Effect of Mid-infrared Free-Electron Laser Irradiation on Refolding of Amyloid-Like Fibrils of Lysozyme into Native Form

Aggregation of lysozyme in an acidic solution generates inactive amyloid-like fibrils, with a broad infrared peak appearing at 1,610?C1,630?cm^?1, characteristic of a ??-sheet rich structure. We report here that spontaneous refolding of these fibrils in water could be promoted by mid-infrared free-electron laser (mid-IR FEL) irradiation targeting the amide bands. The Fourier transform infrared spectrum of the fibrils reflected a ??-sheet content that was as low as that of the native structure, following FEL irradiation at 1,620?cm^?1 (amide I band); both transmission-electron microscopy imaging and Congo Red assay results also demonstrated a reduced fibril structure, and the enzymatic activity of lysozyme fibrils recovered to 70?C90?% of the native form. Both irradiations at 1,535?cm^?1(amide II band) and 1,240?cm^?1 (amide III band) were also more effective for the refolding of the fibrils than mere heating in the absence of FEL. On the contrary, either irradiation at 1,100 or 2,000?cm^?1 afforded only about 60?% recovery of lysozyme activity. These results indicate that the specific FEL irradiation tuned to amide bands is efficient in refolding of lysozyme fibrils into native form.