Model radicals

Radical metalloenzymes is a rapidly growing class of enzymes that use protein radicals in their catalytic cycle. Progress in identification of transient radical species depends, in part, on availability of corresponding model radicals and knowledge of their characteristic properties. We are interested in characterization (both structural and kinetic) of protein radical species in order to understand their role in catalysis. This goal requires developing of simpler models of likely radical species and study of their signature spectroscopic properties.

Sample of a model radical in glowing blue as it is being photolyzed by UV light at 140^oK. Radicals are stable for days at this temperature.

We are working to solve a number of chemical and spectroscopic challenges in characterization of small model radicals. Mobility of isolated free-base radicals is the key factor in controlling their reactivity. We are developing methods of immobilization of radical precursors as a way to prevent cross-reativity of radicals in solution. Immobilization is expected to extend the lifetime of radical species so that their spectroscopic properties can be extensively studies. We are using traditional methods of immobilization in frozen glasses, as well as developing approaches of immobilization on the surfaces at ambient temperatures. Currently we are developing methods of immobilization of hydrophilic carboxylates on structured zirconium phosphate and exploring the possibility of using phospholipid bilayers as a universal substrate for immobilization of molecules from free base amino acids to peptides and proteins. Both methods of immobilization have broad compatibility with spectroscopic and analytical methods.

Studies on isolated radical models are extended and complemented by investigating relatively small and simple protein samples, which range from artificial peptides to relatively well characterized enzymes and proteins such as globins, cytochromes, peroxidases etc. By looking at protein radicals in relatively well characterized systems we can learn, for example, how spectral signatures of radicals are responding to variations in local environment (geometry, protonation, hydrogen bonding etc.)

Development of chemical approaches toward model radical systems is complemented by development of new spectroscopic approaches. Currently our special interest is with far UV absorption spectroscopy (<300 nm), which is a promising extension of visible absorption and spin resonance studies. Until now far UV absorption in proteins in general and by radicals in particular remained largely unexplored.