  Electrons can migrate via proteins over distances that are considered long
for non-conjugated systems. Proteins' nano-scale dimensions and the enormous
flexibility of their structures and chemistry makes them fascinating subjects
for investigating the mechanism of their electron transport (ETp) capacity. One
particular attractive research direction is that of tuning their ETp efficiency
by doping them with external small molecules. Here we report that solid-state
ETp across human serum albumin (HSA) increases by more than two orders of
magnitude upon retinoate (RA) binding to HSA. RA was chosen because optical
spectroscopy has provided evidence for the non-covalent binding of at least
three RA molecules to HSA and indications for their relative structural
positions. The temperature dependence of ETp shows that both the activation
energy and the distance-decay constant decrease with increasing RA binding to
HSA. Furthermore, the observed transition from temperature-activated ETp above
190K to temperature-independent ETp below this temperature suggests a change in
the ETp mechanism with temperature.
