Kinetic analysis of oxidation of dipeptides by sodium N-bromobenzenesulfonamide in acid medium: A mechanistic approach

Article abstract of DOI:10.1246/bcsj.76.73

The kinetics of the oxidation of five dipeptides (DPP) viz., glycylglycine (Gly-Gly), L-alanyl-L-alanine (Ala-Ala), L-valyl-L-valine (Val-Val), L-leucyl-L-leucine (Leu-Leu), and phenylglycyl-phenylglycine (Phg-Phg) by sodium N-bromobenzenesulfonamide or bromamine-B (BAB) in presence of HClO₄ was studied at 40 °C. The five reactions followed identical kinetics with a first-order dependence on [BAB] and fractional order in [DPP]. At [H⁺] > 0.04 mol dm^-3, the rate was inverse fractional in [H⁺], but zero order at lower [H⁺] (≤ 0.04 mol dm^-3). A variation of the ionic strength or dielectric constant of the medium and the addition of halide ions and benzenesulfonamide had no effect on the rate of the reaction. Proton inventory studies were made in H₂O-D₂O mixtures for all five dipeptides. A Michaelis-Menten type mechanism has been suggested to explain the results. The decomposition and equilibrium constants were evaluated. The oxidation products were identified. The isokinetic temperature was 360 K, indicating the enthalpy to be a controlling factor. The rate of oxidation increased in the order Phg-Phg > Ala-Ala > Val-Val > Leu-Leu > Gly-Gly. The kinetics of oxidation of dipeptides was compared with those of their corresponding monomer amino acids, namely Phenylglycine, alanine, valine, leucine, and glycine. A general mechanism was proposed and the derived rate law are consistent with the observed kinetics.

Full text of DOI:10.1246/bcsj.76.73

c. Jpn.
e Chemical © 2003 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 76, 73–80 (2003)
73
ciety of Ja-
n
e Chemical
ciety of Ja-
n
Kinetic Analysis of Oxidation of Dipeptides by Sodium
N-Bromobenzenesulfonamide in Acid Medium: a Mechanistic Approach
Kinetics of Oxidation of Dipeptides by Bromamine-B
Puttaswamy et al.
Puttaswamy* and Nirmala Vaz
03
Department of Post-Graduate Studies in Chemistry, Central College, Bangalore University, Bangalore-560 001, India
(Received January 7, 2002)
SJA8
09-2673
0
0
0
01
The kinetics of the oxidation of five dipeptides (DPP) viz., glycylglycine (Gly-Gly), L-alanyl-L-alanine (Ala-Ala),
L-valyl-L-valine (Val-Val), L-leucyl-L-leucine (Leu-Leu), and phenylglycyl-phenylglycine (Phg-Phg) by sodium N-bro-
2
2
mobenzenesulfonamide or bromamine-B (BAB) in presence of HClO
4
was studied at 40 °C. The five reactions followed
+
−3
identical kinetics with a first-order dependence on [BAB] and fractional order in [DPP]. At [H ] > 0.04 mol dm , the
+
+
−3
rate was inverse fractional in [H ], but zero order at lower [H ] (ꢀ 0.04 mol dm ). A variation of the ionic strength or
dielectric constant of the medium and the addition of halide ions and benzenesulfonamide had no effect on the rate of the
reaction. Proton inventory studies were made in H O–D O mixtures for all five dipeptides. A Michaelis–Menten type
2 2
mechanism has been suggested to explain the results. The decomposition and equilibrium constants were evaluated. The
oxidation products were identified. The isokinetic temperature was 360 K, indicating the enthalpy to be a controlling
factor. The rate of oxidation increased in the order Phg-Phg > Ala-Ala > Val-Val > Leu-Leu > Gly-Gly. The kinetics
of oxidation of dipeptides was compared with those of their corresponding monomer amino acids, namely Phenylgly-
cine, alanine, valine, leucine, and glycine. A general mechanism was proposed and the derived rate law are consistent
with the observed kinetics.
.
.
3
1
The chemistry of N-haloarenesulfonamidates, generally
known as organic haloamines, has evoked considerable inter-
est, because they are sources of halogen cations, hypohalite
species, and N-anions, which act both as bases and nucleo-
philes. The prominent members of this group are chloramine-
T (CAT) and chloramine-B (CAB) and the mechanistic aspects
the active form of BAB in an acid medium. Attempts have
been made to asses the relative rates of oxidation of these
dipeptides and to derive an isokinetic relationship with the
computed activation parameters. Further, the rates of oxida-
tion of these dipeptides were compared with their monomers
by BAB under identical experimental conditions.
1
–3
of many of these reactions have been documented.
Bro-
Experimental
mamine-B (BAB), the bromine analogue of CAB, is gaining
importance as a mild oxidant, and is found to be a better oxi-
15
Materials. Bromamine-B was prepared by the partial de-
bromination of dibromamine-B (DBB), which was obtained as
follows. Pure chlorine was bubbled through an aqueous solution
of chloramine-B (30 g in 560 mL of water) and liquid bromine (6
mL) was added dropwise from a burette with constant stirring. A
yellow precipitate of DBB that formed was thoroughly washed
4
dizing agent than the chloro compound. A review of the liter-
ature shows that kinetic studies using this reagent have been
5
–7
meager.
Although extensive work has been reported on the kinetics
8
–10
of the oxidation of amino acids, using a variety of oxidants,
2
with H O, filtered under suction, and dried in a vacuum desiccator.
there is little information in the literature on the oxidation ki-
netics of dipeptides. Dipeptides are useful biomaterials in
many analytical, biological, pharmaceutical, and synthetic ap-
plications. Glycylglycine, the first member of the dipeptide se-
ries, has been oxidized by manganese(Ⅲ) in an aqueous sulfu-
Dibromamine-B (31.5 g) was digested in small batches with con-
stant stirring in 50 mL of 4 mol dm− NaOH. The mass was
cooled in ice and filtered under suction, and the product was dried
over anhydrous calcium chloride.
3
The purity of BAB was checked iodometrically through its ac-
tive bromine content and the compound was further characterized
1
1
12
ric acid medium and bromamine-T in acid medium. Also,
1
3–14
its hydrolysis kinetics
has been reported. L-valyl-L-valine,
13
by its C FT-NMR spectrum (obtained on a Bruker WH 270-MHz
L-alanyl-L-alanine, L-leucyl-L-leucine, and Phenylglycyl-
Phenylglycine are other dipeptides. A literature survey shows
that there is no report on the oxidation kinetics of these dipep-
tides. In view of this, we have taken up a systematic kinetic
study of the oxidation of important dipeptides, namely, gly-
cylglycine (Gly-Gly), L-valyl-L-valine (Val-Val), L-alanyl-L-
alanine (Ala-Ala), L-leucyl-L-leucine (Leu-Leu), and phenyl-
nuclear magnetic resonance spectrometer) with D
and TMS as the internal standard (ppm relative to TMS) 143.38
C-1, carbon attached to S atom), 134.30 (C-4, para to hetero at-
om), 131.26 (C-2,6), and 129.31 (C-3,5). An aqueous solution of
BAB was standardized iodometrically and preserved in brown
bottles to prevent its photochemical deterioration.
2
O as a solvent
(
Chromatographically pure Gly-Gly and Ala-Ala (Merck), Val-
Val and Leu-Leu (Bachem AG, Switzerland) were used as re-
glycyl-phenylglycine (Phg-Phg) by bromamine-B (C
NBrNa•1.5 H O or BAB) in an acid medium to explore the
mechanistic aspects of these oxidations and also to understand
6 5 2
H SO -
16
2
ceived. Phg-Phg was prepared by following the standard proce-
dure, and was confirmed by its NMR data. Phg-Phg was prepared

7
4
Bull. Chem. Soc. Jpn., 76, No. 1 (2003)
Kinetics of Oxidation of Dipeptides by Bromamine-B
by employing a Boc group for N-protection and methyl ester as a
C-protecting group. The hydrolysis of the methyl ester followed
by removal of Boc group gave the dipeptide. Aqueous solutions
of dipeptides were prepared and employed. All other chemicals
were of analytical grade. Heavy water (99.4%) employed for sol-
vent isotope studies was supplied by the Bhabha Atomic Research
Centre, Mumbai, India. The ionic strength (Ⅰ) of the system was
maintained at a constant high value (0.5 mol dm− ) using a con-
centrated solution of sodium perchlorate. The permittivity of the
reaction mixture was altered by the addition of methanol in vary-
ing proportions (v/v), and the values of the permittivity (dielectric
NH
2
CHRCONHCHRCOOH + 2PhSO
2
NBrNa + 3H
2
O →
+
−
2RCHO + 2CO
2
+ 2NH + 2PhSO NH
3
2
2
+ 2Na + 2Br , (1)
where R = -H for Gly-Gly, -CH
Val, CH CH(CH for Leu-Leu, and - C H
3
for Ala-Ala, -CH(CH
3 2
) for Val-
for Phg- Phg.
2
3
)
2
6 5
The reaction mixture was extracted with ether and the corre-
sponding obtained aldehydes (RCHO, oxidation products of
dipeptides) were detected by the formation of their 2,4-dinitro-
phenylhydrazone derivatives. The hydrazone derivatives were
characterized by their IR spectra. Formaldehyde, acetaldehyde,
isobutyraldehyde, Isovaleraldehyde, and benzaldehyde are the ox-
idation products of Gly-Gly, Ala-Ala, Val-Val, Leu-Leu, Phg-Phg
3
18
17
constant) of methanol-water mixtures reported in the literature
18
were employed. Triply distilled water was used in preparing all
aqueous solutions throughout the studies. A regression analysis of
the experimental data was carried out on an EC-75 statistical cal-
culator to obtain the regression coefficient, (r).
Kinetic Measurements. Kinetic runs were performed under
pseudo first-order conditions with a known excess of the [sub-
strate] over [oxidant] at 40 °C. The reaction was carried out in
stoppered pyrex boiling tubes whose outer surfaces were coated
black to eliminate any photochemical effects. For each run, requi-
and the melting points of the corresponding 2,4-DNP hydrazone
derivatives are 164 °C, 168 °C, 186 °C, 122 °C, and 235 °C, re-
spectively. Benzenesulfonamide (PhSO
2
NH
uct of BAB) was identified by TLC using pet. ether–CHCl
butanol (2:2:1 v/v) as a solvent system and iodine as a spray re-
agent (R = 0.88). NH and CO were identified by conventional
2
, the reduction prod-
19
3
–1-
f
3
2
20
tests. Ammonia was quantitatively estimated by the micro-
kjeldahl procedure.
Results
4
site amounts of solutions of the substrate, acid, NaClO , and the
water (to maintain a constant total volume) were introduced into
the tube and thermostated at 40 ± 0.1 °C until thermal equilibrium
was attained. A measured amount of BAB solution, also thermo-
stated at the same temperature, was added rapidly to the above
mentioned mixture to initiate the reaction. The mixture was peri-
odically shaken to ensure uniform concentration, and the progress
of the reaction was monitored by an iodometric determination of
unreacted BAB in a measured aliquot of the reaction mixture at
different time intervals. The reaction was followed for more than
two half-lives. The pseudo-first order rate constants (kꢀ), calculat-
ed from linear plots of log [BAB] versus time, were reproducible
within ± 3%.
The kinetics of the oxidation of dipeptides (DPP) by BAB
was investigated at several initial concentrations of the reac-
tants in a HClO medium. The same oxidation behavior was
4
observed for all five dipeptides.
Under pseudo-first order conditions of [substrate] ꢀ [oxi-
4
dant] at constant [DPP], [HClO ] and temperature, plots of log
[
BAB] versus time were linear (r > 0.9926) indicating a first-
order dependence of the reaction rate on [BAB]. The rate con-
stant (kꢀ) was not affected by a change in [BAB], confirming
the first-order dependence of the rate on [BAB] (Table 1). The
values of kꢀ increased with an increase in [DPP] (Table 1).
Plots of log kꢀ versus log [DPP] are linear (r > 0.9922) with
fractional slopes (0.42–0.58), indicating a fractional order de-
pendence on [DPP]. Furthermore, plots of kꢀ versus [DPP] are
linear (r > 0.9812) with a y-intercept, confirming the fraction-
al order dependence on [DPP]. The order in [DPP] becomes
Stoichiometry and Product Analysis. Varying ratios of
−
3
(
[oxidant] ꢀ [substrate]) in the presence of 0.04 mol dm HClO
4
were equilibrated at 40 °C for 48 h. An iodometric determination
of the unconsumed BAB indicated that two moles of oxidant were
needed per mole of dipeptide to give the corresponding aldehyde.
The observed stoichiometry is
−
3
zero as [DPP] is increases further (> 0.15 mol dm ), obeying
Table 1. Effect of Variation of Reactant Concentrations on the Rate of Reaction, [HClO
4
] =
−
2
−3
−3
4
.0 × 10 mol dm ; I = 0.5 mol dm ; temp = 313 K
4
2
4
−1
1
0 [BAB]
10 [DPP]
10 kꢀ/s
Leu-Leu Val-Val Ala-Ala Phg-Phg
mol dm−
3
mol dm−3
4.0
Gly-Gly
2.50
2.60
2.55
2.68
2.52
2.64
0.92
1.40
1.90
2.60
3.68
4.35
4.40
4.38
4.30
4.45
2
4
6
8
.0
.0
.0
.0
7.75
7.70
7.60
7.80
7.65
7.84
2.84
3.96
5.50
7.70
9.45
9.50
9.48
9.52
9.56
9.50
2.92
4.40
6.55
9.50
11.9
11.8
11.9
11.7
11.9
11.8
4.10
5.92
7.55
13.3
13.4
13.2
13.3
13.4
13.2
5.45
7.50
10.3
13.4
16.8
20.2
20.3
20.2
20.4
20.3
4.0
4.0
4.0
4.0
4.0
0.5
1.0
2.0
1
1
0.0
2.0
4
4
4
4
4
4
4
4
4
4
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
4.0
8.0
11.8
10.5
14.1
18.0
23.1
23.0
23.2
23.1
23.2
12.0
15.0
20.0
25.0
30.0
12.4
12.5
12.6
12.5
12.4
18.2
18.1
18.2
18.1
18.3

Puttaswamy et al.
Bull. Chem. Soc. Jpn., 76, No. 1 (2003)
Table 2. Effect of Variation of Acid Concentration on the Rate of Reaction, [BAB] = 4.0 ×
75
−
4
−3
−2
−3
−3
1
0
mol dm ; [DPP] = 4.0 × 10 mol dm ; I = 0.5 mol dm ; temp = 313 K
2
4
−1
1
0 [HClO
4
]
10 kꢀ/s
mol dm−
3
Gly-Gly
2.50
2.55
2.62
2.68
2.60
2.22
1.94
1.55
1.20
Leu-Leu
7.75
7.58
7.65
7.55
7.70
6.82
5.90
5.08
3.85
Val-Val
9.42
9.55
9.50
9.58
9.50
8.10
7.25
6.10
4.54
Ala-Ala
11.9
11.8
11.8
11.7
11.8
10.1
8.86
7.30
5.42
Phg-Phg
13.2
13.3
13.4
13.2
13.4
11.5
10.4
8.70
6.60
0
1
2
3
4
5
6
8
.5
.0
.0
.0
.0
.0
.0
.0
1
2.0
Table 3. Proton Inventory Studies for the Oxidation of Dipeptides in H
2
O–D
2
O Mixtures,
−
4
−3
−2
−3
−2
[
BAB] = 4.0 × 10 mol dm ; [DPP] = 4.0 × 10 mol dm ; [HClO ] = 4.0 × 10
4
−
3
−3
mol dm ; I = 0.5 mol dm ; temp = 313 K
4
10 kꢀ
n
/s−1
Atom fraction of
deuterium (n)
Gly-Gly
2.60
2.25
1.90
1.75
Leu-Leu
7.70
6.65
5.88
5.40
Val-Val
9.50
8.40
7.10
6.31
Ala-Ala
11.8
10.1
Phg-Phg
13.4
11.8
10.6
9.89
0
0
0
0
0
.000
.248
.496
.744
.992
9.80
7.92
7.10
1.55
5.08
5.80
9.10
When the ionic strength of the solution was varied by add-
−
3
ing NaClO
4
(0.1–0.8 mol dm ), the rate was unaffected. The
NH
2.0 × 10 –2.0 × 10 mol dm ) and Cl or Br ions in the
addition of reaction product, benzenesulfonamide (PhSO
2
2
−
4
−3
−3
−
−
;
−
4
−3
−3
form of NaCl or NaBr (5.0 × 10 –5.0 × 10 mol dm ) had
a negligible effect on the rate. The rate was not significantly
altered on varying the dielectric constant of the medium by
adding methanol (5–30% v/v). Studies of the reaction rate in a
2
D O medium for Gly-Gly, Leu-Leu, Val-Val, Ala-Ala, and
−
4
−1
2
Phg-Phg, revealed that while kꢀ(H O) = 2.60 × 10 s , 7.70
−
4
−1
−4 −1
−4 −1
−4 −1
×
10 s , 9.50 × 10 s , 11.8 × 10 s , 13.4 × 10 s ,
−
4
−1
−4 −1
−4
2
and kꢀ(D O) = 1.55 × 10 s , 5.08 × 10 s , 5.80 × 10
−
1
−4 −1
−4 −1
s , 7.10 × 10 s , 9.10 × 10 s , respectively. The sol-
vent isotope effect, kꢀ(H O)/kꢀ(D O) = 1.68, 1.51, 1.64, 1.66,
and 1.47 for the five dipeptides. Proton inventory studies were
made in H O–D O mixtures for all five dipeptides; the results
are given in Table 3. Corresponding proton inventory plots for
the rate constant (kꢀ ) in a solvent mixture of deuterium atoms
2
2
2
2
n
fraction (n) are given in Fig. 2.
The reaction was studied at different temperatures (303–323
K). From Arrhenius plots of log kꢀ versus 1/T (r > 0.9855),
composite activation parameters were calculated. These re-
sults are summarized in Table 4. The addition of the reaction
mixture to acrylamide in a nitrogen atmosphere failed to ini-
tiate polymerization of the latter, showing the absence of free-
radical species.
Fig. 1. Plots of log kꢀ versus log [HClO
conditions are as in Table 2.
4
]. Experimental
Michaelis–Menten kinetics (Table 1).
At constant [BAB] and [DPP], the rate was not affected at
+
−3
−2
−3
low [H ] (5.0 × 10 –4.0 × 10 mol dm ), but retardation
+
−2
−2
−3
set in at [H ] = 4.0 × 10 –12.0 × 10 mol dm (Table 2).
Discussion
+
Under the latter conditions, plots of log kꢀ versus log [H ] (Fig.
1
) were linear (r > 0.9968) with negative fractional slopes
Because organic haloamines have similar chemical proper-
+
−2
21–23
(
0.64–0.68). In the low acid range ([H ] = 1.0 × 10 mol
ties,
it is expected that similar equilibria exist in solutions
−
3
dm ), the variation of [DPP] had no effect on the rate (values
are not reported).
of these compounds. Bromamine-B, which is analogous to
chloramine-T and chloramine-B, behaves like a strong electro-

7
6
Bull. Chem. Soc. Jpn., 76, No. 1 (2003)
Kinetics of Oxidation of Dipeptides by Bromamine-B
Table 4. Temperature Dependence and Activation Parameters for the Oxidation of Dipep-
−
4
−3
−2
tides by BAB in HClO
4
Medium, [BAB] = 4.0 × 10 mol dm ; [DPP] = 4.0 × 10
mol dm− ; [HClO
3
] = 4.0 × 10 mol dm ; I = 0.5 mol dm
−2
−3
−3
4
4
−1
Temperature
K
10 kꢀ/s
Gly-Gly
0.82
Leu-Leu
2.51
4.30
7.70
13.4
24.2
86.2
83.6
−38.1
95.5
Val-Val
3.25
Ala-Ala
4.85
Phg-Phg
5.50
3
3
3
3
3
E
∆
∆
∆
03
08
13
18
23
1.41
2.60
4.62
8.30
5.33
9.50
7.25
8.72
11.8
13.4
16.1
26.8
82.1
79.5
−49.4
94.9
19.0
18.9
27.8
27.9
/kJ mol−
1
94.7
92.1
−19.8
98.3
71.1
64.8
a
−
1
H /kJ mol
68.5
62.2
−
1
−1
S /J K mol
−82.6
94.4
−102
94.1
G /kJ mol−
1
Kꢀ
h
PhSO
Kꢀ
2
NHBr + H
2
O
2 2
PhSO NH + HOBr,
= 1.5 × 10− at 25 °C
5
(6)
h
K
a
H+ + OBr , K
−
= 2.0 × 10−9 at 25 °C
HOBr
a
(
7)
(8)
Therefore, the probable oxidizing species in an acid solution of
HOBr + H+
H
2
OBr+
+
BAB are PhSO
experimental rate law for the oxidation of dipeptides by BAB
in HClO at 40 °C is found to be
2 2 2 2
NHBr, PhSO NBr , HOBr, and H OBr . The
4
x
+ −y
Rate = k[BAB] [DPP] [H ] ,
(9)
+
where x = y = 0 in lower [H ] ranges and x and y are fraction-
+
al at higher [H ]. The fractional order in [DPP] tends to be-
−
3
come zero at [DPP] > 0.15 mol dm .
The first-order dependence of the rate on [BAB] and the ad-
dition of PhSO
cate that PhSO
cies ((4) and (6)); further these species were present in very
low concentrations under the experimental conditions em-
ployed. The absence of any ionic- strength effect indicates the
involvement of a neutral species in the rate-limiting step
2
NH
2
having no effect on the reaction rate indi-
2
NBr
2
and HOBr may not be the reactive spe-
Fig. 2. Proton inventory plots for the oxidation of dipeptides
by BAB in H O–D O mixtures. Experimental conditions
are as in Table 3.
2
2
2
1
lyte in both acidic and alkaline media. In general, BAB under-
goes a two-electron change in its reactions. The oxidation po-
tential of BAB/PhSO NH is pH dependent and decreases with
2 2
an increase in the pH of the medium (1.4 V at pH 0.65 and 0.50
V at pH 12.0). In acid solutions, BAB exists in the following
equilibria:
(
r.l.s.). Hence, the effective oxidizing species could be the
conjugate acid, PhSO NHBr.
2
2
4
Glycylglycine exists in the following equilibrium in acidic
solutions:
+
+
NH CH CONHCH COO + H+
−
NH CH CONHCH COOH
3
2
2
3 2 2
cation (SH+)
zwitterion (S°)
PhSO
2
NBrNa
PhSO
2
NBr− + Na+
(10)
(2)
The same equilibria also exist for other dipeptides. The in-
verse dependence of the reaction on the acidity suggests that
there is an equilibrium between the zwitterion (S°) and the pro-
tonated form (SH ), and that it is the zwitterion which takes
part in the reaction.
K
a
PhSO
= 1.5 × 10−5 at 25 °C
2
NBr− + H+
PhSO
2
NHBr,
+
(3)
K
a
K
d
Based on the preceding discussion and experimental obser-
vations, Scheme 1 is proposed to explain the reaction mecha-
2
PhSO
2
NHBr
PhSO
2
NH
2
2 2
+ PhSO NBr ,
K
d
= 0.113 at 25 °C
(4)
(5)
nism for the oxidation of dipeptides by BAB in a HClO medi-
4
um:
K
h
PhSO
2
NBr
2
+ H
2
O
PhSO
2
NHBr + HOBr
Here, X and Xꢀ represent the complex intermediate species,

Puttaswamy et al.
Bull. Chem. Soc. Jpn., 76, No. 1 (2003)
77
dm− HCl and high temperature) are needed to effect the
3
K
1
SH+
S° + H
+
25
26
(i) fast
hydrolysis of a dipeptide bond. Martin has studied the
mechanism of the hydrolysis of glycylglycine in aqueous ace-
tic acid–perchloric acid mixtures at 120 °C. Synge has used
equal volumes of acetic acid and 10 mol dm HCl to effect
hydrolysis. Hence, dipeptides are oxidized rather than hydro-
lyzed under the conditions maintained in the present study.
K
2
S° + PhSO
2
NHBr
X
(ii) fast
2
7
k
3
−3
X
Xꢀ
(iii) slow and r.l.s
(iv) fast
k
4
Xꢀ + PhSO NHBr
2
Products
If [BAB]
t
is the total concentration of BAB, then
NHBr ] + [X], from which
Scheme 1.
[
BAB] = [PhSO
t
2
K
1
+
K
2
[BAB]
[SH+]
H ] + K
The rate of the reaction is given by
t
[
X] =
.
(11)
(12)
1 2
K
[SH+]
[
2 3 t
k [BAB]
[SH+]
−
d[BAB]
K
1
K
+
=
k
3
[X] =
.
dt
[H ] + K
1
K
2
[SH+]
Equation 12 is in the agreement with the experimental results
indicating a first-order dependence of the rate on [BAB]. Frac-
+
tional and inverse fractional orders in [DPP] and [H ], respec-
+
+
1 2
tively, are observed. At higher [DPP], [H ] < K K [SH ], so
that the reaction becomes independent of [DPP], as has been
observed experimentally (Table 1). In the limiting case at low
+
+
[
H ], the reaction is independent of [H ] and [DPP], as in evi-
dent from Eq. 12.
If kꢀ is the observed rate constant, Eq. 12 can be rearranged as
+
1
[H ]
1
=
1 2 3 3
K K k k
[SH+] +
.
(13)
kꢀ
+
Double reciprocal plots of 1/kꢀ versus 1/[SH ] (or 1/[DPP])
were linear (r > 0.9804), and from the intercept and slope of
3 1 2
these plots the values of k and K K were calculated using Eq.
1
3 for each dipeptide at 313 K. Because the rate was fractional
2
8
order in [DPP], Michaelis–Menten kinetics were adopted to
study the effect of [DPP] on the rate at different temperatures
(
3
303–323 K). The decomposition constant (k ) for the rate-
limiting step, and the K values were calculated using Eq. 13
1
K
2
for each dipeptide at different temperatures. The activation pa-
rameters for the rate-limiting step (step (iii) in Scheme 1) were
Scheme 2.
3
evaluated using Arrhenius plots (r > 0.9918) of log k versus
whose structures are shown in Scheme 2, which depict the
probable mode of oxidation of dipeptides by BAB in HClO
medium. Dipeptides are not expected to undergo hydrolysis
1/T. These data are presented in Tables 5 and 6.
The proposed mechanism is supported by the following ex-
perimental facts:
4
before oxidation, since drastic reaction conditions (> 2 mol
The effect of varying the solvent composition on the reac-
Table 5. Values of Decomposition Constant (k
3
) at Different Temperatures and Activation
−
4
−3
Parameters for the Rate-Limiting Step, [BAB] = 4.0 × 10 mol dm ; [HClO
4
] = 4.0
−
2
−3
−3
×
10 mol dm ; I = 0.5 mol dm
4
10 k
3
/s−1
Temperature
K
Gly-Gly
Leu-Leu
4.45
7.30
11.8
Val-Val
5.00
9.09
14.3
Ala-Ala
7.69
12.5
22.2
30.3
Phg-Phg
14.0
19.5
28.7
39.6
3
3
3
3
3
E
∆
∆
∆
03
1.54
2.50
4.35
6.66
08
13
18
23
17.7
20.0
11.1
76.6
74.0
−73.6
97.0
28.2
79.4
76.7
−56.4
94.4
38.4
73.2
70.6
−74.5
93.9
45.5
69.5
66.9
−83.4
93.0
52.0
52.9
50.2
/kJ mol−
1
a
−
1
H /kJ mol
−
1
−1
S /J K mol
−134
92.1
G /kJ mol−
1

7
8
Bull. Chem. Soc. Jpn., 76, No. 1 (2003)
Table 6. Values of Product of Equilibrium Constant (K
Kinetics of Oxidation of Dipeptides by Bromamine-B
1
K
2
) at Different Temperatures, [BAB]
−
4
−3
−2
−3
−3
=
4
4.0 × 10 mol dm ; [HClO ] = 4.0 × 10 mol dm ; I = 0.5 mol dm
Temperature
K
1 2
K K
Gly-Gly
1.6
1.6
2.0
2.0
Leu-Leu
1.9
1.7
2.1
2.0
Val-Val
1.7
1.6
2.0
2.0
Ala-Ala
2.0
1.6
1.7
1.9
Phg-Phg
1.7
1.8
1.6
1.9
3
3
3
3
3
03
08
13
18
23
2.0
1.9
1.7
1.8
2.0
3
4
tion kinetics has been described in detail in well-known mono-
graphs on chemical kinetics. For the limiting case of zero-an-
gle of approach between two dipoles or an ion-dipole system,
comparison with the standard curves indicate the involve-
ment of a single proton or H–D exchange in the reaction se-
quence. Hence, the participation of the H ion in the forma-
tion of transition state is inferred.
+
29
Amis has shown that a plot of log kꢀ versus 1/D gives a
straight line with a negative slope for a reaction between a neg-
ative ion and a dipole, or between two dipoles, while a positive
slope results for a positive ion-dipole interaction. The total ab-
sence of the effect of a varying dielectric constant on the rate in
the present cases cannot be explained by the Amis theory. Ap-
From an inspection of the rate data, the rate of oxidation of
dipeptides follows the order Phg-Phg > Ala-Ala > Val-Val >
Leu-Leu > Gly-Gly. In the case of Phg-Phg, the formation of
the Xꢀ intermediate by ready decarboxylation is favoured due
to resonance stabilization and extended conjugation. In the
case of Ala-Ala, Val-Val and Leu-Leu, the presence of alkyl
groups due to an inductive effect (+I) makes Ala-Ala, Val-Val
and Leu-Leu react more readily with the oxidant. However, a
steric interaction between the alkyl chain and the oxidant
greatly affects the rate of decarboxylation; thus, the order has
been found to be Ala-Ala > Val-Val > Leu-Leu. In the case of
Gly-Gly, because the +I and resonance effects are at a mini-
mum, it is found to be least reactive among of all the dipep-
tides used, and the overall reactivity of these dipeptides has
been found to be Phg-Phg > Ala-Ala > Val-Val > Leu-Leu >
Gly-Gly.
It can be seen from Table 4 that the activation energy is
highest for the slowest reaction, and vice versa, indicating that
the reaction is enthalpy-controlled. Further, the values of ∆H
and ∆S can be correlated linearly (r = 0.9912), resulting in an
isokinetic relationship. From the slope, the value of the isoki-
netic temperature (β) is 360 K, which is much higher than the
experimental temperature, 313 K. The relationship was proved
3
0
plying the Born equation, Laidler has proposed the following
equation for a dipole–dipole interaction:
2
2
2
3
µ

A
3
µ
r
B
µ
r
#

Inkꢀ = Ink
o
+
(2/ D − 1)
+
−
.
3
3
 (14)
8
kT
 r
A
B
#

Here, k
o
is the rate constant in a medium of infinite dielectric
constant and µ and r refer to the dipole moment and radii of re-
actants A and B and the activated complex. It can be seen from
Eq. 14 that the rates should be higher in a medium of lower di-
3
3
3
electric constant when r
≠ A B
> r + r , indicating that the ex-
tent of charge dispersal in the transition state is different. On
3
3
3
the other hand, r
≠
ꢁ r
A
+ r
B
implies the absence of a dielec-
tric effect of the solvent on rate, as was observed in the present
investigations.
The proposed mechanism is supported by a decrease in rate
in D
2
O medium. For acid-catalyzed reactions involving a fast
O)/kꢀ(H O) > 1, since
3 3
D O is a stronger acid than H O . The reverse holds for re-
pre-equilibrium proton transfer, kꢀ(D
2
2
+
31
+
+
35
actions involving retardation by the H ion. Hence, the pro-
to be genuine through the Exner criterion by plotting log
posed mechanism is also supported by the decrease in rate in
kꢀ(313 K) versus log kꢀ(323 K),which gives a straight line (r =
0.9950). The value of β was calculated from the equation β =
T (1–q)/(T /T )−q, where q is the slope of the Exner plot; β
+
D
2
O medium, indicating retardation by H . Proton inventory
studies in H
of the transition state. The dependence of the rate constant
kꢀ ) on n, the atom fraction of deuterium in a solvent mixture
of D
equation, as in
2 2
O–D O mixtures could throw light on the nature
1
1
2
was found to be 366 K, thus proving that the reaction is en-
thalpy-controlled. The moderate energy of activation supports
the proposed mechanism. The negative ∆S values point to-
wards the formation of a more ordered activated complex, and
the near constancy of ∆G shows the operation of similar
mechanisms for the oxidation of these dipeptides by BAB.
The zero effect of halide ions on the rate indicates that no
interhalogen or free bromine is formed. The variation of the
ionic strength of the medium does not alter the rate, indicating
that a neutral species is involved in the rate-limiting step. The
reduction product of the oxidant, benzenesulfonamide, does
not influence the rate, showing that it is not involved in any
pre-equilibrium. These observations are also in conformity
with the proposed mechanism.
(
n
3
2,33
2
O and H
2
O, is given
by a form of the Gross–Butler
TS
π(1 − n + nØ
RS
i
)
)
(15)
kꢀ
o
/ kꢀ
n
=
,
π(1 − n + nØ
j
i j
where Ø and Ø are the isotope fractionation factors for isoto-
pically exchangeable hydrogen sites in the transition state (TS)
and in the reactant state (RS), respectively. Equation 15 allows
one to calculate the fractionation factor of TS if the reactant
fractionation factors are known. However, the curvature of the
proton inventory plot could reflect the number of exchangeable
It was thought worthwhile to compare the rate of oxidation
of these dipeptides with the oxidation of their monomers
(phenylglycine, alanine, valine, leucine, and glycine) under
3
2
protons in the reaction. Plots of kꢀ
n
versus n (Fig. 2) for all
five dipeptides in the present case are curves. These in

Puttaswamy et al.
Bull. Chem. Soc. Jpn., 76, No. 1 (2003)
Table 7. Temperature Dependence and Activation Parameters for the Oxidation of Amino
79
−
4
−3
4
Acids by BAB in HClO medium, [BAB] = 4.0 × 10 mol dm ; [Amino Acid] = 4.0
−
2
−3
−2
−3
−3
×
10 mol dm ; [HClO
4
] = 4.0 × 10 mol dm ; I = 0.5 mol dm
3
−1
Temperature
K
10 kꢀ/s
Valine
Glycine
1.03
Leucine
3.70
5.30
8.25
12.2
18.1
65.7
63.0
−83.9
89.3
Alanine
Phenylglycine
8.82
3
3
3
3
3
E
∆
∆
∆
03
08
13
18
23
4.65
7.35
10.5
15.9
21.6
61.3
58.7
−95.8
88.7
6.05
8.25
1.62
2.50
3.95
6.45
11.1
12.0
15.7
16.1
23.9
53.8
51.8
19.4
23.8
41.8
39.2
/kJ mol−
1
74.1
71.5
−66.9
92.5
a
−
1
H /kJ mol
−
1
−1
S /J K mol
−112
86.1
−155
87.6
G /kJ mol−
1
identical experimental conditions. The rate of oxidation of
amino acids increased in the order phenylglycine > alanine >
valine > leucine > glycine (Table 7), while in the case of
dipeptides the order was found to be Phg-Phg > Ala-Ala >
Val-Val > Leu-Leu > Gly-Gly. It was also found that the ami-
no acids are nearly 10-times faster than the dipeptides. The
change in each case can be ascribed to the increased distance
between the functional groups and consequently weaker elec-
trostatic effects. In the case of dipeptides, a lone pair on nitro-
gen [Scheme 2, (X)] is involved in resonance with the carbonyl
group. Therefore, its nucleophilic character will decrease and,
hence, the rate decreases, where as in the case of monomer
amino acids there is no decrease in the nucleophilic character
and, hence, the rate is much faster compared to dipeptides.
The decrease in the rate for dipeptides compared to their
monomers may also be due to decreased ionization of the
Chem. Kinet., 33, 480 (2001).
B. Thimme Gowda and D. S. Mahadevappa, J. Chem. Soc.,
Perkin Trans. 2, 1983, 323, and references therein.
D. S. Mahadevappa, Puttaswamy, and S. Ananda, Indian J.
Chem., Sect. A, 26, 33 (1987).
Puttaswamy and Nirmala Vaz, Proc. Indian Acad. Sci.,
Chem. Sci., 113, 325 (2001).
D. K. Bhat, B. S. Sherigara, and B. Thimme Gowda, Bull.
Chem. Soc. Jpn., 69, 41 (1996).
T. A. Iyengar and D. S. Mahadevappa, Proc. Indian Acad.
Sci., Chem. Sci., 105, 63 (1993).
S. Akabori, K. Narita, K. Toki, and H. Hanafusa, Nippon
Kagaku Zasshi (J. Chem. Soc. Jpn., Pure Chem. Sec.,) 75, 782
8
9
1
0
1
1
1
2
1
3
(
1954).
E. F. Hammel and S. Glasstone, J. Am. Chem. Soc., 76,
741 (1954).
1
4
3
1
5
M. S. Ahmed and D. S. Mahadevappa, Talanta, 27, 669
COOH group, which is evidenced by an increase in the pK
values (e.g. glycine pK = 2.4; Gly-Gly pK = 3.4). Since the
1
(
1980).
1
1
1
6
M. Bodanszky and A. Bodanszky, “The Practice of Peptide
availability of the COOH group for this reaction is trivial to de-
termine the rate, the rate concurrently decreases due to a lower
concentration of the acidity, resulting in a decrease of the dis-
sociation ability of the COOH group and, hence, a decrease in
the rate of the reaction in the case of dipeptides. The same ar-
gument also holds good for other four dipeptides.
Synthesis,” Springer-Verlag, New York (1984), pp. 145–150.
17 G. Akerloff, J. Am. Chem. Soc., 54, 4125 (1932).
18 A. I. Vogel “Text Book of Practical Organic Chemistry,”
5th ed, ELBS and Longman, London (1989), p. 1332.
19 Puttaswamy and D. S. Mahadevappa, J. Phys. Org. Chem.,
2, 660 (1989).
2
0
A. I. Vogel, “Quantitative Inorganic Analysis,” 4th ed,
ELBS and Longman, London (1978), p. 313.
One of the authors (NV) thanks the Principal and the Man-
agement of Jyoti Nivas College, Bangalore for encouragement
and also the UGC-New Delhi for a Fellowship.
2
2
1
2
E. Bishop and V.J. Jennings, Talanta, 1, 197 (1958).
J. C. Morris, J. A. Salazar, and M. A. Wineman, J. Am.
Chem. Soc., 70, 2036 (1948).
F. F. Hardy and J. P. Johnston, J. Chem. Soc., Perkin Trans.
2
3
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2
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3
(