Oxidative study of gabapentin by alkaline hexacyanoferrate(III) in room temperature in presence of catalytic amount of Ru(III) - a mechanistic approach

Article abstract of DOI:10.1016/j.molstruc.2008.05.006

The kinetics of oxidation of gabapentin by hexacyanoferrate(III) in aqueous alkaline medium at a constant ionic strength of 0.5 mol dm^-3 was studied spectrophotometrically. The reaction is of first order in [HCF(III)] and of less than unit order in [alkali]. The reaction rate is independent upon [gabapentin]. Effects of added products, ionic strength and dielectric constant of the reaction medium have been investigated. Oxidative product of gabapentin was identified. A suitable mechanism has been proposed. The reaction constants involved in the different steps of mechanism are calculated. The activation parameters of the mechanism are computed and discussed.

Full text of DOI:10.1016/j.molstruc.2008.05.006

Journal of Molecular Structure 892 (2008) 121–124
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Oxidative study of gabapentin by alkaline hexacyanoferrate(III) in room
temperature in presence of catalytic amount of Ru(III) – a mechanistic approach
*
Timy P. Jose, Mahantesh A. Angadi, Manjalee S. Salunke, Suresh M. Tuwar
Department of Chemistry, Karnatak Science College, Dharwad 580 001, Karnataka, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The kinetics of oxidation of gabapentin by hexacyanoferrate(III) in aqueous alkaline medium at a constant
ionic strength of 0.5 mol dmꢀ3 was studied spectrophotometrically. The reaction is of first order in
[HCF(III)] and of less than unit order in [alkali]. The reaction rate is independent upon [gabapentin].
Effects of added products, ionic strength and dielectric constant of the reaction medium have been inves-
tigated. Oxidative product of gabapentin was identified. A suitable mechanism has been proposed. The
reaction constants involved in the different steps of mechanism are calculated. The activation parameters
of the mechanism are computed and discussed.
Received 3 December 2007
Received in revised form 5 May 2008
Accepted 5 May 2008
Available online 11 May 2008
Keywords:
Kinetics
Oxidation
Gabapentin
Mechanism
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
combination with other medications for the prevention of seizure
in people suffering from seizure disorders. It is sometimes pre-
Hexacyanoferrate(III) is a weak oxidant; has been widely used
to oxidize numerous organic and inorganic compounds in alkaline
media. The authors [1,2] have suggested that alkaline hexacyano-
ferrate(III) ([Fe(CN)6]3ꢀ) ion simply acts as an electron-abstracting
reagent in redox reactions. However, Speakman and Water [3]
have suggested different paths of oxidation of aldehydes, ketones
and nitroparaffines. Singh and co-workers [4,5], while discussing
the oxidations of formaldehydes, acetones and ethyl methyl ketone
have suggested that the oxidation takes place via an electron trans-
fer process resulting in the formation of a free radical intermediate.
The polarizable cyano ligands around the central atom in ferricya-
nide facilitate electron transfer from electron rich substrates like
gabapentin to the iron.
1-(aminomethyl)cyclohexaneaceticacid, commercially called
gabapentin (GAP), an amino acid is an anticonvulsant that is chem-
ically unrelated to any other anticonvulsant [6] or mood regulating
medication. It is also widely used to treat individuals suffering
from many kinds of pain problems, tremors, restless legs syn-
drome, hot flashes associated with menopause, and various psychi-
atric disorders. Gabapentin seems to be effective in some people
with bipolar mood disorders that have not responded to lithium
or other mood-stabilizers [7].
scribed for the management of neuralgia (nerve pain). Its oxidative
study is reported [8,9] using DPC and DPN where in the results
were entirely different. The oxidation of many organic compounds
using coordination compounds is also well documented [10,11].
Ruthenium(III) is known to be an efficient catalyst in several
redox reactions particularly in alkaline medium [12,13]. The mech-
anism of catalysis can be quite complicated due to the formation of
different intermediate complexes, free radicals and different oxida-
tion states of ruthenium. A micro amount of ruthenium(III) is
sufficient to catalyze the reaction in alkaline medium and a variety
of mechanisms are possible. Thus, to explore the mechanism of
oxidation by hexacyanoferrate(III) in aqueous alkaline medium
and to check the selectivity of gabapentin towards hexacyanofer-
rate(III) in catalyzed system, we have selected ruthenium(III) as a
catalyst. Hence, we have undertaken a detailed study of the title
reaction is to arrive at a plausible mechanism.
2. Experimental
2.1. Materials and reagents
The study of neuroleptic drugs becomes important because of
their biological significance and selectivity towards the oxidant
to yield different products. Gabapentin is prescribed – usually in
Reagent grade chemicals and double distilled H2O were used
throughout this work. The stock solution of gabapentin (s.d. fine-
chem.) was prepared by dissolving a known amount of the sample
in distilled water. A solution of [Fe(CN)6]3ꢀ was prepared by dis-
solving K3[Fe(CN)6] (BDH) in H2O and standardized [14a] by adding
10% KI solution in presence of zinc sulphate and the iodine
* Corresponding author.
E-mail address: sureshtuwar@yahoo.co.in (S.M. Tuwar).
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.05.006

122
T.P. Jose et al. / Journal of Molecular Structure 892 (2008) 121–124
liberated was titrated with standard sodium thiosulphate solution.
The product, [Fe(CN)6]4ꢀ solution was prepared by dissolving pure
sample of K4[(Fe(CN)6] and its concentration was checked [14b]
volumetrically by titrating against standard ceric ammonium sul-
phate solution in sulphuric acid medium; N-phenylantranilic acid
was used as an indicator. Sodium hydroxide and sodium perchlo-
rate (BDH, AnalaR) were employed to maintain the required alka-
linity and ionic strength respectively in reaction solutions. The
ruthenium(III) solution was prepared by dissolving a known
weight of RuCl3 (s.d. fine-chem.) in 0.20 mol dmꢀ3 HCl. Mercury
was added to the ruthenium(III) solution to reduce any ruthe-
nium(IV) formed during the preparation of the ruthenium(III) stock
solution and was used after a day. The ruthenium(III) concentra-
tion was assayed [15] by EDTA titration.
3.2. Reaction orders
The order with respect to [GAP], [Ru(III)] and [alkali] were
found from the graph of log kobs versus log(concentration) plots.
The concentration of hexacyanoferrate(III) was varied in the
range, 8.0 ꢁ 10ꢀ5 to 8.0 ꢁ 10ꢀ4 mol dmꢀ3 at constant [GAP],
[Ru(III)], [OHꢀ] and ionic strength. The non-variation in pseudo-
first order rate constants at various concentrations of hexacyano-
ferrate(III) indicates, the order in [Fe(CN)6]3ꢀ as unity. The linearity
in the plot of log(concentration) versus time also supports the or-
der in [Fe(CN)6]3ꢀ (Table 1).
The substrate, gabapentin was varied in the range of 1.0 ꢁ 10ꢀ3
to 1.0 ꢁ 10ꢀ2 mol dmꢀ3 at 25 °C while keeping all other reactants
concentrations and conditions constant. The order of reaction
was found to be independent on [GAP]. Hence, its order is zero
(Table 1).
2.2. Kinetics measurements
The effect of [alkali] on the rate of reaction was studied at con-
stant concentrations of gabapentin, ruthenium(III) and hexacyano-
ferrate(III) at a constant ionic strength of 0.50 mol dmꢀ3 (Table 2).
The rate constants were increased with the increase in [alkali] and
found that an apparent less than unit orders dependence on [alkali]
(ꢂ0.6).
The oxidation of gabapentin by hexacyanoferrate(III) was fol-
lowed under pseudo-first order conditions where [GAP] was in
excess over [Fe(CN)6]3ꢀ at 25 0.1 °C unless and other wise sta-
ted. The reaction was initiated by mixing the required quantities
of previously thermostatted solution of [Fe(CN)6]3ꢀ and gabapen-
tin which also contained known quantities of ruthenium(III), so-
dium hydroxide and sodium perchlorate used to maintain the
required concentrations of catalyst, alkalinity and ionic strength
respectively. Progress of the reaction was followed by measuring
the absorbance of unreacted hexacyanoferrate(III) spectrophoto-
The catalyst, ruthenium(III), was varied in the concentration
range of 5.0 ꢁ 10ꢀ7 to 5.0 ꢁ 10ꢀ6 mol dmꢀ3 at constant [GAP],
[Fe(CN)6]3ꢀ and [OHꢀ] at ionic strength of 0.50 mol dmꢀ3 (Table 2).
The order with respect to [Ru(III)] was found to be unity.
metrically at 420 nm (e
= 1060 dm3 molꢀ1 cmꢀ3); other constitu-
ents of the reaction mixture do not absorb significantly at this
wavelength. Application of Beer‘s law under the reaction condi-
tions had been verified earlier between 1.0 ꢁ 10ꢀ4 and
Table 1
Effect of variation of [Fe(CN)6]3ꢀ and [gabapentin] on ruthenium(III) catalyzed
oxidation of gabapentin by hexacyanoferrate(III) in aqueous alkaline medium at 25 °C
e
1.0 ꢁ 10ꢀ3 mol dmꢀ3 of [Fe(CN)63ꢀ] ( = 1060 20 dm3 molꢀ1 cmꢀ1).
[Fe(CN)6]3ꢀ ꢁ 104 (mol dmꢀ3
)
[GAP] ꢁ 103 (mol dmꢀ3
)
kobs ꢁ 103 (sꢀ1
)
The first order rate constants, (kobs) were obtained from the plots
of log [Fe(CN)63ꢀ] versus time. The rate constants were reproduc-
ible within 5%.
0.8
1.0
2.0
4.0
6.0
8.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
1.0
2.0
4.0
6.0
8.0
10
2.89
2.84
2.88
2.90
2.81
2.84
2.65
2.89
2.90
2.85
2.70
2.78
3. Results and discussions
3.1. Stoichiometry and product analysis
Different sets of reaction mixtures with different concentrations
of reactants were kept for 6 h at 25 0.1 °C under nitrogen atmo-
sphere in a closed vessel. The remaining [Fe(CN)6]3ꢀ was assayed
spectrophotometrically by measuring the absorbance at 420 nm.
The product [Fe(CN)6]4ꢀ was analyzed by titrating against Ce(IV)
solution [14b]. The results indicated that four moles of hexacyano-
ferrate(III) were reacted with one mole of gabapentin as in equa-
tion (1)
[OHꢀ] = 0.10; [Ru(III)] = 1.0 ꢁ 10ꢀ6; I = 0.50/mol dmꢀ3
.
Table 2
Effect of variation of [Ru(III)] and [OHꢀ] on ruthenium(III) catalyzed oxidation of
gabapentin by hexacyanoferrate(III) in aqueous alkaline medium at 25 °C
HOOC
CH2COOH
CH2COOH
4Fe(CN) ] +
[Ru(III)] ꢁ 106 (mol dmꢀ3
)
[OHꢀ] (mol dmꢀ3
)
kobs ꢁ 103 (sꢀ1
)
H2NH2C
Found
Calculated*
4Fe(CN) ]4 -
4OH-
3-
+
+
6
+
6
0.5
0.8
1.0
2.0
4.0
5.0
1.0
1.0
1.0
1.0
1.0
1.0
0.10
0.10
0.10
0.10
0.10
0.10
0.05
0.08
0.10
0.20
0.40
0.50
1.76
2.51
2.89
5.90
11.9
15.0
1.87
2.56
2.89
4.21
5.84
6.20
1.50
2.38
2.97
5.95
11.90
14.90
1.83
2.57
2.97
4.20
5.61
6.00
NH3
+
2H2O
ð1Þ
The main oxidative products were submitted to spot tests. The
main reaction product was identified as 1-(carboxymethyl) cyclo-
hexanecarboxylic acid by spot test [16] for free dicarboxyl groups.
The product was extracted with ether and purified. The IR spec-
trum of the compound revealed the presence of two carboxylic
groups, one aliphatic and other cyclo alkane. Another product,
ammonia was identified [14c] by spot test. It was observed that,
the products do not undergo further oxidation under prevailing ki-
netic conditions.
[Fe(CN)6]3ꢀ = 4.0 ꢁ 10ꢀ4; [GAP] = 4.0 ꢁ 10ꢀ3; I = 0.50/mol dmꢀ3
.
*‘kobs
’
are calculated using rate law (6) and ‘k’ and ‘K’ were
7.94 ꢁ 103 dm3 molꢀ1 sꢀ1 and 6.0 dm3 molꢀ1, respectively.

T.P. Jose et al. / Journal of Molecular Structure 892 (2008) 121–124
123
3.3. Effect of [Clꢀ]
3.7. Effect of temperature
Since, ruthenium chloride was used as the catalyst, the effect of
Clꢀ on rate of reaction was studied. The concentration of KCl was
varied in the range of 1.0 ꢁ 10ꢀ5 to 1.0 ꢁ 10ꢀ4 mol dmꢀ3 keeping
all other reactants concentration and conditions constant. It was
found that the added chloride did not affect the rate of reaction.
The rate of reaction was measured at different temperatures at
30,
35,
40
and
;
45 °C
with
[Fe(CN)6]3ꢀ = 4.0 ꢁ 10ꢀ4
;
[GAP] = 4.0 ꢁ 10ꢀ3
[Ru(III)] = 1.0 ꢁ 10ꢀ6
;
[OHꢀ] = 0.10
and
I = 0.50/mol dmꢀ3. The rate was found to increase with increase
in temperature. The first order rate constants kobs ꢁ 103 sꢀ1 were
obtained as 2.89, 3.33, 3.90 and 4.58 at 30, 35, 40 and 45 °C, respec-
tively. These values lead to activation parameters as
3.4. Effect of ionic strength and dielectric constant
Ea = 23.8 0.2 kJ molꢀ1
,
DH
# = 21.4 0.2 kJ molꢀ1
,
DS
# = ꢀ221
The effect of ionic strength was studied by varying the sodium
perchlorate concentration from 0.50 to 1.50 mol dmꢀ3 at constant
concentrations of hexacyanoferrate(III), gabapentin, ruthenium(III)
and alkali. It was found that the rate constants were increased with
10 JKꢀ1 molꢀ1 # = 88 4 kJ molꢀ1 and logA = 1.63 0.2.
, DG
Ruthenium(III) is known [19,20] to catalyze many oxidation
reactions both in acid and alkaline media. Its catalytic ability is
due to the exhibition [21] of multivalent states as ꢀ2, ꢀ1, 0, +1,
+2, +3, +4, +5, +6, +7 and +8. Nevertheless, ruthenium compounds
are widely used as RuCl3. Its aqueous solutions are normally pre-
pared in HCl and hence, Ru(III) is expected to be existed in its chlo-
increase in concentration of NaClO4 and the plot of logkobs versus
p
I
was linear with positive slope (0.6) (Fig. 1) (r > 0.9888,
S < 0.017).
The dielectric constant (D) effect was studied by varying the t-
ride complexes [21] viz., [Ru(H2O)5Cl]2+
,
[Ru(H2O)4Cl2]+,
butyl alcohol–water content in the reaction mixture with all other
conditions being maintained constant. The dielectric constants
were computed from the values of pure liquids [17,18]. The solvent
did not react alone with oxidant and catalyst or in the mixture of
both under the experimental conditions. The rate constants, kobs
were increased with decrease in the dielectric constant of the med-
ium. The plot of log kobs versus 1/D was linear with positive slope
(Fig. 1) (r > 0.9983, S < 0.0341).
[Ru(H2O)2Cl4]ꢀ and [RuCl6]3ꢀ in addition to its aqueous species
as [Ru(H2O)6]3+. The chloride complexes are possible [22] only
when on heating or aging in presence of higher concentration of
HCl. In the present study the fresh solutions are used and solutions
are prepared in low concentration of HCl. Hence, the chloride com-
plexes are excluded. Alternatively it may be in the form of
[Ru(H2O)6]3+. This has been supported by its UV–vis spectrum
which was identical with reported study [21] for [Ru(H2O)6]3+
.
However, in alkaline medium, ruthenium(III) is known [23] to
exist as its hydroxylated species with general formula
3.5. Effect of initially added products
[Ru(OH)x(H2O)6 ꢀ x]
3ꢀx, where x < 6 and variable depending upon
The initially added product such as hexacyanoferrate(II) was
studied in the concentration range from 4.0 ꢁ 10ꢀ5 to
4.0 ꢁ 10ꢀ4 mol dmꢀ3 keeping all other reactant concentrations
constant. It was found that added product had a negligible effect
on the rate of reaction.
the [OHꢀ] used. Under the conditions [OHꢀ] > [Ru(III)], it is mainly
present as [Ru(H2O)5(OH)]2+ as shown in equation (2),
K
[Ru(H2O)5(OH)]2+
ð2Þ
[Ru(H2O)6]3+ + OH-
Hence, active species of ruthenium(III) has been considered as
[Ru(H2O)5(OH)]2+, which was supported by the positive fractional
order in [OHꢀ]. The zero order dependency on [GAP], first order
each on [Fe(CN)6]3ꢀ and [Ru(III)] and fractional order in [OHꢀ]
can be accommodated in the following scheme.
Mechanism written as in scheme 1 is in accordance with the
fact that ruthenium is known to be better catalyst in which hydride
ion [24] is during oxidation of substrate.
3.6. Polymerization study
The reaction mixture was mixed with acrylonitrile, a monomer
and kept for 2 h in an inert atmosphere. On diluting with methanol
there was no precipitation formed, indicating the intervention of
free radicals in the reaction was absent.
The results of absence of intervention of free radical also indi-
cate that the interaction between substrate and one equivalent oxi-
dant is ruled out. The zero order dependency on [substrate] also
supports the reaction between substrate and catalyst in fast steps.
The interaction between Ru(IV) and GAP is very remote because of
the fact that such interaction leads intervention of free radical,
which is obscure. The similar [15] mechanism has been proposed
for Ru(III) catalyzed oxidation of DMSO by [Fe(CN)6]3ꢀ. Since, the
order with respect to catalyst and oxidant is one each and increase
in rate with increase in ionic strength explains the interaction be-
tween Ru(III) and [Fe(CN)6]3ꢀ through inner sphere mechanism
wherein the electrons are transferred through a common bridging
ligand CNꢀ from Ru(III) to Fe(III).
2
1/D x 10
1.35
1.25
0.85
1.30
1.40
1.45
1.50
0.8
0.75
0.65
0.55
0.45
0.35
0.7
0.6
0.5
0.4
0.3
The product, [Fe(CN)6]4ꢀ is generally known to retard the rate of
reaction. However, in Ru(III) catalysis its effect has not been ob-
served which may be due to the formation of highly reactive inter-
mediates Ru(IV) or Ru(V). The unaltered in the rate by adding Clꢀ
can also be explained on the basis of formation Ru(III)-chloride
complexes which are very remote as they are formed [22] only at
high concentration of Clꢀ.
0.6
0.8
1.0
.2
1.4
1
I
√
Increasing the dielectric constant (D) of the reaction medium
decrease in rate was observed. This might be due to the stabiliza-
tion of activated complex at high dielectric constant of the
Fig. 1. Effect of variation of ionic strength (I) and dielectric constant on the ruth-
enium(III) catalyzed oxidation of gabapentin by hexacyanoferrate(III) in aqueous
alkaline medium at 25 °C.
medium. The large negative value of
D
S# and large positive value

124
T.P. Jose et al. / Journal of Molecular Structure 892 (2008) 121–124
K
-6
3
-1
[Ru(H2O)5(OH)]2+
[Ru(H2O)6]3+ + OH-
1/[Ru(III)] x10 (dm mol )
fast
1
0.0
.5
1.0
.5
2.0
2.5
0
k
[Ru(H2O)5(OH)]3+ + [Fe(CN)6]4-
[Ru(H2O)5(OH)]4+ + [Fe(CN)6]4-
[Fe(CN)6]3-
+
6.0
5.0
4.0
3.0
2.0
1.0
0.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
[Ru(H2O)5(OH)]2+
slow
fast
[Ru(H2O)5(OH)]3+ + [Fe(CN)6]3-
H2NH2C CH2COO-
OHC CH2COO-
fast
[Ru(H2O)5(OH)]4+
+
+ H2O
+ [Ru(H2O)5(OH)-H]3+ + H+ + NH3
[Ru(H2O)5(OH)]2+ + H2O
fast
[Ru(H2O)5(OH)-H]3+ + OH-
0.0
5.0
10.0
15.0
3
20.0
25.0
-OOC
CH2COO-
OHC CH2COO-
fast
-
-1
1/[OH ](dm mol )
[Ru(H2O)5(OH)]4+
+
+ H2O
Fig. 2. Verification of rate law (6) of ruthenium(III) catalyzed oxidation of gabap-
entin by hexacyanoferrate(III) in aqueous alkaline medium at 25 °C (conditions as in
Table 2).
+ [Ru(H2O)5(OH)-H]3+ + 2H+
[Ru(H2O)5(OH)]2+ + H2O
fast
tween oxidant and catalyst was proposed in the rate determining
step to give Ru(IV) and further oxidizes to Ru(V) followed by oxida-
tion of substrate; the interaction between oxidant and substrate is
of inner sphere complex type which was supported by the small
value of frequency factor and large value of second order rate con-
stant (7.9ꢁ103 dm3 molꢀ1 sꢀ1). The non-intervention of free radical
during the reaction revealed that oxidation of substrate occurs in
the complementary reaction of two equivalent change in interme-
diate, Ru(V) and a substrate in a single step. The catalyst was acted
as hydride ion abstracting agent.
[Ru(H2O)5(OH)-H]3+ + OH-
Scheme 1.
of
DG
# support the loss of degree of freedom. The small value of the
frequency factor supports the inner sphere interaction
of species.
The rate law for scheme 1 can be derived as follows.
3ꢀ
ꢀd½FeðCNÞ6ꢃ
3ꢀ
2þ
¼ rate ¼ k½FeðCNÞ6ꢃf ½RuðH2OÞ5ðOHÞꢃf
ð3Þ
ð4Þ
dt
References
2þ
kobs ¼ k½RuðH2OÞ5ðOHÞꢃf
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1051.
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(2007) 246.
After calculating the total concentration of Ru(III), [Ru(III)]T and
total concentration [OHꢀ]T using the first equilibrium as in the
scheme, kobs becomes
kK½RuðIIIÞꢃT½OHꢃT
kobs
¼
ð5Þ
1 þ K½OHꢀꢃ
The denominator of the above equation also contains the term
(1+K[Ru(III)]T) but it is neglected as [Ru(III)]T is very small and
hence becomes unity.
The rate constant ‘k’ and equilibrium constant ‘K’ can be evalu-
ated using the equation (5) after omitting the subscript T in the
above equation, the equation (6) follows.
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Chemical 267 (2007) 65.
[12] A.M. Balado, B.C. Galan, F.J.P. Martin, Ann. Quimica 88 (1992) 170.
[13] H.S. Singh, R.K. Singh, S.M. Singh, A.K. Sisodia, J. Phys. Chem. 81 (1977) 1044.
[14] [a] J. Mendham, R.C. Denney, J.D. Barnes, M.J.K. Thomas, Vogel’s Text Book of
Quantitative Chemical Analysis, sixth ed., Pearson Education, Delhi, 2003.
p. 438;
1
1
1
¼
þ
ð6Þ
kobs kK½RuðIIIÞꢃ½OHꢀꢃ k½RuðIIIÞꢃ
The plot of 1/kobs versus 1/[Ru(III)] should be a straight line with
zero intercept; and the 1/kobs versus 1/[OHꢀ] plot should be non-
zero intercept and found to be so in Fig. 2. From the slope and
intercept of such graph, the ‘k’ and ‘K’ are evaluated as
k = 7.937 ꢁ 103 dm3 molꢀ1 sꢀ1 and K = 6.0 dm3 molꢀ1. The value
of ‘K’ is very close to the literature value [12,13]. Hence, the mech-
anism is proved. The ‘k’ and ‘K’ derived as above are used to regen-
erate the kobs values for different experimental conditions and are
found to be neighborhood of experimental values (Table 2).
[b] J. Mendham, R.C. Denney, J.D. Barnes, M.J.K. Thomas, Vogel’s Text Book of
Quantitative Chemical Analysis, sixth ed., Pearson Education, Delhi, 2003.
p. 426;
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4. Summary
The title reaction is zero order dependency in substrate and first
order each in oxidant and catalyst. Hence, the results suggest that
oxidation of substrate occurred in the fast step. The interaction be-