Characterization of Supramolecular (H2O)18 Water Morphology and Water-Methanol (H2O)15(CH3OH)3 Clusters in a Novel Phosphorus Functionalized Trimeric Amino Acid Host

Abstract

Phosphorus functionalized trimeric alanine compounds (l)- and (d)-P(CH₂NHCH(CH₃)COOH)₃2 are prepared in 90% yields by the Mannich reaction of Tris(hydroxymethyl)phosphine 1 with (l)- or (d)- Alanine in aqueous media. The hydration properties of (l)-2 and (d)-2 in water and water−methanol mixtures are described. The crystal structure analysis of (l)-2·4H₂O, reveals that the alanine molecules pack to form two-dimensional bilayers running parallel to (001). The layered structural motif depicts two closely packed monolayers of 2 each oriented with its phosphorus atoms projected at the center of the bilayer and adjacent monolayers are held together by hydrogen bonds between amine and carboxylate groups. The water bilayers are juxtaposed with the H-bonded alanine trimers leading to 18-membered (H₂O)₁₈ water rings. Exposure of aqueous solution of (l)-2 and (d)-2 to methanol vapors resulted in closely packed (l)-2 and (d)-2 solvated with mixed water−methanol (H₂O)₁₅(CH₃OH)₃ clusters. The O−O distances in the mixed methanol−water clusters of (l)-2·3H₂O·CH₃OH and (d)-2·3H₂O·CH₃OH (O−O(average) = 2.857 Å) are nearly identical to the O−O distance observed in the supramolecular (H₂O)₁₈ water structure (O−O(average) = 2.859 Å) implying the retention of the hydrogen bonded structure in water despite the accommodation of hydrophobic methanol groups within the supramolecular (H₂O)₁₅(CH₃OH)₃ framework. The O−O distances in (l)-2·3H₂O·CH₃OH and (d)-2·3H₂O·CH₃OH and in (H₂O)₁₈ are very close to the O−O distance reported for liquid water (2.85 Å).