Porphyrin Amphiphiles as Templates for the Nucleation of Calcium Carbonate

Abstract

The processes of biogenic crystal growth are of interest both to derive an understanding of the biochemistry for the development of mesoscopic structures and as an inspiration to devise new strategies for materials fabrication.1 Template-mediated nucleation of calcite, a predominant polymorph in these structures, has been investigated using either proteins2,3 or simple monofunctional amphiphiles.4-6 Model studies with small amphiphilic molecules at compressed Langmuir interfaces, pioneered by Mann et al.,4 have yielded important insights on the molecular recognition affecting the nucleation process of the inorganic phase. However, the morphological control mechanisms and numerous subtleties associated with protein templates remain unknown at the molecular level. We have synthesized amphiphilic tricarboxyphenylporphyrin iron(III) μ-oxo dimers, surface films of which mimic several aspects of nucleation and growth of calcium carbonate observed with native biological templates: (1) highly oriented crystals have been obtained; (2) self-assembly of the template occurs with no external mechanical forces to modulate crystal growth or morphology; (3) highly patterned excavations resulting in “chiral” crystals are observed. The porphyrin presents a semirigid surface array of carboxylate groups7 intermediate in complexity between protein matrices and simple molecules. This approach could delineate the effects of cooperativity2 and spatial-stereochemical effects,8 and, especially, determine what minimal structure is required to mimic the controlled nucleation observed in biological systems. Monolayers of the μ-oxo iron(III) porphyrin dimer 17b were formed at the air/water interface using a solution of 1 in 3:1 chloroform/methanol (1 mg/mL) to spread the film. Isotherms obtained over a supersaturated calcium bicarbonate subphase by compression produced a limiting area of 56-58 A2/porphyrin (Figure 1), corresponding to an approximately vertical orientation of the porphyrin plane at the interface.7,9 A limiting area of only 46-48 A2/molecule (Figure 1) was obtained over distilled, deionized water. The higher limiting area in the presence of calcium ions is indicative of strong cation binding to the anionic template.5 The total Ca concentration was estimated by EDTA titrimetric analysis to be ∼9 mmol dm-3.