Reduction of Escherichia coli ribonucleotide reductase subunit R2 with eight water-soluble ferrocene derivatives

摘要

Water soluble ferrocenes [Fe(Cp)(CpL)], where Cp- is the eta(5)-cyclopentadienide ligand and the side chain L is (a) the carboxylic acid group -(CH2)(x)CO2H with x = 0-4 (I-V); (b) the complex x = 2 with the beta-methylene mono-methyl substituted (VI); (c) the amine hydrochloride derivative with L = CH(Me) NH3+ (VII); and (d) the complex with two Cp rings bridged by the amine hydrochloride -CH(NH3+)CH2CH2- (VIII); have been prepared, and are used as one-equivalent reductants for the active-R2 subunit of Escherichia coli ribonucleotide reductase. Formal reduction potentials E-1(o')(25 degreesC) of the carboxylates of acids I-VI in 20 mM NaOH, and of the amine hydrochlorides VII and VIII in water were determined by cyclic voltammetry, and are in the range 0.308-0.550 V versus nhe, I= 0.100 M (NaCl). Second-order rate constants k(12) (25 degreesC) for the reduction of active-R2 were determined by UV-Vis spectrophotometry, and are in the range 0.15-0.50 M-1 s(-1) at I = 0.100 M. A free-energy plot of log k(12) versus E-o' values gives no clearcut unidirectional trend. Since from present information the electron self-exchange rate constant for the [Fe(Cp)(2)](+)/[Fe(Cp)(2)] couple is favourable ( > 7 x 10(6) M-1 s(-1) in methanol at 25 degreesC), it would appear that electron-transfer from the ferrocenes via Trp-48, Asp-237, His-118 to the Fe-2(III) site on R2 is much slower than expected, and smaller than with the organic radical reductants previously studied. Electron-transfer from some other position on the protein surface to the Tyr* is considered as an alternative. (C) 2002 Elsevier Science B.V. All rights reserved. [References: 40]