125
* Corresponding author. e-mail: ahmed41_chem@yahoo.com
Kinetics and Mechanism of Oxidation of
Chromium(III)-Guanosine Complex By Periodate
HASSAN A. EWAIS, SAMAH A. AHMED
a
AND AHMED A. ABDEL-KHALEK
*
Chemistry Department, Faculty of Science at Beni-Suef, Cairo University, Beni-Suef City, Egypt
Received on July 24, 2003; in final form October 14, 2003
The oxidation kinetics of the chromium(III)-guanosine complex, [Cr
III
(G)(H
2
O)
4
]
3+
(G =
guanosine) with periodate in aqueous solution were studied and found to obey the rate law:
Rate = [Cr
III
]
T
[IO
4
-
] {k
1
K
5
+ ( k
2
K
1
K
6
/[H
+
])} / {1 + (K
1
/[H
+
]) + K
5
[IO
4
-
] + (K
1
K
6
[IO
4
-
]/
[H
+
])} where K
1
, K
5
and K
6
are the deprotonation of [Cr
III
(G)(H
2
O)
4
]
3+
and pre-equilibrium
formation constants [Cr
III
(G)(H
2
O)
3
-OIO
3
]
2+
and[Cr
III
(G)(H
2
O)
2
(OH)OIO
3
]
+
precursor
complexes respectively. Thermodynamic activation parameters have been calculated. It is
proposed that electron transfer proceeds through an inner-sphere mechanism via coordination
of IO
4
-
to chromium(III).
Keywords: Chromium(III)-guanosine complex; Periodate; UV-spectrophotometer; Thermodynamic
activation parameters
INTRODUCTION
Periodate oxidations have been reported to play an
important role in biological determinants [1, 2]. They
are used to degrade carbohydrate determinants in
proteins without altering protein or lipid epitopes [1,
2]. Although interactions of DNA with a large number
of metals ions have been investigated, the interaction
of chromium(III) with DNA has apparently escaped
detailed attention, despite the fact that chromium(III)
complexes draw particular attention owing to their
environmental toxicity and the carcinogenic nature of
higher valent chromium [3]. The oxidation of
hexacyanoferrate(II) by periodate has been
investigated previously in weakly alkaline and neutral
phosphate buffer solution and the kinetic were
observed to be simple [4]. The oxidation kinetics of
hexacyanoferrate(II) by periodate in acetate buffer was
also studied [5]. It was reported that the mechanism of
oxidation greatly differed on going from alkaline to
acidic solution, whereas in acidic solutions the
reaction rate was not only independent of periodate,
Inorganic Reaction Mechanisims, Vol. 5, pp. 125-133 © 2004 Old City Publishing, Inc.
Reprints available directly from the publisher Published by license under the OCP Science imprint,
Photocopying permitted by license only a member of the Old City Publishing Group.
a
Abstracted from the M.Sc. Thesis of Samah A. Ahmed.
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