Cold Denaturation of Proteins in the Absence of Solvent

The effect of temperature on the stability of proteins is well explored for high temperatures, but harder to track below the freezing point of water. This challenge is met with the use of variable temperature ion mobility mass spectrometry (VT IM-MS), which allows the structure of isolated, solvent free molecules to be measured at sub ambient temperatures in the form of their collision cross section (CCS). Here we monitor conformational changes that occur to two isotypes of monoclonal antibodies over a temperature range from 295 to 165 K. For each we observe a large increase in the magnitude of the CCS at 250K (-20 °C) substantially above that predicted. This loss of structure in the absence of bulk solvent is attributed to a change in the strength of stabilizing intermolecular interactions, causing rearrangement. At 190 K (-80 °C) the CCS distribution narrows which we attribute to better resolution. These findings indicate that in vacuo deep-freezing minimizes denaturation and maintains the gas phase native fold supporting this practice in vitro. Comparing the data for each isotype suggests that the disulfide bridging influences thermal structural rearrangement and taken together we show that this method provides unique insights to the phenomenon of cold denaturation.

Implicit water model within the Zimm-Bragg approach to analyze experimental data for heat and cold denaturation of proteins

Studies of biopolymer conformations essentially rely on theoretical models that are routinely used to process and analyze experimental data. While modern experiments allow study of single molecules in vivo, corresponding theories date back to the early 1950s and require an essential update to include the recent significant progress in the description of water. The Hamiltonian formulation of the Zimm-Bragg model we propose includes a simplified, yet explicit model of water-polypeptide interactions that transforms into the equivalent implicit description after performing the summation of solvent degrees of freedom in the partition function. Here we show that our model fits very well to the circular dichroism experimental data for both heat and cold denaturation and provides the energies of inter- and intra-molecular H-bonds, unavailable with other processing methods. The revealed delicate balance between these energies determines the conditions for the existence of cold denaturation and thus clarifies its absence in some proteins.

On the liquid demixing of water + elastin-like polypeptide mixtures: bimodal re-entrant phase behaviour

Water + Elastin-like Polypeptides (ELPs) exhibit a transition temperature below which the chains transform from collapsed to expanded states, reminiscent of cold denaturation. This conformational change coincides with liquid-liquid phase...