Sharon Burgmayer

This work provides the first extensive study of the redox reactivity of the pyranopterin system that is a component of the catalytic site of all molybdenum and tungsten enzymes possessing molybdopterin. The pyranopterin system possesses certain characteristics typical of tetrahydropterins, such as a reduced pyrazine ring; however, it behaves as a dihydropterin in redox reactions with oxidants. Titrations using ferricyanide and dichloroindophenol (DCIP) prove a 2e−/2H+ stoichiometry for pyranopterin oxidations. Oxidations of pyranopterin by Fe(CN)63− or DCIP are slower than tetrahydropterin oxidation under a variety of conditions, but are considerably faster than observed for oxidations of dihydropterin. The rate of pyranopterin oxidation by DCIP was studied in a variety of media. In aqueous buffered solution the pyranopterin oxidation rate has minimal pH dependence, whereas the rate of tetrahydropterin oxidation decreases 100-fold over the pH range 7.4–8.5. Although pyranopterin reacts as a dihydropterin with oxidants, it resists further reduction to a tetrahydropterin. No reduction was achieved by catalytic hydrogenation, even after several days. The reducing ability of the commonly used biological reductants dithionite and methyl viologen radical cation was investigated, but experiments showed no evidence of pyranopterin reduction by any of these reducing agents. This study illustrates the dual personalities of pyranopterin and underscores the unique place that the pyranopterin system holds in the spectrum of pterin redox reactions. The work presented here has important implications for understanding the biosynthesis and reaction chemistry of the pyranopterin cofactor in molybdenum and tungsten enzymes.

The interactions of five bis(bipyridyl) Ru(II) complexes of pteridinyl-phenanthroline ligands with calf thymus DNA have been studied. The pteridinyl extensions were selected to provide hydrogen-bonding patterns complementary to the purine and pyrimidine bases of DNA and RNA. The study includes three new complexes [Ru(bpy)2(L-pterin)]2+, [Ru(bpy)2(L-amino)]2+, and [Ru(bpy)2(L-diamino)]2+ (bpy is 2,2′-bipyridine and L-pterin, L-amino, and L-diamino are phenanthroline fused to pterin, 4-aminopteridine, and 2,4-diaminopteridine), two previously reported complexes [Ru(bpy)2(L-allox)]2+ and [Ru(bpy)2(L-Me2allox)]2+ (L-allox and L-Me2allox are phenanthroline fused to alloxazine and 1,3-dimethyalloxazine), the well-known DNA intercalator [Ru(bpy)2(dppz)]2+ (dppz is dipyridophenazine), and the negative control [Ru(bpy)3]2+. Reported are the syntheses of the three new Ru–pteridinyl complexes and the results of calf thymus DNA binding experiments as probed by absorption and fluorescence spectroscopy, viscometry, and thermal denaturation titrations. All Ru–pteridine complexes bind to DNA via an intercalative mode of comparable strength. Two of these four complexes—[Ru(bpy)2(L-pterin)]2+ and [Ru(bpy)2(L-allox)]2+—exhibit biphasic DNA melting curves interpreted as reflecting exceptionally stable surface binding. Three new complexes—[Ru(bpy)2(L-diamino)]2+, [Ru(bpy)2(L-amino)]2 and [Ru(bpy)2(L-pterin)]2+—behave as DNA molecular “light switches.”