First-order phase transition in protein dynamics of ferritin

Detailed Mössbauer spectra of⁵⁷Fe in the iron storage protein, ferritin, in the temperature range between 250 and 280 K reveal a first-order phase transition with a thermal hysteresis loop of 7 K width. While the temperature is raised from 90 K to 271 K, Mössbauer spectra composed of a narrow line quadrupole doublet, typical for solids, are observed. Above this temperature, each spectrum is composed of the narrow line subspectrum and a broad line subspectrum whose relative intensity increases with temperature. The intensity of the narrow line subspectrum decreases by a factor of five at the critical temperature and thus shows a large increase in the mean square displacements at T_up=271 K. While decreasing the temperature, the bounded diffusive motions, expressed in the spectra by the coexistence of the narrow and broad lines, survive down to T_down=264 K, where again the spectral shapes and areas undergo a discontinuous jump. The narrow line subspectrum increases in intensity and the broad line subspectrum disappears. These phenomena may be understood in terms of supercooling of the water in the free channels and in the cavity of the ferritin molecule.