Radical reactions of nitric oxide synthases

NOSs (nitric oxide synthases) are flavohaem enzymes that function broadly in human health and disease. We are combining mutagenesis, crystallographic and rapid kinetic methods to understand their mechanism and regulation. The NOSs create a transient tetrahydrobiopterin radical within the enzyme to generate their free radical product (NO). Recent work is revealing how critically important this process is at all levels of catalysis. This article will synthesize four seemingly disparate but related aspects of NOS tetrahydrobiopterin radical formation: (i) how it enables productive O2 activation by providing an electron to the enzyme haem, (ii) what structural features help to regulate this electron transfer, (iii) how it enables NOS to synthesize NO from its diamagnetic substrate and (iv) how it allows NOS to release NO after each catalytic cycle instead of other nitorgen oxide-containing products.

Electron paramagnetic resonance spectra of some sulfur-containing radicals

The e.p.r. spectra from the low temperature matrix photolysis of several mercaptans, disulfides, and cyclic sulfides have been recorded and compared to those obtained in the photolysis of these materials adsorbed on a porous Vycor glass. The spectra of the adsorbed radicals are characterized by an enhanced thermal stability and a shift of the highest sulfur g-value to higher fields. The only free radical product in the photolysis of mercaptans and disulfides is the thiyl radical. The cyclic sulfides [Formula: see text] appear to yield•CH2—(CH2)n—S•type diradicals, while the cyclic disulfide [Formula: see text][Formula: see text] gives the [Formula: see text] diradical.