~ Pergamon 0277-5387(94)00376-9 Polyhedron Vol. 14. No. 9, pp. 1207 1211, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0277 5387/95 $9.50+0.00 KINETICS AND MECHANISM OF THE OXIDATION OF D-GALACTOSE BY COPPER(II) IN ACIDIC MEDIUM LUIS F. SALA,* LAURA CIULLO, RUBI~N LAFARGA and SANDRA SIGNORELLA Area Inorgfinica, Departamento de Quimica Fisica, Universidad de Rosario, Suipacha 531 (2000) Rosario, Argentina (Received 9 August 1994 ; accepted 12 September 1994) Abstract--The oxidation of D-galactose by Cu n has been studied in acetate buffers within the pH range 4-4.75 at l l0°C. The observed pseudo-first-order rate constant is given by kobs = {ka+kb/[H +]+kc/[H+] 2} [gal]/{1 + (kd+ke/[U+])[gal]} where k n = 2.77 × 10 -3 M -I s -l, kb= (1.22+0.11)x 10 -7 s -1, kc= (1.12+0.16)× l0 -12 M s -1, kd= 2.08 M -1, ke -- (2.75 + 0.33) x 10 -5. This rate law corresponds to the formation of galactonic acid and Cu I when a l0 fold or higher excess of o-galactose over Cu n is employed. The results are discussed in terms of a possible mechanism of oxidation of the enodiol of galactose by CuOAc ÷, present in solution. After Ca n binding, Cutl binding to carboxylates has been the most studied. This is in part due to several observations of biologically significant interactions between Cu n and sugars. Cu H carbohydrate com- plexes have been extensively studied because of their potential efficacy in the treatment of rheu- matoid arthritis. 1,2 However, the possibility of redox reactions between sugars and Cu n may lead to the production of free radicals involved in car- cinogenic and ageing cellular processes. 3 We have therefore studied the oxidation of sugars by Cun. 4 In this work we have investigated the kinetics of oxidation of D-galactose (Scheme 1) at low pH in the absence of oxygen. EXPERIMENTAL Kinetics The reaction mixture was prepared by mixing Cu(CIO4)2 in appropriate solutions of NaOAc- AcOH and variable amounts of D-galactose (gal) * Author to whom correspondence should be addressed. (Sigma grade) (Scheme 1) in order to obtain the desired ratio of oxidant/reductant. Ionic strength was maintained at I = 0.59 M throughout all exper- iment employing sodium perchlorate (Fluka grade). Oxygen was purged from the solutions by bubbling nitrogen through the solutions protected by serum caps. Residual dioxygen in the nitrogen was removed by passing the nitrogen through scrubbers containing pyrogallol. All solutions were prepared with deionized water which had been dis- tilled from alkaline permanganate in glass appar- atus. The sealed solutions were brought to the desired temperature within 15 s using an electrically regulated thermostat. A set of 20 ampoules for each gal/Cu Hratio was prepared. The reaction was quen- ched by quickly cooling the solutions. The Cu20 produced in the reaction was filtered off and 50% NH3 was added to the filtrate. Unreacted Cu 11 was determined spectrophotometrically as the [Cu(NH3)4] 2+ complex ('~max ~--- 620 nm, e = 42 M -l cm -1) on a Shimadzu UV-140 double beam spec- trophotometer. The nature of the precipitate was confirmed by treating it with hydrochloric acid in the presence of glycerine to stabilize Cu I, thereby avoiding its dismutation to Cu 1I and Cu °. The com- plete dissolution of the precipitate confirms the for- mation of Cu20 and the absence of Cu °. A large excess of gal (10-100 fold) was used for all the 1207