The Effect of the Protein Dynamical Transition on Intramolecular Vibrations

Abstract

Protein structural motion is necessary for function. A typical measure of protein flexibility is the average mean squared atomic displacement (MSD). Both X-ray and neutron scattering have found a strong increase in the MSD at 200-220 K for a number of proteins. This increase is often referred to as the dynamical transition (DT) [1]. Function ceases for many proteins below the DT. It was suggested that the DT indicated the temperature at which the motions necessary for function are frozen out. A question has been raised as to how long-range motions are affected by the transition. Specifically, would the long-range intramolecular vibrations associated with conformational changes be present at low frequencies, and then at higher temperatures red shift due to anharmonicity, as is seen in conventional solids? The different DT measurement techniques do not distinguish between local particle excitations and coherent long-range motions. Direct optical measurement of the long-range intramolecular vibrations requires spectroscopy within the energy range of 1-100 cm−1 (ν = 0.03-3 THz). Our recently developed technique, CATM (Crystal Anisotropy Terahertz Microscopy) [2], uses polarization dependent absorption on aligned molecules to isolate the long-range intramolecular vibrations from the local excitations. Temperature dependent CATM from 180K to 295K is used to investigate how the DT effects long-range vibrations. We find that a sharp 70 cm-1 band grows in as the temperature increases. We suggest that this new band above DT demonstrates that in fact the surrounding solvent acts as a frozen cage preventing long-range correlated motions, and as the surrounding solvent becomes more mobile, the large-scale motions necessary for function can occur.1.Doster, W., et al. Physical Review Letters, 2010. 104(9): p. 098101.2.Acbas, G., et al. Nature Communications, 2014. 5, 3076 http://dx.doi.org/10.1038/ncomms4076.