Solution Conformation of the Ionophore A204A Na⁺‐Salt

Abstract

Abstract Most of the parameters of the Na + ‐salt of A204A could be extracted from its 300 MHz H‐nmr spectrum in CDCl 3 , by consecutive double irradiation and INDOR experiments. A comparison of the so‐obtained solution conformation (Figure 4) with that reported 2 for the solid state disclose their striking analogy. The backbone forms a pseudocyclic structure, closed by head‐to‐tail hydrogen bond between OH‐15 and COO θ (1). The cation is fixed in the center by at least six ligands, forming an irregular octahedron. Rather OH‐3 or 0‐4 (instead of the second oxygen atom of the carboxylic function as proposed 2 for the crystaline state) seems to be involved as one of the ligands. Also, the rotameric states of some side chains (methoxy groupings and ring G) behave presumably different in solution. The outside periphery of the pseudo cycle is not entirely hydrophobic and four methoxy groups, OH‐15 and at least three ether/acetal oxygen atoms point away from the central cavity into the surroundings. Most of these ambident functions are concentrate around one region, e. g. the dangling sugar‐like ring G. This is a chemically mobile entity and attached via an acetal function to the remaining architecture, the latter thus being the sole skeleton responsible for chelation.