SURFACE-PROMOTED THIOETHER LINKAGE BETWEEN PROTO-PORPHYRINS OR HEMATO-PORPHYRINS AND THIOL-SILANIZED QUARTZ - FORMATION OF SELF-ASSEMBLED MONOLAYERS AND INTERACTION WITH IMIDAZOLE AND CARBON-MONOXIDE

Free base protoporphyrin IX (PP),(1) 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 monol ayers (SAMs) were characterized by UV/vis absorption and fluorescence spectroscopy. Evidence for chemisorption through the formation of thio ether linkages between the immobilized thiols at the surface and the v inyl or the hydroxyethyl groups of the porphyrins is as follows: (1) O nly porphyrins that possess either vinyl or hydroxyethyl groups show t he characteristic absorption and properties of a SAM; porphyrins free of vinyl or hydroxyethyl groups do not form SAMs. (2) Attachment to th e surface through the propionates of the metal is ruled out since the presence of both metal and propionates is not mandatory for the format ion 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 cond itions between thiols and the vinyl or the hydroxyethyl groups of porp hyrin. This is analogous to the covalent attachment of heme in cytochr ome c through the poorly understood reaction between the vinyl groups of the heme and the thiol of cysteine. Furthermore, this surface-promo ted chemistry provides a simple and direct method for attaching unmodi fied porphyrins onto solid substrates to form remarkably robust monola yers. Linear dichroism revealed that the Fe(III)PPs in the monolayer a re tilted in average at an angle of 33 degrees relative to the substra te, shifting to 46 degrees after ligation to imidazole. Carbon monoxid e 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.