Photon echo spectroscopy as a probe of dynamics in the immune system

Photon echo spectroscopy has been used to resolve the amplitudes and time scales of reorganization resulting from electronic excitation of the chromophore in three fluorescein-binding antibodies. The spectral density of nuclear motions derived by fitting the data serves as a characterization of protein flexibility. The three antibodies show motions that range in time scale from tens of femtoseconds to nanoseconds. Relative to the others, one antibody, 4-4-20, possesses a rigid binding site, that likely results from a short and inflexible HCDR3 loop and residue TyrL32 acting as a 'molecular splint,' to rigidify the Ag across its most flexible degree of freedom. The remaining two antibodies possess binding sites that are considerably more flexible, possibly due to the increased length of the HCDR3 loops. These variations in binding site flexibility may result in differing mechanisms of antigen recognition, including lock-and-key, induced-fit, and conformational selection.