Electrical conductivity measurements were performed on single apoferritin and holoferritin molecules by conductive atomic force microscopy. Conductivity of self-assembled monolayer films of ferritin molecules on gold surfaces was also measured. Holoferritin was 5-15 times more conductive than apoferritin, indicating that for holoferritin most electron-transfer goes through the ferrihydrite core. With 1 V applied, the average electrical currents through single holoferritin and apoferritin molecules were 2.6 pA and 0.19 pA, respectively.

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(17), and an E m ϭ -790 mV (18) was estimated for the 1ϩ͞0 couple, which is in line with E m estimates of all-ferrous Fe-S clusters by using discrete Fourier transform calculations (19). Indeed, this latter study raised the question of whether [Fe 4 S 4 ] 0 Av2 can even be made in vivo (1). A value of -790 mV is not consistent with turnover potential measured by Ti(III) and other reductants and with reported potentials for the Ti(IV)͞Ti(III) couple (7,20). If an E m ϭ -460 mV for the 1ϩ͞0 couple (16)

Mammalian ferritin from horse spleen undergoes an electrochemical or chemical reduction reaction in which each iron atom present is reduced by one electron (2300 electrons per ferritin molecule containing 2300 Fe3+ ions). Midpoint potentials of -190 mV, -310 mV, and -416 mV were determined at pH 7.0, 8.0, and 9.0. This variation of potential with pH indicates that approximately 2 H+ are transferred to the core for each Fe3+ reduced to Fe2+. Mössbauer measurements of partially reduced ferritin give spectra that consist of a ferric quadrupole doublet with a superposed ferrous quadrupole doublet. The relative intensities of these doublets are consistent with the electrochemically determined degree of reduction.