Figure 2. Fluorescence titrations are shown of Xenopus interphotoreceptor retinol-binding protein (xIRBP) binding all-trans retinol. The concentration of xIRBP was 0.89 μM in each panel. A: The titration of IRBP with all-trans retinol in this panel is followed by monitoring the increase in retinol fluorescence compared to a fluorescence matched solution of N-acetyl-L-tryptophanamide (excitation: 330 nm; emission: 480 nm). B: The titration shown follows the quenching of intrinsic protein fluorescence by bound retinol (excitation: 280 nm; emission: 340 nm). C: The titration follows ligand binding by monitoring energy transfer (excitation: 280 nm; emission: 480 nm).