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Contributions to cytochrome c inner- and outer-sphere reorganization energy† † Electronic supplementary information (ESI) available. See DOI: 10.1039/d1sc02865k

ABSTRACT: Cytochromes c are small water-soluble proteins that catalyze electron transfer in metabolism and energy conversion processes. Hydrogenobacter thermophilus cytochrome c552 presents a curious case in displaying fluxionality of its heme axial methionine ligand; this behavior is altered by single point mutation of the Q64 residue to N64 or V64, which fixes the ligand in a single configuration. The reorganization energy (λ) of these cytochrome c552 variants is experimentally determined using a combination of rotating disc electrochemistry, chronoamperometry and cyclic voltammetry. The differences between the λ determined from these complementary techniques helps to deconvolute the contribution of the active site and its immediate environment to the overall λ (λTotal). The experimentally determined λ values in conjunction with DFT calculations indicate that the differences in λ among the protein variants are mainly due to the differences in contributions from the protein environment and not just inner-sphere λ. DFT calculations indicate that the position of residue 64, responsible for the orientation of the axial methionine, determines the geometric relaxation of the redox active molecular orbital (RAMO). The orientation of the RAMO with respect to the heme is key to determining electron transfer coupling (HAB) which results in higher ET rates in the wild-type protein relative to the Q64V mutant despite a 150 mV higher λTotal in the former. Efficient delocalization of the redox-active molecular orbital (RAMO) in HtWT results in an increase in HAB value which in turn accelerates the electron transfer (ET) rate in spite of the higher reorganization energy (λ) than the HtQ64V mutant.

SUBMITTER: Chattopadhyay S

PROVIDER: S-EPMC8442690 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Superparamagnetic iron oxide nanoparticles, MNPs, are under investigation as stimulus-responsive nanocarriers that can be tracked by magnetic resonance imaging. However, fundamental questions remain, including the effect of differing surface chemistries on MR image contrast efficacy (relaxivity), both initially and over time in the biological environment. The effects of pH and ligand type on the relaxivity of electrostatically and sterically stabilized spherical 8.8 nm superparamagnetic MNP suspensions are described. It is shown for the first time that across the pH ranges, within which the particles are fully dispersed, increasing acidity progressively reduces relaxivity for all ligand types. This effect is stronger for electrostatically (citrate or APTES) than for sterically stabilized (PEG5000) MNPs. NMR relaxation profiles (relaxivity as a function of 1H Larmor frequency) identified an inner-sphere effect, arising from the protonation of bare oxide or low-molecular-weight-bound species, as the cause. The suppression is not accounted for by the accepted model (SPM theory) and is contrary to previous reports of increased relaxivity at lower pH for paramagnetic iron oxide nanoparticles. We propose that the suppression arises from the orientation of water molecules, with the oxygen atom facing the surface increasingly preferred with increasing surface protonation. For APTES-stabilized MNPs, pendant amines and the silane layer confer exceptional chemical and colloidal stability at low pH. Dissolution of these particles at pH 1.8 was monitored over several months by combining in situ measurements of relaxation profiles with dynamic light scattering. It was shown that particles are magnetically intact for extended periods until they rapidly dissolve, once the silane layer is breached, in a process that is apparently second order in particle concentration. The findings are of interest for tracking MNP fate, for quantitation, and for retention of magnetic responsiveness in biological settings.

Dataset Information

Contributions to cytochrome c inner- and outer-sphere reorganization energy† † Electronic supplementary information (ESI) available. See DOI: 10.1039/d1sc02865k

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