Reversible binding of metal ions onto bacterial layers revealed by protonation-induced ATR-FTIR difference spectroscopy

The ability of microorganisms to adhere to abiotic surfaces and the potentialities of attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy have been exploited in order to study protonation and heavy metal binding events onto bacterial surfaces. This work represents the first attempt to apply on bacteria the recently developed method known as perfusion-induced ATR-FTIR difference spectroscopy. Such technique allows measurement of even slight changes in the infrared spectrum of the sample, deposited as a thin layer on an ATR crystal, while an aqueous solution is perfused over its surface. Solutions at different pH have been used for inducing protonation/deprotonation of functional groups lying on the surface of Rhodobacter sphaeroides cells, chosen as a model system. The interaction of Ni2+ with surface protonable groups of this microorganism has been investigated with a double-difference approach exploiting competition between nickel cations and protons. Protonation-induced difference spectra of simple model compounds have been acquired in order to guide band assignment in bacterial spectra, thus allowing identification of major components involved in proton uptake and metal binding. The data collected reveal that carboxylate moieties on the bacterial surface of R. sphaeroides play a role in extracellular biosorption of Ni2+, establishing with this ion relatively weak coordinative bonds.