Mechanisms of HNO Reactions with Ferric Heme Proteins

Mechanistic Origin of Favorable Substituent Effects in Excellent Cu Cyclam Based HNO Sensors

HNO has broad chemical and biomedical properties. Metal complexes and derivatives are widely used to make excellent HNO sensors. However, their favorable mechanistic origins are largely unknown. Cu cyclam is a useful platform to make excellent HNO sensors including imaging agents. A quantum chemical study of Cu cyclams with various substitutions was performed, which reproduced diverse experimental reactivities. Structural, electronic, and energetic profiles along reaction pathways show the importance of HNO binding and a proton-coupled electron transfer mechanism for HNO reaction. Results reveal that steric effect is primary and electronic factor is secondary (if the redox potential is sufficient), but their interwoven effects can lead to unexpected reactivity, which looks mysterious experimentally but can be explained computationally. This work suggests rational substituent design ideas and recommends a theoretical study of a new design to save time and cost due to its subtle effect.

HNO to NO Conversion Mechanism with Copper Zinc Superoxide Dismutase, Comparison with Heme Protein Mediated Conversions, and the Origin of Questionable Reversibility

The interconversion of NO and HNO, via copper zinc superoxide dismutase (CuZnSOD), is important in biomedicine and for HNO detection. Many mechanistic questions, including the decades-long debate on reversibility, were resolved in this work. Calculations of various active-site and full-protein models show that the basic mechanism is proton-coupled electron transfer with a computed barrier of 10.98 kcal mol^-1 , which is in excellent agreement with experimental results (10.62 kcal mol^-1 ), and this nonheme protein-mediated reaction has many significant mechanistic differences compared with the conversions mediated by heme proteins due to geometric and electronic factors. The reasons for the irreversible nature of this conversion and models with the first thermodynamically favorable and kinetically feasible mechanism for the experimental reverse reaction were discovered. Such results are the first for nonheme enzyme mediated HNO to NO conversions, which shall facilitate other related studies and HNO probe development.