A catecholate-type artificial siderophore with a terminal-NH2 group (1) and its Fe(3+) complex (2) were prepared. Siderophore 1 was characterized by (1)H NMR, FT-IR, and ESI-TOF MS spectroscopy. The corresponding Fe(3+) complex 2 was obtained by reaction of 1 with Fe(acac)3. The absorption band at 500 nm (ε = 4670 M(-1) cm(-1) at pH 7.0) of the electronic absorption spectrum of 2 is assignable as the LMCT (O(catecholate) → Fe(3+)) absorption band. This band indicates the formation of the Fe(3+) complex of 1. The biological activity of 2 with respect to Escherichia coli was clearly confirmed by observing that it permeates into the cell membrane. The self-assembled monolayer of 2 on an Au substrate, 2/Au, was prepared and its preparation was confirmed by FT-IR reflection-absorption spectroscopy (IR-RAS) and cyclic voltammetry (CV). Furthermore, a quartz crystal microbalance (QCM) chip modified with 2 effectively adsorbed E. coli. M. flavescens, an organism which is incapable of synthesizing siderophores and must therefore use exogenous hydroxamate-type siderophores for growth, did not adsorb on 2/Au. In contrast, E. coli did not adsorb on the hydroxamate-type artificial siderophore-Fe(3+) complex (3)-modified Au substrate, 3/Au. These results provide preliminary evidence that microbes recognized Fe(3+) ion-bound siderophores on the surface. The detection limit of 2/Au was ∼10(4) CFU mL(-1).
The hydroxamate-typed artificial siderophores, tris [2-{3-(N-acyl-N-hydroxamino)propylamido}propyl]aminomethane (L1-L3, acyl-R group = Me, Et, and Ph, respectively) were prepared and their Fe III complexes, Fe III (L1-L3) (1-3) were modified on Au electrode surfaces. Complex 1-3 indicated biological activity for Microbacterium flavescens, which is hydroxamate-typed siderophore auxotrophic gram-positive microorganism, suggesting that these complexes were able to permeate the cell membrane of microorganism. The growth curve indicated the permeation ability of 1-3 depended on their terminal acyl groups. The modification of these complexes was carried out by stepwise self-assembling method. The cyclic voltammetry of the resultant Au electrodes, 1-3/Au confirmed the surface modification of 1-3. The adsorption experiments of M. flavescens with 1-3/Au were clearly showed that 1-3/Au could immobilize microorganisms. The images of the adsorption of M. flavescens were obtained by various microscopic methods. Quartz crystal microbalance (QCM) measurements also suggested that 1-3/Au was able to adsorb M. flavescens. The adsorption ability of 1-3/Au for microorganisms depended on the terminal bulky acyl group. The adsorption ability is due to the interaction between 1-3 on Au electrodes and receptor/binding protein in the cell membrane.
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