Relative hydrophilicities of cis and trans formamides.

Secondary formamides are widely encountered in biology and exist as mixtures of both cis and trans isomers. Here, we assess hydrophilicity differences between isomeric formamides through direct competition experiments. Formamides bearing long aliphatic chains were sequestered in a water-soluble molecular container having a hydrophobic cavity with an end open to the aqueous medium. NMR spectroscopic experiments reveal a modest preference (<1 kcal/mol) for aqueous solvation of the trans formamide terminals over the cis isomers. With diformamides, the supramolecular approach allows staging of intramolecular competition between short-lived species with subtle differences in hydrophobic properties.