50
RANGAPPA ET AL.
conditions including the HClO4 medium and the tem-
perature. The main objective of this investigation was
twofold: (1) to ascertain whether the neutral ␣-amino
acids follow a common mechanism of oxidation, which
they do, and (2) to compare these new kinetic results
with those of the earlier studies [1,2] so that the main
differences could be identified. The following differ-
ences between the earlier studies and the present work
are noted:
differences described above justify the present
study.
EXPERIMENTAL
Chloramine-T (Merck) was purified by a standard
method[3]anditspuritywascheckedbyiodometryand
by its 1H and 13C NMR spectra. An aqueous solution of
the compound was prepared and preserved in a brown
bottle to prevent its photochemical deterioration. The
chromatographically pure glycine (S.D. Chem.), DL-
valine, L-alanine, and L-phenylalanine (Loba Chemie)
were further assayed by the acetous perchloric acid
method [4]. All other chemicals used were of analytical
reagent grade. Solvent-isotope studies were made with
D2O (99.4%) supplied by BARC, Trombay, Mumbai,
India. Triply distilled water was used in the prepara-
tion of aqueous solutions. The dielectric constant of the
medium was varied by the addition of MeOH and the
values of dielectric constant for MeOH–water mixtures
reported in literature [5,6] were employed.
i. Unlike the present case, in the earlier studies
[1,2], oxidation kinetics were performed on indi-
vidual amino acids under different reaction con-
ditions, including acid and base media, resulting
in different experimental rate laws and mecha-
nisms. For example, one of the previous studies
[1] involved the oxidation of L-threonine by CAT,
in HCl, HClO4, and H2SO4 media, while the
present study involves the oxidation of four neu-
tral amino acids in HClO4 medium by CAT. The
threonine oxidation shows the following kinetic
orders, which differ from the present data: a first-
order in [CAT], a negative first-order in [H+],
and a fractional-order each in [threonine] and
[Cl ].
Kinetic Procedure
The reaction was carried out in glass-stoppered Pyrex
boiling tubes whose outer surface was coated black to
eliminate photochemical effects. Requisite amounts of
the solutions of the amino acid and perchloric acid were
taken in the tube and thermostated at 30 C. A measured
amount of CAT solution, also thermostated at the same
temperature, was rapidly added with stirring to the mix-
ture in the tube. The progress of the reaction was moni-
tored by the iodometric determination of the unreacted
CAT in aliquots of the reaction mixture withdrawn at
different intervals of time. The course of the reaction
was studied for at least two half-lives. The pseudo first-
order rate constants, kobs, calculated from the plots of
log [CAT] vs. time were reproducible within 3%.
Regression analysis of experimental data, to obtain
regression coefficient r and the standard deviation s,
was performed with an EC-72 statistical calculator.
ii. Similar differences can be found between the ki-
netic results and mechanisms for several amino
acids reported earlier [2].
iii. A part of the earlier study [2] involved the ki-
netics of oxidation of glycine, valine, alanine,
and phenylalanine with CAT performed in HCl
medium, but not in HClO4 medium. Hence the
two sets of kinetic data in two acid media for the
same four amino acids are different.
iv. There are differences in the reaction stoichiome-
tries and in the products formed. Under the exper-
imental conditions used in this work, the mole-
to-mole ratio of the amino acid to CAT is 1 : 1
with NH4+ ion, CO2, and the corresponding alde-
hyde as oxidation products of the substrate. In
contrast, under the experimental conditions used
in the previous studies [2], the stoichiometry in-
volved 2 mol of CAT consumed per mole of the
amino acid, yielding a nitrile and CO2 as oxida-
tion products.
v. In the present study, a proton-inventory experi-
ment has been performed showing the involve-
ment of H+ in the transition state. The earlier
studies [1,2] do not report this experiment.
vi. The applicabilities of the Taft equation and
single-parametercorrelationshavebeenexplored
to elucidate stereochemical information of the
reactions in the present study. These data are
lacking in the earlier investigations [1,2]. The
Stoichiometry and Product Analysis
The reaction mixtures containing the amino acid and
perchloric acid with excess of CAT were kept for 24 h
at 30 C. Iodometric determination of the unconsumed
CAT showed that 1 mol of the oxidant was consumed
per mole of the amino acid forming the correspond-
ing aldehyde. The stoichiometry of the reaction can be
represented by Eq. (1).
R0CH(NH2)COOH + RNCl + H3O+
→ R0CHO + NH4+ + CO2 + RNH2 + Cl (1)