Probing Surface Properties of Cytochrome c at Au Bionanoconjugates

Bionanoconjugates were created with cytochrome c from horse heart (HCc) or yeast (YCc) and citrate-stabilized gold nanoparticles (AuNPs). Evidence for the formation of stable HCc-AuNP and YCc-AuNP bionanoconjugates came from a 5 nm red-shift of the surface plasmon resonance band of the AuNPs, increase of zeta-potential, and direct visualization by atomic force microscopy. Langmuir isotherm fittings of zeta-potential data indicated that higher enthalpy changes are involved in the formation of the HCc-AuNP than in YCc-AuNP. UV-vis and circular dichroism studies of pH-induced aggregation of the bionanoconjugates revealed distinct protonation patterns with an aggregation pH of 8.8 and 6.2 for YCc-AuNP and HCc-AuNP, respectively. No appreciable changes were observed in the secondary structure of HCc in HCc-AuNP. In contrast, YCc in YCc-AuNP presented a decrease in cc-helix content upon AuNP binding and an increase in beta-sheet content upon pH-induced aggregation. Data discussion is based on the distinct binding modes of both proteins to the AuNPs via a covalent bond (Cys 102) for YCc and via electrostatic interaction for HCc.