Nanoconstruction of Microspheres and Microcapsules Using Proton-Induced Phase Transitions:  Molecular Self-Recognition by Diamide Diacids in Water

Bis(N^α-amido-l-phenylalanine)-1,1-cyclobutane dicarboxylate (5) was studied by Fourier transform infrared (FTIR) spectroscopy, variable-temperature NMR (VT-NMR), transmission electron microscopy, X-ray crystallography, Raman microscopy, and a novel imaging technique known as “soft” X-ray microscopy (XRM). Diamide diacid 5 was shown to self-associate into solid microspheres during a proton-induced phase transition from the solvated state to the desolvated assembled state. These diverse techniques allowed for the delineation of the molecular recognition events involved in the assembly process. X-ray crystallography revealed that 5 packs in a bundled helical array comprised of two types of intermolecular hydrogen bonds (i.e., OCO···HN and COOH···OCN). VT-NMR and IR measurements of 5 (1 mM in CDCl₃) revealed the small temperature dependence of the amide NH chemical shift (Δδ/ΔT = −1.1 ppb/K) and the availability of the “free” amide NH of 5 to form intermolecular hydrogen bonds. Supramolecular rodlike structures were observed during the aqueous assembly of 5 into microspheres by XRM. Raman microscopy confirmed that nearly identical bonding patterns are present in the assembled microsphere and the crystal architecture of 5. Collectively, these observations provide compelling evidence that the assembly of 5 occurs via crystalline supramolecular intermediates, which are similar in shape and have complementary bonding motifs for proper self-recognition. Competition experiments involving varying concentrations of 5 and its microcapsule-forming cyclopropane analogue 3 revealed that molecular fidelity was less important to the microsphere-forming process than the related capsule-forming process.