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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