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Molecular mechanisms underlying the repair of nitrosylated [Fe-S] clusters by the microbial protein YtfE remain poorly understood. The X-ray crystal structure of YtfE, in combination with EPR, magnetic circular dichroism (MCD), UV, and (17) O-labeling electron spin echo envelope modulation measurements, show that each iron of the oxo-bridged Fe(II) -Fe(III) diiron core is coordinatively unsaturated with each iron bound to two bridging carboxylates and two terminal histidines in addition to an oxo-bridge. Structural analysis reveals that there are two solvent-accessible tunnels, both of which converge to the diiron center and are critical for capturing substrates. The reactivity of the reduced-form Fe(II) -Fe(II) YtfE toward nitric oxide demonstrates that the prerequisite for N2 O production requires the two iron sites to be nitrosylated simultaneously. Specifically, the nitrosylation of the two iron sites prior to their reductive coupling to produce N2 O is cooperative. This result suggests that, in addition to any repair of iron centers (RIC) activity, YtfE acts as an NO-trapping scavenger to promote the NO to N2 O transformation under low NO flux, which precedes nitrosative stress.

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The coupling of one-carbon (C]) fragments to form carbon-carbon bonds has been studied on a Cu(11O) surface. In these studies, methyl (CH 3 ) and methylene (CH 2 ) groups have been generated on Cu( 110) by the dissociative adsorption of CH3I and CH 2 I 2 , respectively. Formation of CH 3 (a) below 200 K on this surface is inferred from the lack of molecular desorption as well as the lack of recombinative hydrogen desorption in temperature-programmed reaction (TPR) experiments. Similar low temperature C-I bond dissociation in CH 2 I 2 to form CH 2 (a) is implicated based on the evolution of ethylene at 300 K in TPR studies. By studying the reactions of CD 3 (a) and CH 2 (a) coadsorbed and adsorbed separately on Cu( 110), three C-C bond forming reactions have been identified: methyl coupling above 400 K to form ethane, methylene coupling at -300 K to form ethylene, and methyl! methylene coupling (methylene insertion) at 300-350 K to produce ethyl groups. To our knowledge, this is the first time that methylene insertion, a potentially important chain growth step in hydrocarbon syntheses, has been definitively established on metal surfaces.