Reductive nitrosylation aquacobalamin

NO adds reversibly to reduced cobalamin, Cbl(II).156 It does not react directly with aquacobalamin(III), (0blni(H2O)), but it does add to Cbl,n(N02 ) and Cblm(NO).175 Acid hydrolysis of the dinitroso species releases nitrite, and binding of nitrite to Cblln(H20) generates Cbln,(. 02 ). This sequence thus affords a nitrite-catalyzed mechanism for NO substitution at Cblln(H20). The reaction of NO with Com porphyrins is quite complex.176 In the first step, NO displaces an axial water ligand to form a weakly bound mono NO complex this mono NO complex reacts with a second molecule of NO to form nitrite and a reduced Co-NO complex. This latter process is called reductive nitrosylation. Manganese(II) porphyrins bind NO very rapidly.177 Stability constants have been measured for the formation of mono and bis NO complexes of Cun(dithiocarbamate)2.157... 

Scheme 13 Proposed mechanism for reductive nitrosylation of aquacobalamin at low pH (129).

In summary, the results of this study enabled to draw several important conclusions regarding reductive nitrosylation reactions of aquacobalamin (i) Cbl(H20) cannot react with NO even at low pH (ii) the active species toward NO is Cbl(N02 ) and not Cbl(H20) (iii) the kinetics of reductive nitrosylation of aquacobalamin is dominated by its back reaction, and this is the reason for its strange NO and HNO2 concentration dependences (iv) the mechanism of reductive nitrosylation of Cbl(H20) is similar to that of Co porphyrins, but completely different from that known for the Fe analogues and (v) the high instabihty of the Co -(NO) complexes seems to be responsible for the observed distinctive reactivity behavior and also explains why Cbl(H20) does not react with nitric oxide. 

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