X-Ray Photoelectron Spectroscopy (XPS) of Bacteriorhodopsin Analogues Synthesized from Fluorophenyl Retinals

The two external point-charge (TEPC) model for bacteriorhodopsin (bR) has been examined by X-ray photoelectron spectroscopy (XPS) and CNDO/S molecular orbital calculations. A main concern was given to a point charge near the β-ionone ring. XPS measurements were carried out on fluorophenyl retinal (F-ret) and their derivatives (Schiff base, protonated Schiff base and bacteriorhodopsin analogues (F-bR)), paying close attention to the chemical shift of the F 1 s core level. No meaningful differences were observed among these species although numerical calculations on an assumption of the TEPC model have predicted the chemical shift of about 3 eV between F-ret and F-bR. This fact has arisen a serious question to the validity of the TEPC model. The same conclusion has been reached by the present study of absorption maxima of F-ret and their derivatives.