Self-Assembled monolayers as templates for heme crystallization

Homogeneous self-assembled monolayers (SAMs) of alkanethiols (HS(CH ₂)_nX) on Au(111) were used as substrates for crystallization of ferriprotoporphyrin IX (heme) in acidic aqueous solution. Different terminal functional groups (X = OH, COOH, NH₂, CH ₃) were used on the SAMs as models of sites where heme crystallization takes place in blood-feeding organisms. Atomic force microscopy, X-ray diffraction (XRD), and X-ray absorption near edge spectroscopy (XANES) were employed to characterize particle morphology, density, crystallographic orientation, and the coordination environment. It was found that the morphology and extent of growth of particulates were strongly affected by the environment in which they crystallize. As has been previously observed, acicular crystals form in DMSO-methanol solution, whereas irregular aggregates of crystals form in acidic aqueous solution. Here tabular crystals were found to form on -NH ₂ and -OH terminated SAMs, whereas inclined crystals formed on -COOH and -CH₃ terminated substrates. Particulate coverage on these SAMs decreased in the order of-NH₂, -COOH, -CH₃, and -OH. Chloroquine, a widely used antimalaria drug, slowed particle nucleation rate on the SAMs with varying efficacy but was most efficient on the -COOH SAM. XANES measurements showed that the coordination environment surrounding iron in the particles was found to be the same, regardless of the preparation method and matches existing spectra of hemozoin produced in vivo and synthetic β-hematin. Different crystallographic planes were found to be expressed depending on the identity of the SAM using XRD. The interaction between the terminal functional group of the SAM and the density and orientation of crystals is discussed.