.; Schwan, A. J. Sulfur Chem. 2004, 25, 183-211.
+
(
27) Davies, K. J. A. NATO Advanced Study Institute Series A296; 1998, pp
2
53-266.
exhibits the same rates for X ) Cl and Br (Figure 1) between
pH 10 and 12, and overall second-order kinetics: first-order
each in [oxidized cysteine intermediate] and [CySH]. In addition,
the pH-independent reaction was also observed as a minor
reaction at pH > 12 (data not shown). The pH-independent
reaction will be attributed later to the reaction of cysteine
(
(
(
28) Pattison, D. I.; Davies, M. J. Chem. Res. Toxicol. 2001, 14, 1453-1464.
29) Pattison, D. I.; Davies, M. J. Biochemistry 2004, 43, 4799-4809.
30) Pattison, D. I.; Hawkins, C. L.; Davies, M. J. Chem. Res. Toxicol. 2003,
1
6, 439-449.
31) Hawkins, C. L.; Pattison, D. I.; Davies, M. J. Amino Acids 2003, 25, 259-
74.
(
(
(
(
2
32) Giles, G. I.; Tasker, K. M.; Collins, C.; Giles, N. M.; O’Rourke, E.; Jacob,
C. Biochem. J. 2002, 364, 579-585.
33) Giles, G. I.; Tasker, K. M.; Jacob, C. Gen. Physiol. Biophys. 2002, 21,
6
5-72.
34) Giles, G. I.; Tasker, K. M.; Jacob, C. Free Radical Biol. Med. 2001, 31,
(37) Nagy, P.; Ashby, M. T. Chem. Res. Toxicol. 2005, 18, 919-923.
(38) Nagy, P.; Ashby, M. T. Chem. Res. Toxicol. 2007, 20, 79-87.
(39) Nagy, P.; Lemma, K.; Ashby, M. T. J. Org. Chem. 2007. In press.
(40) Armesto, X. L.; Canle L, M.; Fernandez, M. I.; Garcia, M. V.; Santaballa,
J. A. Tetrahedron 2000, 56, 1103-1109.
1
279-1283.
(
35) Giles, G. I.; Jacob, C. Biol. Chem. 2002, 383, 375-388.
(
36) Jacob, C.; Lancaster, J. R.; Giles, G. I. Biol. Soc. Trans. 2004, 32, 1015-
1
017.
J. AM. CHEM. SOC.
9
VOL. 129, NO. 45, 2007 14083