Marked changes in electron transport through the blue copper protein azurin in the solid state upon deuteration

TL;DR

This study investigates how deuteration affects electron transport through azurin in the solid state, revealing a temperature-dependent shift and high kinetic isotope effects that shed light on the underlying transport mechanism.

Contribution

It demonstrates that deuteration changes the temperature dependence of electron transport in azurin, providing new insights into the molecular mechanisms involved.

Findings

Deuteration induces a transition from temperature-independent to temperature-dependent ETp above 180K.

KIE values range from 1.8 at 340K to 9.1 at 180K, indicating a significant isotope effect.

Results suggest a transport mechanism involving through-bond pathways in the protein's structure.

Abstract

Measuring electron transport (ETp) across proteins in the solid-state offers a way to study electron transfer (ET) mechanism(s) that minimizes solvation effects on the process. Solid state ETp is sensitive to any static (conformational) or dynamic (vibrational) changes in the protein. Our macroscopic measurement technique extends the use of ETp meas-urements down to low temperatures and the concomitant lower current densities, because the larger area still yields measurable currents. Thus, we reported previously a surprising lack of temperature-dependence for ETp via the blue copper protein azurin (Az), from 80K till denaturation, while ETp via apo-(Cu-free) Az was found to be temperature de-pendent \geq 200K. H/D substitution (deuteration) can provide a potentially powerful means to unravel factors that affect the ETp mechanism at a molecular level. Therefore, we measured and report here the kinetic deuterium isotope effect (KIE) on ETp through holo-Az as a function of temperature (30-340K). We find that deuteration has a striking effect in that it changes ETp from temperature independent to temperature dependent above 180K. This change is expressed in KIE values between 1.8 at 340K and 9.1 at \leq 180K. These values are particularly remarkable in light of the previously reported inverse KIE on the ET in Az in solution. The high values that we obtain for the KIE on the ETp process across the protein monolayer are consistent with a transport mechanism that involves through-(H-containing)-bonds of the {\beta}-sheet structure of Az, likely those of am-ide groups.