We observed the mid-infrared
(MIR) response of a single pigment
of bacteriochlorophyllaat 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.