Synthetic Modeling of Nitrite Binding and Activation by Reduced Copper Proteins. Characterization of Copper(I)−Nitrite Complexes That Evolve Nitric Oxide

Abstract

In an effort to provide precedence for postulated intermediates in copper-protein-mediated nitrite reduction, a series of novel complexes containing the Cu I -NO 2 -unit, including monocopper(I), dicopper(I,I), and mixed valence dicopper(I,II) and copper(I)-zinc(II) species, were prepared, fully characterized, and subjected to reactivity studies designed to probe their ability to produce nitric oxide. Treatment of solutions of [LCu(CH 3 CN)]PF 6 (L ) L i-Pr 3 , 1,4,7-triisopropyl-1,4,7-triazacyclononane, or L Bn 3 , 1,4,7-tribenzyl-1,4,7-triazacyclononane) in MeOH with excess NaNO 2 yielded the novel dicopper(I,I) complexes [(LCu) 2 (µ-NO 2 )]PF 6 . The complex with L ) L i-Pr 3 was cleaved by PPh 3 to afford [L i-Pr 3 Cu(PPh 3 )]PF 6 and L i-Pr 3 Cu(NO 2 ), a structural model for the substrate adduct of copper nitrite reductase. Oxidation of the dicopper(I,I) compound (L ) L i-Pr 3 ) with (Cp 2 Fe)(PF 6 ) in CH 2 Cl 2 yielded the deep red, mixed valent, dicopper(I,II) species [(L i-Pr 3 Cu) 2 (µ-NO 2 )](PF 6 ) 2 , which was structurally characterized as its [B(3,5-(CF 3 ) 2 C 6 H 3 ) 4 ] -salt (crystal data: triclinic space group P1 h, a ) 13.439(8) Å, b ) 13.777(5) Å, c ) 14.471(8) Å, R ) 108.22(4)°, β ) 92.08(5)°, γ ) 90.08(4)°, Z ) 1, T ) 177 K, R ) 0.074, and R w ) 0.070). A diamagnetic heterodinuclear Cu I Zn II analog, [L i-Pr 3 Cu(µ-NO 2 )ZnL i-Pr 3 ](O 3 SCF 3 ) 2 , was assembled by mixing L i-Pr 3 Cu(NO 2 ), Zn(O 3 -SCF 3 ) 2 , and L i-Pr 3 and was shown to adopt a structure similar to that of its Cu I Cu II relative (crystal data: monoclinic space group P2 1 /c, a ) 10.8752(1) Å, b ) 15.6121(3) Å, c ) 25.8020(5) Å, β ) 90.094(1)°, Z ) 4, R1 ) 0.0472, and wR2 ) 0.1082). Both compounds exhibit an intense electronic absorption feature that was assigned as a Cu I f NO 2 -MLCT transition on the basis of resonance Raman spectroscopic results.

Redox-coupled proton transfer mechanism in nitrite reductase revealed by femtosecond crystallography

“…The difference of the NO 2 − coordination modes between Cu(I) and Cu(II) is not surprising, because model complexes of Cu(I)-NO 2 − generally show an N-coordination (7, 28, 50-52), not the O-coordination observed in Cu(II)-NO 2 − . Our present data, however, did not show a rearrangement from the O-coordination to the N-coordination, which was expected by model complexes and computational chemistry (28,(50)(51)(52)(53).…”

Structure of Nitrite Bound to Copper-containing Nitrite Reductase from Alcaligenes faecalis

“…A similar mechanism has been proposed for copper NIR whereby nitrite is N-coordinated to a reduced copper followed by protonation, dehydration and production of a copper nitrosyl (Cu ϩ -NO ϩ ) intermediate (8). Model compounds of NIR with both N-coordinate bound to Cu(I) and O-coordinate nitrite (12)(13)(14) bound to Cu(I) and Cu(II) have been characterized. Protonation of the nitrite adduct of the N-bound Cu(I) compound in nonaqueous solution results in the production of NO (12).…”

Spectroscopic Characterization of Reaction Intermediates in a Model for Copper Nitrite Reductase

“…[2][3][4] In this mechanism, the copper(i) nitrite complex is a key intermediate in the reduction by NiR. While there are numerous reports of copper nitrite complexes, [5] the first and only structurally characterized model of the copper(i) nitrite complex of NiR, namely, [(iPr 3 tacn)Cu I -(NO 2 )] (1, iPr 3 tacn = 1,4,7-triisopropyl-1,4,7-triazacyclononane), was reported by Tolman et al [6,7] Compound 1 is a good functional model of NiR and reacts with two equivalents of acetic acid to yield NO and a copper(ii) acetate complex, [(iPr 3 tacn)Cu II (O 2 CCH 3 ) 2 ] (2 a, Scheme 2). The structure of the nitrite complex of reduced NiR was recently determined by incubating nitrite with crystals of reduced NiR at low temperature.…”

“…A previous study reported that the reaction of 1 with trimethylsilyl triflate (Me 3 SiO 3 SCF 3 ), a proton equivalent, initially yielded a mixed-valent dinuclear copper(i,ii) nitrite complex as the result of a side reaction of 1 and the copper(ii) reactant, [7] but the absorption spectrum of the new species is different from that of the dinuclear copper(i,ii) nitrite complex. The time course for the reaction, monitored at 290, 423, and 680 nm, is poorly described by a singleexponential function, but better fitted by a biexponential function with k fast and k slow ( Figure 1 and Supporting Information).…”