Synthesis, Biological Evaluation, and Molecular Modeling of Aza-Crown Ethers

Synthetic and natural ionophores have been developed to catalyze ion transport and have been shown to exhibit a variety of biological effects. We synthesized 24 aza- and diaza-crown ethers containing adamantyl, adamantylalkyl, aminomethylbenzoyl, and ε-aminocaproyl substituents and analyzed their biological effects in vitro. Ten of the compounds (8, 10-17, and 21) increased intracellular calcium ([Ca²⁺]_i) in human neutrophils, with the most potent being compound 15 (N,N'-bis[2-(1-adamantyl)acetyl]-4,10-diaza-15-crown-5), suggesting that these compounds could alter normal neutrophil [Ca²⁺]_i flux. Indeed, a number of these compounds (i.e., 8, 10-17, and 21) inhibited [Ca²⁺]_i flux in human neutrophils activated by N-formyl peptide (fMLF). Some of these compounds also inhibited chemotactic peptide-induced [Ca²⁺]_i flux in HL60 cells transfected with N-formyl peptide receptor 1 or 2 (FPR1 or FPR2). In addition, several of the active compounds inhibited neutrophil reactive oxygen species production induced by phorbol 12-myristate 13-acetate (PMA) and neutrophil chemotaxis toward fMLF, as both of these processes are highly dependent on regulated [Ca²⁺]_i flux. Quantum chemical calculations were performed on five structure-related diaza-crown ethers and their complexes with Ca²⁺, Na⁺, and K⁺ to obtain a set of molecular electronic properties and to correlate these properties with biological activity. According to density-functional theory (DFT) modeling, Ca²⁺ ions were more effectively bound by these compounds versus Na⁺ and K⁺. The DFT-optimized structures of the ligand-Ca²⁺ complexes and quantitative structure-activity relationship (QSAR) analysis showed that the carbonyl oxygen atoms of the N,N'-diacylated diaza-crown ethers participated in cation binding and could play an important role in Ca²⁺ transfer. Thus, our modeling experiments provide a molecular basis to explain at least part of the ionophore mechanism of biological action of aza-crown ethers.