Cryogenic Single-Molecule Fluorescence Detection of the Mid-Infrared Response of an Intrinsic Pigment in a Light-Harvesting Complex

[Image: see text] We observed the mid-infrared (MIR) response of a single pigment of bacteriochlorophyll a at the B800 binding site of a light-harvesting 2 complex. At a temperature of 1.5 K, a single complex in a spatially isolated spot in a near-infrared (NIR) fluorescence image was selected and was simultaneously irradiated with MIR and NIR light. We found that the temporal behavior of the NIR fluorescence excitation spectrum of individual pigments in a single complex was modulated by the MIR irradiation at 1650 cm(–1). The MIR modulation of a single pigment was linearly proportional to the MIR intensity. The MIR linear response was detected in the range from 1580 to 1670 cm(–1).