J. Chil. Chem. Soc., 58, Nº 2 (2013)
KINETICS AND MECHANISM OF BASE HYDROLYSIS OF A-AMINOACID ESTERS CATALYSED BY [Pd(1,3-
DIAMINO-2-HYDROXYPROPANE)(H2O)2]2+ COMPLEX
Al-QALAF . F. Ac , Al BASSAM. A.Ac. AND. SHOUKRY. M.Ma,b*.
a) Department of Chemistry, Faculty of Science, Islamic University-Madinah, Saudi Arabia.
b) Department of Chemistry, Faculty of Science, Cairo University, Giza, P.O. Box 12611, Egypt.
c) Deparment of Chemistry, Faculty of Snience, Public Authority for Applied Education and Training, Kuwait.
(Received: July 23, 2012 - Accepted: January 5, 2013)
ABSTRACT
Amino acid esters (L) react with [Pd(DHP(H2O)2]2+ , (DHP = 1,3-diamino-2-hydroxopropane) giving mixed ligand [Pd(DHP)L]2+ The kinetics of hydrolysis
of [Pd(DHP)L]2+ have been studied by pH-stat technique and rate constants were obtained. Rate acceleration observed for glycine methyl ester is high. The effect
with methionine methyl ester and histidine methyl ester are much less marked, as the mixed-ligand complexes with these ligands do not involve alkoxycarbonyl
donors. Possible mechanisms for these reactions are considered. Activation parameters have been determined for glycine methyl ester.
Keywords: 1,3-diamino-2-hydroxopropane, Amino acid ester hydrolysis, Pd(II), pH-stat technique.
obtained. Anal. Calcd. for C3H10N2OPdCl2 (267.3) :C, 13.6; H, 3.7; N, 10.5.
Found: C, 13.5; H,4.0; N, 10.3%.
INTRODUCTION
Metal ions in metalloenzymes such as carboxypeptidase A1, carbonic
anhydrase2 and alkaline phosphatase3 play a key role in many biochemical
processes.4 In these metalloenzymes, the metal ions at the active sites are
considered to serve as a primary catalytic considered to serve as a primary
catalytic centre for bringing substrate and nucleophile together through
formation of a coordination complex, to activate the substrate carbonyl group
facilitating attack of the nucleophile in caroxypeptodase A5 or to activate the
water molecule in the reversible hydrated process of carbon dioxide in carbonic
anhydrase6 and to activate the serine hydroxyl group in alkaline phosphatase.7
In order to probe the mechanism by which the metalloenzyme may operate and
consequently provide a theoretical base for designing high effective artificial
metalloenzyme, previous reports8-11 have developed biomimetic models
for metalloenzyme which catalyse the hydrolysis of carboxylic acid esters
in biomemetic models for certain metalloenzymes, as the metalloenzyme-
substrate complex .
Aqueous solutions of the diaqua form of the Pd(DHP)Cl2 complex were
prepared in situ by the addition of slightly less than two mole equivalents of
AgNO3 to a solution of a known amount of the dichloro complex and stirred
over night. The white precipitate of AgCl that formed was filtered off using a
0.1 mm pore membrane filter. Great care was taken to ensure that the resulting
solution was free of Ag+ ion and that the dichloro complex had been converted
completely into the diaqua species. The ionic strength of the solutions was
adjusted to 0.1 M with NaNO3 (Acros, p.a.).
Kinetic measurements
The kinetics of hydrolysis was monitored using a Metrohm 751 Titrino
operated with the SET mode (titration to a preset end point). The titroprocessor
and electrode were calibrated with standard buffer solutions according to
NIST.19 Hydrolysiskineticsofglycine-, methionine-, andhistidinemethylesters
Work in our laboratory12-17 has focused on catalysis of the hydrolysis of
various amino acid esters by metal complexes. The mixed ligand complex
[Pd(en)L]2+ , where a five-membered chelate ring is formed, undergoes
hydrolysis by water and hydroxide ion.18 It is therefore of considerable
interest to extend this work to the mixed ligand complex with 1,3-diamino-2-
hydroxypropane , where a six- membered chelate ring. The increase of chelate
ring size may affect the elecrophilicity of the Pd(II) ion and tune the reactivity
of this metal centre in possible catalytic and biological applications as the
hydrolysis of the ester group.
in the presence of [Pd(DHP)(H2O)2]2+ is investigated by pH-stat technique20,21
.
After equilibrating a solution mixture (40 cm3) containing [Pd(DHP)(H2O)2]2+
(2.5x10-3M) , ester (2.5x10-3M) and NaNO3 (0.1M) at the required temperature
under nitrogen flow and the pH was brought to the desired value by the addition
of 0.05 M NaOH solution. The hydrolysis was then followed by the automatic
addition of 0.05 M NaOH solution to maintain the given pH constant. The data
fitting was performed with the OLIS KINFIT set of programs22 as described
previously23. The precision of the kinetic data was estimated from plot as
obtained from the OLIS program output. The accepted residual values are less
than 10-2. Values of the hydroxide ion concentration were estimated from the
pH using pKw = 13.997 and an activity coefficient of 0.772 was determined
from the Davies equation24 . At the variable temperature studies, the following
values of pKw and g were employed25, at 15oC ( pKw = 14.35, g = 0.776), at 20o
C ( pKw = 14.16, g = 0.774) at 25o C ( pKw = 14.00, g = 0.772) at 30oC ( pKw
= 13.83, g = 0.770), at 35oC ( pKw = 13.68, g = 0.768)
EXPERIMENTAL
Materials and reagent
All reagents were of Analar grade. K2PdCl4 and 1,3-diamine-2-
hydroxopropane are provided by Aldrich. The glycine-, histidine-, and
methionine methyl esters were purchased from Fluka. Carbonate-free NaOH
was prepared and standardized against potassium hydrogen phthalate solution.
All solutions were prepared in deionized H2O.
RESULTS AND DISCUSSION
a-amino acid esters react with [Pd(DHP)(H2O)2]2+ according to the
equilibrium (1). The equilibrium constant is expected to be >> 1. This is due
to the high affinity of PdII ion to react with N-ligands26. The resulting mixed-
ligand complexes [Pd(DHP)L]2+ [L = NH2CH(R)CO2R`] undergo hydrolysis
by water and hydroxide ion according to Eq. (2) and (3) .
Apparatus and measuring techniques
Pd(DHP)Cl2 was prepared by dissolving K2PdCl4 ( 2.82 mmol) in 10 ml
water with stirring. The clear solution of [PdCl4]2- was filtered and 1,3-diamino-
2-hydroxopropane ( 2.82 mmol), dissolved in 10 ml H2O was added drop wise
to the stirred solution. The pH was adjusted to 2-3 by the addition of HCl
and/or NaOH. A yellowish -brown precipitate of Pd(DHP)Cl2 was formed and
stirred for a further 30 minute at 50 °C. After filtering off the precipitate, it was
thoroughly washed with H2O, ethanol and diethyl ether. A yellow powder was
(1)
e-mail: shoukrymm@hotmail.com
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