Spectrophotometric methods for the determination of osmium-III Reaction of osmium tetroxide with 1:5-diphenylcarbo- hydrazide in aqueous solutions followed by extraction of the complex

-A spectrophotometric method has been developed which is applicable to the determination of extremely small quantities of osmium. Osmium is oxidised to the octovalent state, then added to an acidic aqueous solution containing 1:5-diphenylcarbohydrazide (DPC). After heating the aqueous solution to 65°, the osmium-DPC complex is extracted with chloroform. A molar absorbancy index of about 150,000 is obtained. From 7 to 25 μg of osmium can be determined with a coefficient of variation of 6%. It was established that Fe^III, Cu^II, Ru^III and Au^III seriously interfere in the determination of osmium by this method, while Cr^VI, Ni^II, Mo^VI, Ir^III and chloride interfere only when present in relatively high concentrations.     

Immobilizing a Fluorescent Dye Offers Potential to Investigate the Glass/Resin Interface

Silane coupling agents are commonly applied to glass fibers to promote fiber/resin adhesion and enhance durability in composite parts. In this study, a coupling agent multilayer on glass was doped with trace levels of the dimethylaminonitrostilbene (DMANS) fluorophore. The fluorophore was immobilized on the glass surface by tethering the molecule to a triethoxy silane coupling agent, creating the DMANS/silane coupling agent molecule (DMSCA). DMSCA was then diluted with commonly used coupling agents and grafted to a glass microscope coverslip to create a model composite interface. A 53-nm blue shift in fluorescence from the immobilized DMSCA can be followed during cure of an epoxy resin overlayer, giving this technique potential to monitor the properties of the fiber/resin interface during composite processing. Contact angle measurements on these coupling agent layers were similar in the presence or absence of the DMSCA molecule, suggesting that trace levels of the fluorescent probe did not affect the structure of the layer. The immobilized DMSCA molecule behaved similarly to the DMANS precursor in solution. Both showed longer wavelength fluorescence in more polar environments. Copyright 2000 Academic Press.     

Kinetics and mechanism of *NO2 reacting with various oxidation states of myoglobin

Nitrogen dioxide ((*)NO(2)) participates in a variety of biological reactions. Of great interest are the reactions of (*)NO(2) with oxymyoglobin and oxyhemoglobin, which are the predominant hemeproteins in biological systems. Although these reactions occur rapidly during the nitrite-catalyzed autoxidation of hemeproteins, their roles in systems producing (*)NO(2) in the presence of these hemeproteins have been greatly underestimated. In the present study, we employed pulse radiolysis to study directly the kinetics and mechanism of the reaction of oxymyoglobin (MbFe(II)O(2)) with (*)NO(2). The rate constant of this reaction was determined to be (4.5 +/- 0.3) x 10(7) M(-1)s(-1), and is among the highest rate constants measured for (*)NO(2) with any biomolecule at pH 7.4. The interconversion among the various oxidation states of myoglobin that is prompted by nitrogen oxide species is remarkable. The reaction of MbFe(II)O(2) with (*)NO(2) forms MbFe(III)OONO(2), which undergoes rapid heterolysis along the O-O bond to yield MbFe(V)=O and NO(3-). The perferryl-myoglobin (MbFe(V)=O) transforms rapidly into the ferryl species that has a radical site on the globin ((*)MbFe(IV)=O). The latter oxidizes another oxymyoglobin (10(4) M(-1)s(-1) < k(17) < 10(7) M(-1)s(-1)) and generates equal amounts of ferrylmyoglobin and metmyoglobin. At much longer times, the ferrylmyoglobin disappears through a relatively slow comproportionation with oxymyoglobin (k(18) = 21.3 +/- 5.3 M(-1)s(-1)). Eventually, each (*)NO(2) radical converts three oxymyoglobin molecules into metmyoglobin. The same intermediate, namely MbFe(III)OONO(2), is also formed via the reaction peroxynitrate (O(2)NOO(-)/O(2)NOOH) with metmyoglobin (k(19) = (4.6 +/- 0.3) x 10(4) M(-1)s(-1)). The reaction of (*)NO(2) with ferrylmyoglobin (k(20) = (1.2 +/- 0.2) x 10(7) M(-1)s(-1)) yields MbFe(III)ONO(2), which in turn dissociates (k(21) = 190 +/- 20 s(-1)) into metmyoglobin and NO(3-). This rate constant was found to be the same as that measured for the decay of the intermediate formed in the reaction of MbFe(II)O(2) with (*)NO, which suggests that MbFe(III)ONO(2) is the intermediate observed in both processes. This conclusion is supported by thermokinetic arguments. The present results suggest that hemeproteins may detoxify (*)NO(2) and thus preempt deleterious processes, such as nitration of proteins. Such a possibility is substantiated by the observation that the reactions of (*)NO(2) with the various oxidation states of myoglobin lead to the formation of metmyoglobin, which, though not functional in the gas transport, is nevertheless nontoxic at physiological pH.     

(Z)-3-p-tolylsulfinylacrylonitrile as a chiral dienophile: diels-alder reactions with furan and acyclic dienes

The behavior of (Z)-3-p-tolylsulfinylacrylonitrile (1) as a chiral dienophile has been evaluated from its reactions with furan and acyclic dienes. Electrostatic interactions of the cyano group with the sulfinyl one restrict the conformational mobility around the C-S bond, thus controlling the pi-facial selectivity, which is almost complete in all cases, the approach of the diene from the less-hindered face of the dienophile (that bearing the lone electron pair) in the predominant rotamer being the favored one. The regioselectivity is also completely controlled by the cyano group. Additionally, the reactivity of compound 1 as well as its endo-selectivity are both higher than those observed for the corresponding (Z)-3-sulfinylacrylates, thus proving the potential of sulfinylnitriles as chiral dienophiles.     

The reaction of 4-(p-nitrobenzyl)pyridine with some electrophiles

The nucleophile 4-(p-nitrobenzyl)pyridine was allowed to react with four carcinogenic alkylating agents, chloromethyl methyl ether, bis(chloromethyl) ether, glycol sulfate and propane sultone, one carcinogenic acylating agent, N,N-dimethylcarbamyl chloride, and one noncarcinogenic electrophile, perchlorocyclobutenone. The structures of the major products formed, which are substituted pyridinium salts or 1,4-dihydropyridines, were determined.