J. CHEM. RESEARCH (S), 1998 99
1
1
yields k1=27.0 0.9 mol dm3
s
and 104(kK2+k3K3) =
fast
k1
4–
5–
Fe2(CN)10 + cysteine
Fe2(CN)10 + cysteine+•
4.4 1.5 s 1. Treatment of the data by non-linear regression
analysis yields 26.8 1.0 and 4.5 1.0 respectively. As the
concentrations of the tautomeric species B and C must
always be in the constant ratio K2/K3 we cannot obtain sep-
arate values for k2 and k3 except by some ad hoc postulate
such as assuming that one of the tautomers is very much
more reactive than the other. Thus if k2wk3 then k2 is
Fe2(CN)10 + A+•
5–
6–
Fe2(CN)10 + A
k2
k3
Fe2(CN)10 + B+•
5–
6–
Fe2(CN)10 + B
Fe2(CN)10 + C+•
5–
6–
Fe2(CN)10 + C
1
1
1.5 Â105 mol dm3
s
at 25 8C. It should be noted that
2A+• (or B+• or C+•
)
cystine
although we have assigned k1 to HSCH2CH(NH3+)CO2
there must also be a small concentration of the tautomer
SCH2CH(NH3+)CO2H. From the fact that the pKa values
for CO2H and SH dissociation dier by some 6.5 log units
suggests the concentration of this tautomer may be some
106.5 times smaller than the main carbohydrate ionised
tautomer. However this would still give a bimolecular rate
constant well below the encounter limit. A similar argument
applies to HSCH2(NH2)CO2H. Further analysis of the data
is not justi®ed. With other reductants that contain a nucleo-
philic sulfur centre reaction with [Fe2(CN)10]4 can give rise
to coloured pentacyanoferrate(III) complexes with a sixth
ligand bound to iron through sulfur. We do not see this
with L-cysteine, possibly because over our pH range the sulf-
hydryl group is only slightly ionised, and reduction to
[Fe2(CN)10]6 is faster than nucleophilic substitution.
Scheme 1
copper catalysis for the second reaction we cannot exclude
the possibility that it may contribute to the ®rst reaction,
though the fact that the ®rst reaction was still fast in the
presence of added edta argues against it. In the absence
of kinetic data we cannot go further in discussing the ®rst
reaction.
For the second reaction the variation in kobs with pH
must surely be due to the acid/base equilibria of L-cysteine.
It has three ionisable groups, carboxyl (CO2H), amino
(NH3 ) and sulfhydryl (SH). In aqueous solution L-cysteine
can exist in ®ve dierent forms depending on the pH of the
solution [eqns. (2)±(6)]. The values for the respective dis-
sociation constants7 are pK1=2.0; pK2=8.53; pK3=8.86;
pK4=10.36; pK5=10.03.
Experimental
Materials.ÐThe complex, [Fe2(CN)10
HSCH2CHꢀNH3 CO2H
4
]
,
was prepared as
560 nm and concen-
described previously.4 Solutions had
K1
max
HSCH2CHꢀNH3 CO2 H ꢀ2
1
trations were calculated using = 1600 M cm 1. L-cysteine hydro-
chloride was supplied by Aldrich Chemical Co., USA and used
without further puri®cation.
HSCH2CHꢀNH3 CO2
K2
Kinetic Studies.ÐReactions were followed by stopped-¯ow spec-
a Hi-Tech Scienti®c SF-51 stopped-¯ow
SCH2CHꢀNH3 CO2 H
ꢀ3
ꢀ4
ꢀ5
ꢀ6
trophotometry using
attached to a Hi-Tech Scienti®c SU-40 spectrophotometer unit. The
machine was attached to a Haake GH constant temperature water
bath ®tted with a Haake D8 circulating pump. The pH was varied
using acetate and disodium hydrogen orthophosphate±citric acid
buers, the pH being measured with an Orion Research Expandable
HSCH2CHꢀNH3 CO2
K3
HSCH2CHꢀNH2CO2 H
SCH2CHꢀNH3 CO2
Ion Analyzer EA 920 ®tted with a Cole-Parmer combination elec-
trode. Ionic strength was maintained at 1.0 mol dm
K4
3
using
NaClO4. In all cases, the reaction was investigated under pseudo
®rst order conditions with [cysteine]r[complex].
SCH2CHꢀNH2CO2 H
HSCH2CHꢀNH2CO2
Analysis for Copper.ÐThis was carried out using a Perkin Elmer
2380 AAS instrument, calibrated with standard copper sulfate sol-
ution. A good Beer Lambert Law plot was obtained up to 10 ppm
of copper. All reagents including reactants, buers and distilled
water were checked and found to contain less than 20 ppb, our
detection limit.
K5
SCH2CHꢀNH2CO2 H
Over our pH range it is clear from the pKa values that
the main component of cysteine, more than 98%, is
HSCH2CH(NH3 )CO2 with only minor contributions from
other species. The fact that kobs is almost constant from
pH 3.63 to 4.36 at 25 8C suggests that the bimolecular rate
constant for HSCH2CH(NH3 )CO2 must be close to 0.59/
This work was supported by the Board of Postgraduate
Studies, University of the West Indies through scholarships
to F.A.B. and D.B. We thank a referee for valuable
comments.
1
1
0.02 = 29.5 mol dm3
s
and that undissociated cysteine
has negligible reactivity under our experimental conditions.
The increase in kobs with pH must be due to contributions
from conjugate base species.
Received, 15th September 1997; Accepted, 22nd October 1997
Paper E/7/06692I
Scheme 1 shows the proposed mechanism, and from
this rate law (7) may be written, where A, B and C
References
are HSCH2CH(NH3+)CO2
HSCH2CH(NH2)CO2 respectively.
,
SCH2CH(NH3 )CO2 and
1 A. D. James, W. C. E. Higginson and R. S. Murray, J. Chem.
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3 F. A. Beckford, T. P. Dasgupta and G. Stedman, J. Chem. Soc.,
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5
Rate ꢀk1A k2B k3CFe2ꢀCN
ꢀ7
10
Now in our pH range [A] = [cys]T[H+]/([H+] + K2+K3),
[B] = [cys]TK2/([H+] + K2+K3)
and
[C] = [cys]TK3/
([H+] + K2+K3) where [cys]T is the total, stoichiometric
concentration of L-cysteine. On substituting into eqn. (7)
and rearranging eqn. (8) was obtained.
kobsꢀH K2 K3 cys k1H k2K2 k3K3 ꢀ8
T
When eqn. (8) is plotted using the data in Table 2 a
straight line plot is obtained, and least squares analysis
7 R. E. Benesch and R. Benesch, J. Am. Chem. Soc., 1955, 77,
5877.