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Fig. 1 nirFAST, a versatile and tunable near-infrared fluorescent chemogenetic reporter.a Principle of nirFAST. b Structure of the anionic state of HPAR-3OM and HPAR-3,5DOM in interaction with residues Y42 and Q46. c Absorption (dashed line, left axis) and emission (solid line, right axis) spectra of HPAR-3OM free (black) or bound (orange) to nirFAST. Spectra were recorded in pH 7.4 PBS at 25?°C. The image on the right shows the fluorescence of solutions of HPAR-3OM free or bound to nirFAST under illumination with far-red light. d Absorption (dashed line) and emission (solid line) spectra of HPAR-3,5DOM free (black) or bound (dark red) to nirFAST. Spectra were recorded in pH 7.4 PBS at 25?°C. The image on the right shows the fluorescence of solutions of HPAR-3,5DOM free or bound to nirFAST under illumination with far-red light. e Properties of nirFAST with HPAR-3OM (nirFAST680) and with HPAR-3,5DOM (nirFAST715) in PBS pH 7.4. KD thermodynamic dissociation constant, ?abs wavelength of maximal absorption, FWHMabs full width at half-maximal absorption, ? molar absorptivity at ?abs, ?em wavelength of maximal emission, FWHMem full width at half-maximal emission, FQY fluorescence quantum yield. f Engineering of nirFAST from frFAST670: the thermodynamic dissociation constant KD, absorption wavelength, emission wavelength, and brightness of the main engineering intermediates are indicated. g, h Structural model of nirFAST715 generated by homology modeling and molecular dynamics using the crystal structure of the Halorhodospira halophila Photoactive Yellow Protein (PYP) (PDB: 6P4I). HPAR-3,5DOM is in red, while Gln46 is in blue. h Interactions networks involved in HPAR-3,5DOM binding and recognition within nirFAST. Residues introduced during the engineering process are shown in magenta. See also Supplementary Fig. 6 and Supplementary Data for additional details. Source data are provided as a Source Data file.