Molecular structure of rubidium six-coordinated dihydrate complex with monensin A

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Abstract

Crystal structure of monensin A rubidium salt dihydrate, [Rb(C36H61O11)·2H2O], has been studied by X-ray diffraction, FT-IR spectroscopy and PM5 semiempirical methods. The crystal space group is P212121 with a = 12.6153(7), b = 16.4841(10), c = 19.4840(12) and Z = 4. The Rb–O bond lengths are between 2.788(7) and 2.901(6) Å. The carboxyl group of monensin A is deprotonated and engaged in two intramolecular O(11)–H···O(1) of 2.52(1) Å and O(10)–H···O(2) of 2.60(1) Å hydrogen bonds with hydroxyl groups, accompanied with formation of a pseudo-cyclic structure. This structure is stabilised by the coordination of the Rb+ cation by oxygen atoms. Two water molecules are involved in the weak intermolecular hydrogen bond between the different species forming a supramolecule. The IR spectrum of the crystal is consistent with the results obtained by the X-ray study and provides spectroscopic evidence for the complex formation. The calculated structure and the structural parameters of the monensin A rubidium salt are comparable with those determined by the X-ray study.

Introduction

Ionophorous antibiotics belong to a group of highly bioactive molecules, because they are able to transport monovalent and bivalent metal cation across natural and artificial lipid membranes. Monensin A (see Scheme 1) isolated from Streptomyces cinnamonensis is a well-known representative of this class of compounds. It is able to form pseudomacrocyclic complexes with monovalent cations and to transport these cations across cell membranes. Monensin regulates many cellular functions, including apoptosis. It causes collapse of sodium and potassium gradients at the plasma membrane, blocks intracellular protein transport, and exhibits antibiotic, antimalarial, and other important biological activities [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Monensin A is used as a growth-promoting agent and as a coccidiostat in beef cattle, sheep, chickens and turkeys [4], [12], [13], [14], [15], [16].

The crystal structures of monensin complexes with several cations have been determined by X-ray diffraction [17], [18], [19], [20], [21], [22], [23], [24], [25]. In these structures the coordination of the cation by monensin A was always accompanied with the appearance of a pseudo-cyclic structure stabilized by the intramolecular hydrogen bonds.

Recently, we have studied the crystal structures of monensin A sodium salt as well as monensin A lithium salt [24], [25]. As a continuation of the studies a new crystal of monensin A rubidium dihydrate has been obtained and studied by X-ray diffraction and FT-IR spectroscopy. In this report, the structure of rubidium complex of monensin A determined by X-rays is compared with that calculated using the PM5 semiempirical method.

Section snippets

Experimental

Monensin A sodium salt (90–95%) was purchased from Sigma. CH3CN spectral-grade solvent was stored over 3 Å molecular sieves for several days. All manipulations with the substances were performed in a carefully dried and CO2-free glove box. Rubidium hydroxide hydrate RbOH·xH2O (99.995%) was purchased from Aldrich.

X-ray crystallography

As expected, the rubidium cation is complexed by the monensin A anion, as shown in Fig. 1. The cation is sixfold coordinated, by four tetrahydrofurane oxygen atoms and two hydroxyl oxygen atoms, the Rb···O distances range between 2.788 and 2.901 Å (Table 3). The anion, with the carboxyl group deprotonated is present in the pseudo-ring conformation. The selected torsion angles describing the monensin A anion conformation are listed in Table 4. The terminal carboxylate and two hydroxyl groups

Acknowledgement

Adam Huczyński wishes to thank the Foundation for Polish Science for fellowship.

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