Influence of carboxylic group or methyl ester group on the interactions of copper cation with aromatic system of naproxen, naphthalene acetic acids and their methyl esters

• Solution containing naproxen, its methyl ester and Cu2+ was analyzed by ESI-MS.
• Molecular ions of naproxen and its ester were generated ([Nap]+ and [NapCH3]+).
• Formation of [Nap]+ was more efficient than formation of [NapCH3]+.
• Analogical result was obtained for naphthaleneacetic acids (α, β) and their esters.
• It was also found that β isomer is more prone to form molecular ions than α isomer.

Solutions containing naproxen, its methyl ester and Cu2+, or naphthalene acetic acid, its methyl ester and Cu2+ was analyzed by using electrospray ionization mass spectrometry. As a result of interaction between Cu2+ and aromatic system (Cu2+–π interaction), the respective molecular ions were obtained. The interactions of Cu2+ with –COOH and –COOCH3 groups compete with that of Cu2+ with naphthalene rings, affecting the formation of molecular ions. Because of inductive effect of –CH3 moiety, –COOCH3 group interacts with Cu2+ stronger than –COOH group. ESI mass spectrum obtained for a mixture of isotopically labeled 1-naphthalene acetic acid methyl ester, 2-naphthalene acetic acid methyl ester and Cu2+ showed that second isomer is more prone to form molecular ions than the first.

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