Electron transfer with myoglobin in free and strongly confined regimes: disclosing diverse mechanistic role of the Fe-coordinated water by temperature- and pressure-assisted voltammetric studies

Abstract

The naturally occurring electron-transfer (ET) event for myoglobin (Mb) can be mimicked through its functionalization at diversely modified metal platforms to allow for the electron exchange either in freely diffusing or immobilized regimes. In this work, horse muscle Mb was involved in the electron exchange with Au electrodes modified by dissimilar, thin or thick alkanethiol SAMs, terminated either by unicomponent (–OH) or 1 : 1 mixed (–OH/–COOH) functional (externally exposed) entities, respectively. The systematic, temperature- and pressure-supported cyclic voltammetry studies perfectly confirmed certainty of two kinds of ET patterns for Mb, embodying: (a) different operational kinetic regimes (including protein’s freely diffusing and strongly confined motifs) and (b) different intrinsic physical mechanisms (including dynamically controlled and non-adiabatic modes). Our analysis of obtained and published data for Mb and the reference redox-active protein, cytochrome c, specified further the central mechanistic role of the Fe-(heme-)coordinated water whose displacement is directly coupled to ET, and can be, in turn, controlled by the conformational organization and intrinsic fluctuational mobility of the Mb macromolecule.

Graphical Abstract

Keywords:

• Met-myoglobin
• Iron-coordinated water
• Self-assembled monolayer films
• Protein film voltammetry
• Freely diffusing and confined regimes
• Long- and short-range electron transfer
• Non-adiabatic and dynamically controlled mechanisms
• Key role of the water translocation

Acknowledgements

Research grants from the Alexander von Humboldt Foundation, Germany (in the framework of the “Research Group Linkage Program”, 3.4-IP-SER/1070196; 2014-2016), Volkswagen Foundation, Germany (I-85 642; 2011-2013), Shota Rustaveli National Science Foundation, Georgia (FR/771/7-230/11; 2012-2015) are kindly acknowledged.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

Dedicated to Professor Rudi van Eldik on the occasion of his 70th birthday.

Additional information

Funding

This work was supported by the Alexander von Humboldt-Stiftung [3.4-IP-SER/1070196]; Shota Rustaveli National Science Foundation [FR/771/7-230/11]; Volkswagen Foundation [I-85 642].