Power-law dependence of the melting temperature of ubiquitin on the volume fraction of macromolecular crowders

The dependence of the melting temperature increase (Delta T-m) of the protein ubiquitin on the volume fraction (phi) of several commonly used macromolecular crowding agents (dextran 6, 40, and 70 and ficoll 70) was quantitatively examined and compared to a recently developed theoretical crowding model, i.e., Delta T-m similar to (R-g/R-c)(alpha)phi(alpha/3). We found that in the current case this model correctly predicts the power-law dependence of Delta T-m on phi but significantly overestimates the role of the size (i.e., R-c) of the crowding agent. In addition, we found that for ubiquitin the exponent alpha is in the range of 4.1-6.5, suggesting that the relation of alpha = 3/(3 nu -1) is a better choice for estimating alpha based on the Flory coefficient (nu) of the polypeptide chain. Taken together these findings highlight the importance of improving our knowledge and theoretical treatment of the microcompartmentalization of the commonly used model crowding agents. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3556671]