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Intermolecular Electron Transfer Reactivity and Dynamics of Cytochrome c – Nanoparticle Adducts

Interprotein electron transfer (ET) is crucial for natural energy conversion and a fundamental reaction in the pursuit of understanding the broader problem of proteinprotein interactions and reactivity. Simplifying the complicated nature of these natural systems has driven development of biomimetic approaches. Functionalized gold nanoparticles offer simplified, tunable surfaces that can serve as a proxy to study the reactivity and dynamics of proteins. Amino-acid functionalized gold nanoparticles (Au-TX) served as a complementary partner to cytochrome c (Cyt c) and catalyzed its ET reactivity without altering the native structure. Redox mediator and EPR experiments confirmed that the redox potential and coordination environment of the heme were unaltered. Varying the functionality of Au-TX under limiting redox reagent concentrations resulted in distinct ET reactivity. These conditions reflected the collision of a small redox reagent with the Cyt c/Au-TX adduct, introducing the possibility of Cyt c/Au-TX dynamics to modulate ET. Under high ionic strength conditions, the rate enhancement ranged from 0.0870 " 1011 for Cyt c/Au-TAsp to 1.95 " 1011 M-1 s-1 for Cyt c/Au-TPhe. Au-TAsp binds to a larger surface of the front face of Cyt c than Au-TPhe, likely reducing heme access and resulting in attenuated ET reactivity.Site-directed spin-labeling characterized the dynamic interactions and motion of Cyt c with Au-TX. Several mutants of Cyt c were utilized to extract information about the different dynamics of the Cyt c/Au-TPhe and Cyt c/Au-TAsp systems. Cyt c appeared to have a highly dynamic binding interaction with the surface of Au-TPhe while binding to Au-TAsp resulted in a more rigid interface, particularly at the heme crevice. The dynamic interaction of Cyt c/Au-TX at the heme crevice could promote a gated ET mechanism between Cyt c and its redox partner. Thus, the reduced reactivity of Cyt c/Au-TAsp is likely a result of both slower global dynamics and more rigid binding near the heme crevice.
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