Electron tunneling in single crystals of Pseudomonas aeruginosa azurins.
Rates of reduction of Os(III), Ru(III), and Re(I) by Cu(I) in His83-modified Pseudomonas aeruginosa azurins (M-Cu distance approximately 17 A) have been measured in single crystals, where protein conformation and surface solvation are precisely defined by high-resolution X-ray structure determinations: 1.7(8) x 10(6) s(-1) (298 K), 1.8(8) x 10(6) s(-1) (140 K), [Ru(bpy)2(im)(3+)-]; 3.0(15) x 10(6) s(-1) (298 K), [Ru(tpy)(bpy)(3+)-]; 3.0(15) x 10(6) s(-1) (298 K), [Ru(tpy)(phen)(3+)-]; 9.0(50) x 10(2) s(-1) (298 K), [Os(bpy)2(im)(3+)-]; 4.4(20) x 10(6) s(-1) (298 K), [Re(CO)3(phen)(+)] (bpy = 2,2'-bipyridine; im = imidazole; tpy = 2,2':6',2' '-terpyridine; phen = 1,10-phenanthroline). The time constants for electron tunneling in crystals are roughly the same as those measured in solution, indicating very similar protein structures in the two states. High-resolution structures of the oxidized (1.5 A) and reduced (1.4 A) states of Ru(II)(tpy)(phen)(His83)Az establish that very small changes in copper coordination accompany reduction but reveal a shorter axial interaction between copper and the Gly45 peptide carbonyl oxygen [2.6 A for Cu(II)] than had been recognized previously. Although Ru(bpy)2(im)(His83)Az is less solvated in the crystal, the reorganization energy for Cu(I) --> Ru(III) electron transfer falls in the range (0.6-0.8 eV) determined experimentally for the reaction in solution. Our work suggests that outer-sphere protein reorganization is the dominant activation component required for electron tunneling.
Crane
B R
BR
Beckman Institute, California Institute of Technology, Pasadena, California 91125, USA.
Di Bilio
A J
AJ
Winkler
J R
JR
Gray
H B
HB
D001400:Azurin / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D003300:Copper / Q000737:chemistry / Q000378:metabolism; D003460:Crystallization; D018360:Crystallography, X-Ray; D004579:Electron Transport; D008958:Models, Molecular; D009992:Osmium / Q000737:chemistry / Q000378:metabolism; D010084:Oxidation-Reduction; D011487:Protein Conformation; D011550:Pseudomonas aeruginosa / Q000737:chemistry / Q000378:metabolism*; D012211:Rhenium / Q000737:chemistry / Q000378:metabolism; D012428:Ruthenium / Q000737:chemistry / Q000378:metabolism
D001400:Azurin; D009992:Osmium; D012211:Rhenium; D003300:Copper; D012428:Ruthenium