Naproxen is one of the most consumed nonsteroidal anti-inflammatory drugs and marketed as S-naproxen since R-naproxen is hepatotoxic. In this study, chiral recognition of naproxen has been investigated by tandem mass spectrometry (MS/MS). Among all diastereomeric complexes formed between naproxen and the examined chiral selectors, including cyclodextrins (α/β/γ-CD), modified phenylalanines ( N-acetyl-phenylalanine, N-t-butoxycarbonyl-phenylalanine, N-9-fluorenylmethyloxycarbonyl-phenylalanine), amino acids (Trp, Phe, Tyr, His), glucose, tartaric acid, and vancomycin, a novel binuclear metal bound diastereomeric complexes [(M(II))₂( S/ R-naproxen)(l-His)₂-3H]⁺ (M = Cu, Ni, or Co with Cu being the best) could allow effective identification of the absolute configuration of naproxen and determination of its enantiomeric excess ( ee) through MS/MS analysis. The key candidate structure of [(Cu(II))₂( S/ R-naproxen)(l-His)₂-3H]⁺ has been revealed by means of collision-induced dissociation, ion mobility mass spectrometry and density functional theory calculations, indicating an interesting and unusual self-assembled compact geometry with the two Cu(II) ions bridged closely together (Cu-Cu distance is 3.04 Å) by the carboxylate groups of the two histidines. It was shown that the difference in dissociation efficiency between the two diastereomers was attributed to the interaction between the NH₂ bond of the amino group of one histidine and the naphthyl ring of naproxen. The present report is the first to observe and characterize the complex of (Cu(II))₂(His)₂ with aromatic acid, which could contribute to the chiral recognition of other chiral aromatic acids, design of catalysts based on binuclear copper bound complex, as well as the better understanding of metal ion complexation by His or His-containing ligands.

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