Photoinduced electron transfer in silica-supported self-assembled thin films containing a 1,4,5,8-naphthalenetetracarboxylic diimide and cytochrome c

Abstract

Self-assembled thin films containing an inner monolayer of N,N′-bis(2-phosphonoethyl)-1,4,5,8-naphthalenetetracarboxylic diimide (PNDI) and an outer monolayer of the cationic protein cytochrome c (cyt c) were prepared on the surface of non-porous silica gel particles. Silica particles modified with PNDI alone or with cyt c alone were also prepared for comparison. The materials were characterized by elemental and thermogravimetric analysis, as well as UV-visible absorption, fluorescence and infrared spectroscopies. Binding of PNDI to the silica resulted in a low density, sub-monolayer coverage, in contrast to cyt c adsorption which gave a rather compact protein layer. The loading of cyt c found in the new materials (ca. 12 µmol g⁻¹) was higher than that found in the literature for porous silica gel. Irradiation of the materials at the imide absorption band resulted in the reduction of the cyt c heme iron, showing that the protein retained its activity upon binding to the modified silica gel surface. The proposed mechanism involves electron transfer from a photogenerated imide radical anion to the heme iron of the protein in the adjacent layer.