The molecule combines three chemically different binding regions.
- sodium: the oxygen-rich calixarene/polyether region, including the ether oxygens and suitably oriented ester carbonyl oxygens. sodium is a hard Lewis acid and favors multidentate coordination by hard oxygen donors. The scaffold holds several donors near one another and can reduce the conformational cost of binding.
- nitrate: the two urea groups. Their NH groups donate hydrogen bonds to nitrate oxygens. Several NH donors can cooperate around an anion; the carbonyl oxygen of a urea is an acceptor and is not the proposed nitrate-binding donor.
- silver: the terminal bipyridyl nitrogen donors. The pyridine lone pairs coordinate the softer silver ion. The actual coordination may involve donor sites on more than one arm or more than one receptor, depending on geometry and concentration.
These assignments identify the preferred local sites, rather than guaranteeing three independent binding constants: occupancy can alter conformation, electrostatics and the affinity of neighboring sites.
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