Spin Delocalization Over Type Zero Copper

Abstract

Hard-ligand, high-potential copper sites have been characterized in double mutants of Pseudomonas aeruginosa azurin (C112D/M121X (X = L, F, I)). These sites feature a small A_(zz)(Cu) splitting in the EPR spectrum together with enhanced electron transfer activity. Due to these unique properties, these constructs have been called "type zero" copper sites. In contrast, the single mutant, C112D, features a large A_(zz)(Cu) value characteristic of the typical type 2 Cu^(II). In general, A_(zz)(Cu) comprises contributions from Fermi contact, spin dipolar, and orbital dipolar terms. In order to understand the origin of the low A_(zz)(Cu) value of type zero Cu^(II), we explored in detail its degree of covalency, as manifested by spin delocalization over its ligands, which affects A_(zz)(Cu) through the Fermi contact and spin dipolar contributions. This was achieved by the application of several complementary EPR hyperfine spectroscopic techniques at X- and W-band (~9.5 and 95 GHz, respectively) frequencies to map the ligand hyperfine couplings. Our results show that spin delocalization over the ligands in type zero Cu^(II) is different from that of type 2 Cu^(II) in the single C112D mutant. The ^(14)N hyperfine couplings of the coordinated histidine nitrogens are smaller by about 25–40%, whereas that of the ^(13)C carboxylate of D112 is about 50% larger. From this comparison, we concluded that the spin delocalization of type zero copper over its ligands is not dramatically larger than in type 2 C112D. Therefore, the reduced A_(zz)(Cu) value of type zero Cu^(II) is largely attributable to an increased orbital dipolar contribution that is related to its larger g_(zz) value, as a consequence of the distorted tetrahedral geometry. The increased spin delocalization over the D112 carboxylate in type zero mutants compared to type 2 C112D suggests that electron transfer paths involving this residue are enhanced.

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