1241 Pure Appl. Chem., Vol. 81, No. 7, pp. 1241–1249, 2009. doi:10.1351/PAC-CON-08-08-23 © 2009 IUPAC, Publication date (Web): 29 June 2009 Electron transfer in non-oxovanadium(IV) and (V) complexes: Kinetic studies of an amavadin model* Jeremy M. Lenhardt 1 , Bharat Baruah 2,3 , Debbie C. Crans 2,‡ , and Michael D. Johnson 1,† 1 Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, NM 88003, USA; 2 Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA; 3 Department of Chemistry and Biochemistry, Kennasaw State University, Kennesaw, GA 30144, USA Abstract: Electron-transfer reactions of the eight-coordinate vanadium complex, bis-(N- hydroxyiminodiacetate)vanadium(IV) [V(HIDA) 2 ] 2– , a synthetic analog of amavadin with ascorbic acid and hexachloroiridate(IV), have been studied. The self-exchange rate constant for this analog has been calculated from oxidation and reduction cross-reactions using Marcus theory and directly measured using 51 V NMR paramagnetic line-broadening tech- niques. The average self-exchange rate constant for the bis-HIDA vanadium(IV/V) couple equals 1.5 × 10 5 M –1 s –1 . The observed rate enhancements are proposed to be due to the small structural differences between the oxidized and reduced forms of the HIDA complex and inner-sphere reorganizational energies. The electron-transfer reaction of this synthetic analog is experimentally indistinguishable from amavadin itself, although significant differences exist in the reduction potential of these compounds. This suggests that ligand modification effects the thermodynamic driving force and not the self-exchange process. Keywords: vanadium; electron transfer; kinetics; amavadin; non-oxo; model complexes. INTRODUCTION Amavadin has been found in the mushroom Amanita muscaria where vanadium concentrations exceed 400 times those normally found in plants [1–6]. It was first reported in 1972 by Kneifel and Bayer [7,8] and later shown by Meisch and coworkers [9] that only one species of the more than 200 in the genus Amanita contained elevated vanadium concentrations. Although it was isolated over 30 years ago, the function of amavadin remains unknown. Many roles have been proposed, which include a redox medi- ator or an oxygen carrier. It has reversible redox chemistry with a one-electron reduction potential (+0.03 V in dimethylsulfoxide (DMSO) and +0.53 V in H 2 O, both vs. SCE) [10]. Amavadin possesses an unusual non-oxovanadium(IV) center within an eight-coordinate environ- ment formed through the 1:2 complexation of the vanadium metal with the natural (1) (not shown) or *Paper based on a presentation at the 6 th International Symposium on Chemistry and Biological Chemistry of Vanadium, 17–19 July 2008, Lisbon, Portugal. Other presentations are published in this issue, pp. 1187–1330. ‡ Corresponding author: E-mail: crans@lamar.colostate.edu † E-mail: johnson@nmsu.edu