Monitoring redox-dependent contribution of lipids in Fourier transform infrared difference spectra of complex I from Escherichia coli

Biochemical and crystallographic studies have shown that phospholipids are essential for the integrity and activity of membrane proteins. In the study presented here, we use electrochemically induced Fourier transform infrared (FTIR) spectroscopy to demonstrate variations occurring upon the presence and absence of lipids in NADH:ubiquinone oxidoreductase (complex I) from Escherchia coli by following the C=O vibration of the lipid molecule. Complex I is activated in the presence of lipids. Interestingly, in electrochemically induced FTIR difference spectra of complex I from E. coli, a new signal at 1744/1730 cm(-1) appears after addition of E. coli polar lipids, concomitant with the oxidized or reduced form, respectively. Absorbance spectra of liposomes from mixed lipids at different pH values demonstrate shifts for the carbonyl vibration depending on the environment. On this basis we suggest that lipids, though not redox active themselves, contribute in reaction-induced FTIR difference spectra, if a change occurs in the direct environment of the lipid during the observed reaction or coupled processes.