Structure of 1-naphthylmethyl ester of monensin A complexes with monovalent metal cations

A new 1-naphthylmethyl ester of monensin A (MON8) has been synthesized and its capability of complex formation with Li+, Na+ and K+ cations has been studied by mass spectrometry, spectroscopic and PM5 semiempirical methods. ESI mass spectrometry indicates that MON8 forms complexes of exclusively 1:1 stoichiometry with these cations. The fragmentation pathways of MON8 complexes are discussed. The FT-IR spectra of MON8 complexes with Li+ and Na+ cations indicate an equilibrium between two complex structures, in which the carbonyl ester group is either involved or not involved in the complexation process, whereby the equilibrium is shifted towards the latter structure. The structures of the MON8 complexes with Li+, Na+ and K+ cations are stabilized by intramolecular hydrogen bonds in which the OH groups are always involved. The structures are visualized using PM5 semiempirical method and discussed in detail. We show that MON8–Na+ complex has a pseudo-crown ring structure which increases the affinity of MON8 to this cation.