Scanning calorimetric investigation of the polymerization of the coat protein of tobacco mosaic virus.

The endothermic polymerization of the coat protein of tobacco mosaic virus has been studied by high-sensitivity differential scanning calorimetry, with control experiments involving turbidimetry and sedimentation velocity measurements. The variation of the apparent extent of polymerization under conditions close to equilibrium as the temperature is raised follows a course which is difficult to duplicate on the basis of simple models for the process. The enthalpy of polymerization at low protein concentration varies from 12.5 kcal (mol of monomer)-1 (17500 daltons) under conditions where the product is largely a mixture of short helical rods to 6.0 kcal ol-1 for the formation of double disks containing 34 monomer units. In the former case, the polymerization is accompanied by a decrease in apparent heat capacity of 350 cal K-1 mol-1 while in the latter there is an increase of 150 cal K-1 mol-1. These results constitute evidence that these two types of polymerization involve intersubunit bonds of quite different chemical character.