Urea Decomposition Mechanism by Dinuclear Nickel Complexes.

Area of Science:

• Bioinorganic Chemistry
• Enzyme Mimicry
• Nickel Coordination Complexes

Background:

• Urease enzyme catalyzes urea hydrolysis via a dinuclear nickel active site.
• Inorganic models with similar active sites do not replicate urease's mechanism.
• The discrepancy in urea decomposition pathways remains unexplained.

Purpose of the Study:

• To synthesize and investigate bis-nickel complexes as biomimetic models of urease.
• To elucidate the mechanistic differences between urease and its inorganic models.
• To evaluate the role of reaction intermediates in urea hydrolysis.

Main Methods:

• Synthesis of two bis-nickel complexes ([Ni2L(OAc)] and [Ni2L(Cl)(Et3N)2]) with varying ligand lability.
• Experimental and theoretical evaluation of reaction intermediates, including the aquo intermediate.
• Kinetic studies comparing ammonia production rates of complexes and urease.
• Substrate reaction analysis to assess isocyanate hydrolysis.

Main Results:

• Both synthesized nickel complexes produced ammonia from urea, but at rates significantly slower than urease.
• The aquo intermediate was identified as crucial, facilitating outer-sphere coordination of urea.
• Isocyanate formation was observed, indicating an activated water molecule acting as a base.
• Biomimetic complexes demonstrated hydrolysis of isocyanate.

Conclusions:

• Outer-sphere complex formation in urease analogues may explain the mechanistic divergence from the native enzyme.
• The study provides insights into the unique catalytic mechanism of urease.
• The synthesized complexes serve as valuable tools for understanding metalloenzyme function.