3.2.1. Azurin

*Pseudomonas aeruginosa* azurin is a simple blue copper protein that undergoes single-ET between the Type 1 copper atom and the electrode, now developed as a single-molecule "core" target [6,132–136], as for cyt *c* characterized and mapped in unique detail. A hydrophobic patch and the disulfide group at opposite ends of the azurin molecule are both critical for well-defined orientations on the electrode. Chi, Ulstrup, Zhang and associates conducted extensive investigations of the electrochemical behavior of azurin [39,43,51,137]. *In situ* STM disclosed arrays of well-organized self-assembled azurin monolayers on single-crystal Au(111)-electrodes mapped to single-molecule resolution [51]. Hydrophobic alkanethiol monolayers were employed for gentle immobilization by hydrophobic interactions with the hydrophobic patch of azurin [43]. Notably, exponential decay of the ET rate constant with increasing chain length was observed for chain lengths longer than six carbon atoms, reflecting a dual mechanism, with tunneling dominating for the longer chains [39]. The ET kinetics and redox mechanism of azurin have been analyzed theoretically at different levels, as discussed in detail elsewhere [6,138]. Inserting 3–4 nm coated AuNPs as for horse heart cyt *c* [83] results in a 20-fold enhancement of *kapp* (220 ± 16 s−<sup>1</sup> ) for azurin compared with the AuNP-free system (10.2 ± 0.4 s−<sup>1</sup> ) [51]. Armstrong and associates compared the ET kinetics of azurin on the electrode modified with a synthetic {3,5-diethoxy-4-[(E)-2-(4-ethylphenyl)vinyl]-phenyl} methanethiol or commercial-CH<sup>3</sup> terminal alkanethiol [139]. The front hydrophobic ethyl group of SAMs served as the protein-binding bridge on the surface, yielding a very fast ET rate with a *kapp* over 1600 s−<sup>1</sup> .
