A light-induced transient change in the ionic interaction between chloride and arginine in the transmembrane anion pump halorhodopsin (hR) is detected with infrared absorption spectroscopy. In the IR difference spectrum of hR and one of its photoproducts (hL), only a few bands have frequencies that depend on the particular halide ion (Cl−, Br−, or I−) present. Three of the halide-sensitive negative difference bands (at 1695, 1610, and 1170 cm−1) correspond in frequency to arginine C‒N vibrations and undergo anion-dependent shifts that match those seen in ethylguanidinium halide model compounds. These shifts reflect the different strengths of the ionic interactions formed with the various halides. We conclude that a halide‒arginine ion pair is present in the hR state; this interaction appears to be disrupted by photoconversion to hL.
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