Redox-neutral S-nitrosation Mediated by a Dicopper Center

An unprecedented redox-neutral <i>S</i>-nitrosation of thiol has been achieved at dicopper(I,I) center. Treatment of dicopper (I,I) complex with excess NO^• and thiol generates a dicopper (I,I) di-<i>S</i>-nitrosothiol complex [Cu^ICu^I(RSNO)₂]²⁺ or dicopper (I,I) mono-<i>S</i>-nitrosothiol complex [Cu^ICu^I(RSNO)]²⁺, which readily release RSNO in 88-94% yield. The <i>S</i>-nitrosation reaction proceeds through a mixed-valence [Cu^IICu^III(<i>m</i>-O)(<i>m</i>-NO)]²⁺<i> </i>species, which deprotonates RS-H at the basic <i>m</i>-O site and nitrosates the RS^- at the <i>m</i>-NO site. The [Cu^IICu^III(<i>m</i>-O)(<i>m</i>-NO)]²⁺ complex is also competent for <i>O</i>-nitrosation of MeOH, which is isoelectronic to thiol. In this case, a rare [Cu^IICu^II(<i>m</i>-NO)(OMe)]²⁺ intermediate has been isolated and fully characterized, suggesting the <i>S</i>-nitrosation proceeds through the intermediary of analogous [Cu^IICu^II(<i>m</i>-NO)(SR)]²⁺ species. The redox- and proton-neutral <i>S-</i>nitrosation process reported here represents the first functional model of ceruloplasmin in mediating <i>S</i>-nitrosation of external thiols, adding further implications for biological copper sites in the interconversion of NO^•/RSNO.

Redox-neutral S-nitrosation Mediated by a Dicopper Center

An unprecedented redox-neutral <i>S</i>-nitrosation of thiol has been achieved at dicopper(I,I) center. Treatment of dicopper (I,I) complex with excess NO^• and thiol generates a dicopper (I,I) di-<i>S</i>-nitrosothiol complex [Cu^ICu^I(RSNO)₂]²⁺ or dicopper (I,I) mono-<i>S</i>-nitrosothiol complex [Cu^ICu^I(RSNO)]²⁺, which readily release RSNO in 88-94% yield. The <i>S</i>-nitrosation reaction proceeds through a mixed-valence [Cu^IICu^III(<i>m</i>-O)(<i>m</i>-NO)]²⁺<i> </i>species, which deprotonates RS-H at the basic <i>m</i>-O site and nitrosates the RS^- at the <i>m</i>-NO site. The [Cu^IICu^III(<i>m</i>-O)(<i>m</i>-NO)]²⁺ complex is also competent for <i>O</i>-nitrosation of MeOH, which is isoelectronic to thiol. In this case, a rare [Cu^IICu^II(<i>m</i>-NO)(OMe)]²⁺ intermediate has been isolated and fully characterized, suggesting the <i>S</i>-nitrosation proceeds through the intermediary of analogous [Cu^IICu^II(<i>m</i>-NO)(SR)]²⁺ species. The redox- and proton-neutral <i>S-</i>nitrosation process reported here represents the first functional model of ceruloplasmin in mediating <i>S</i>-nitrosation of external thiols, adding further implications for biological copper sites in the interconversion of NO^•/RSNO.

Development and validation of a simple spectrophotometric method for the determination of methyldopa in both bulk and marketed dosage formulations

A simple, precise, sensitive, rapid, specific and economical spectrophotometric method was developed to determine methyldopa (MTD) content in bulk and pharmaceutical dosage formulations. The proposed method was based on the formation of a colored product from the nitrosation reaction of MTD with sodium nitrite in an acid medium. The resultant nitroso derivative species reacts further with sodium hydroxide and is converted it into a more stable compound. This yellow nitrosation product exhibited an absorption maximum at 430 nm. Beer's Law was obeyed in a concentration range of 6.37 to 82.81 μg mL-1 MTD with an excellent coefficient of determination (R2 = 0.9998). No interference was observed from common excipients in formulations. The results showed the method to be simple, accurate and readily applied for the determination of MTD in pure form and in pharmaceutical preparations. The analytical results obtained for these products using the proposed method are in agreement with those of the Brazilian Pharmacopoeia procedure at a 95% confidence level.

The Influence of Phosphate Buffer on the Formation of N-Nitrosodimethylamine from Dimethylamine Nitrosation

Buffer solutions were widely used for almost all the investigations concerning N-nitrosodimethylamine (NDMA), a member of powerful mutagenic and carcinogenic compounds which are ubiquitous in the environment. However, whether or how the buffer matrixes influence NDMA formation is still unknown. The effect of buffer solutions on NDMA formation from the nitrosation of dimethylamine (DMA) by nitrite (NaNO2) was investigated at pH 6.4 in four kinds of buffer solutions, that is, Na2HPO4/C6H8O7, Na3(C6H5O7)/C6H8O7, NaH2PO4/NaOH, and NaH2PO4/Na2HPO4. Our observations demonstrate an unexpected inhibitory effect of the buffer solutions on NDMA formation and the phosphate buffer plays a more significant role in inhibiting NDMA formation compared to the citrate buffer. Moreover, the amount of the phosphate in the buffer was also found to greatly impact the formation of NDMA. A further investigation indicates that it is the interaction between NaH2PO4and reactant NaNO2rather than DMA that leads to the inhibitory effect of phosphate buffer during the DMA nitrosation reaction. This study expands the understanding of the influence of buffer solution on nitrosamines formation through the nitrosation pathway and further gives a hint for water plants to reduce the formation of nitrosamines.