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D. Sudha:
"Kinetic And Mechanistic Studies On The Oxidation Of L-Serine And L-Threonine";
Betreuer/in(nen), Begutachter/in(nen): W. Linert, G. Rao, P. Vani; Andhra University, Dept Chemistry, 2017; Rigorosum: 26.09.2017.

L-Serine is an aliphatic, non-essential, proteinogenic amino acid
which contains alcoholic hydroxyl group in its side chain. It is synthesized
in the body from other metabolites like D-3-phosphoglycerate. It occurs in
the active sites of many enzymes like chymotrypsin, trypsin and serves as a
nucleophile in a number of proteolytic enzymes as it has a primary
alcoholic group.
L-threonine is an aliphatic, essential amino acid which also
contains alcoholic hydroxyl group in its side chain. It supports
cardiovascular, liver and central nervous system. It also helps to keep
connective tissues and muscles throughout the body strong and elastic. It
supports the immune system by aiding in the production of antibodies.
In view of the biological importance of L-serine and L-threonine,
the author has chosen tetrachloroaurate(III) and periodate for the oxidation
of L-serine and L-threonine and manganese(III) (ruthenium(III)-catalysed)
for the oxidation of L-threonine.
The work is presented in six chapters. The references cited in the
text of each chapter are given at the end of that chapter. Chapter -I gives a
brief account of the biological role of L-serine and L-threonine with an
introductory survey of the existing literature on the kinetic and mechanistic
patterns involved in their oxidations.
The remaining thesis is organized into three parts. Part-A is divided
into two chapters - chapter-II dealing with the oxidation of L-serine and
chapter-III with that of L-threonine by tetrachloroaurate(III).
In Chapter-II, the author presents the kinetic study of oxidation of
L-serine by tetrachloroaurate(III) in sodium acetate-acetic acid buffer. The
reaction is first order in [oxidant] and fractional order in [substrate]. The
increase in oxidation rate with increase in pH suggests that zwitterionic
form of serine SerZ and [AuCl3(OH)]- of oxidant are the reactive species. 2-
hydroxy acetaldehyde is identified to be the oxidation product. The
proposed mechanism involves the decomposition of the complex in the rate
determining step.
Chapter-III comprises the observations of the author pertaining
to kinetic study of oxidation of L-threonine by tetrachloroaurate(III) in
sodium acetate-acetic acid buffer. The reaction is first order each in
[threonine] and [gold(III)]. The rate of the reaction increases with increase
in pH and 2-hydroxy propanaldehyde is found to be the product of
oxidation. A suitable mechanism is proposed with the zwitterionic form of
L-threonine, Thrz and [AuCl3(OH)]- as the reactive species of the substrate
and tetrachloroaurate(III) respectively.
Part-B is also divided into two chapters and it incorporates the
oxidation of L-serine (chapter-IV) and L-threonine (chapter-V) by
periodate.
In Chapter-IV, the author presents the influence of surfactant,
sodium dodecyl sulphate (SDS) on the oxidation of L-serine by periodate in
aqueous perchloric acid medium by mimicking the conditions existing
inside the protein molecules. The rate of the reaction increases with
increase in the surfactant concentration upto a certain level and thereafter
remains constant. In presence of surfactant, the reaction is first order each
in [substrate] and [oxidant]. 2-hydroxy acetaldehyde is found to be the
oxidation product. The mechanism proposed involves the binding of the
oxidant, H5IO6 and protonic form of serine with the micelle.
Chapter-V incorporates the kinetic investigations of the author on
the oxidation of L-threonine by periodate(a two electron oxidant ) in
alkaine medium. The reaction is first order in [oxidant] and fractional order
in [substrate]. The rate increases with increase in [OH-]. The reaction
follows Michaelis - Menten type of kinetics with the formation of a 1:1
complex between the periodate and threonine. The increase in the oxidation
rate with increase in [alkali] suggests that [H2IO6]3- and anionic form of
threonine are the reactive species. The product of oxidation (2-hydroxy
propanaldehyde) is found to be the same as in the case of oxidation with
tetrachloroaurate(III). The proposed mechanism involves the reaction of the
complex with the oxidant in the rate determining step.
Part-C comprises only one chapter(chapter-VI) which embodies the
work on the ruthenium(III)-catalysed oxidation of L-threonine by
manganese(III) (a one electron oxidant) in sulphuric acid medium. The
reaction is first order each in [substrate], [oxidant] and [catalyst]. The rate
of reaction is not altered with acid concentration leading to the presumption
that both Mn3+ and Mn(OH)2+are the reactive species of the oxidant. 2-
hydroxy propanaldehyde is found to be the product of oxidation as in the
case of other oxidants. Protonated species of L-threonine, HThr and
ruthenium(III) are considered to be the reactive species of the substrate and
the catalyst respectively.
Finally, a brief summary on the mechanism of oxidation of Lserine
and L-threonine with different oxidants and the conclusions of the
author are presented at the end of the thesis. In the oxidation of two amino
acids by tetra chloroaurate (III) the difference in order with the substratefractional
with L-serine and first order with L- threonine is most likely due
to complexation of the oxidant with the former and lack of complexation
with the latter. Probably, the additional -CH3 group in threonine is
hindering complexation.
From the products of oxidation of the two amino acids-2-hydroxy
acetaldehyde in the case of L-serine and 2-hydroxy propanaladehyde in
case of L-threonine it may be inferred, that the -CH2OH group in the
former and the -CH(OH)CH3 in the latter are inert and not getting oxidised.
Thus, the general mechanism of oxidation of the two amino acid molecules
seems to be similar and it involves decarboxylation and deamination as in
case of other amino acid oxidations.

redox kinetics, biological important species