  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.
