Selective metal-ion incorporation and ligand substitution are employed to control whether electrons tunnel over intra-or intermolecular separations in crystals of P. aeruginosa azurin modified with Ru-polypyridine complexes. Cu1+-to-Ru3+ electron transfer (ET) across a specific protein-protein interface in the crystal lattice has a time constant 5-10 times longer than ET between the same donor and acceptor within a single protein (tau(ET) = 5 vs 0.5-1.0 mu s). Slower intermolecular ET agrees well with a longer distance between redox centers across the inter-protein (18.9 angstrom) compared to the intra-protein separation (17.0 angstrom) and indicates that the closest donor/acceptor pair dominates crystal ET. Lowering the crystal pH accelerates inter-protein ET (tau(ET) = 1.0 mu s) but not intra-protein ET. Faster inter-protein ET likely results from a pH-induced peptide bond flip that perturbs hydrogen bonding in the path between Ru and Cu centers on adjacent molecules.