Surface-Promoted Thioether Linkage between Proto- or Hemato porphyrins and Thiol-Silanized Quartz:  Formation of Self-Assembled Monolayers and Interaction with Imidazole and Carbon Monoxide

Free base protoporphyrin IX (PP), Zn(II)protoporphyrin IX (Zn(II)PP), Fe(III)protoporphyrin IX (Fe(III)PP), and Fe(III)protoporphyrin IX dimethylester (Fe(III)PPDME) as well as free base hematoporphyrin were selectively chemisorbed onto thiol-silanized quartz in aqueous media at neutral pH values and at room temperature. The self-assembled monolayers (SAMs) were characterized by UV/vis absorption and fluorescence spectroscopy. Evidence for chemisorption through the formation of thioether linkages between the immobilized thiols at the surface and the vinyl or the hydroxyethyl groups of the porphyrins is as follows: (1) Only porphyrins that possess either vinyl or hydroxyethyl groups show the characteristic absorption and properties of a SAM; porphyrins free of vinyl or hydroxyethyl groups do not form SAMs. (2) Attachment to the surface through the propionates of the metal is ruled out since the presence of both metal and propionates is not mandatory for the formation of SAMs. (3) None of the porphyrins form SAMs on bare quartz or on quartz silanized with propyltrimethoxysilane. To our knowledge, this is the first description of a direct reaction under physiological conditions between thiols and the vinyl or the hydroxyethyl groups of porphyrin. This is analogous to the covalent attachment of heme in cytochrome c through the poorly understood reaction between the vinyl groups of the heme and the thiol of cysteine. Furthermore, this surface-promoted chemistry provides a simple and direct method for attaching unmodified porphyrins onto solid substrates to form remarkably robust monolayers. Linear dichroism revealed that the Fe(III)PPs in the monolayer are tilted in average at an angle of 33° relative to the substrate, shifting to 46° after ligation to imidazole. Carbon monoxide coordinates to Fe(II) in SAMs of FePP and FePPDME. SAMs of Fe(III)PP and Fe(III)PPDME ligate imidazole cooperatively and, after reduction, bind CO reversibly. In contrast, only one imidazole ligates to Zn(II)PP as a SAM, and no interaction with CO was observed.