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Heat Capacities from 11 to 305° K and Entropies of Hydrated and Anhydrous Bovine Zinc Insulin and Bovine Chymotrypsinogen A

ENTROPY CHANGE FOR FORMATION OF PEPTIDE BONDS

From the ¹ From the Departments of Physiology and Chemistry and the James Franck Institute, University of Chicago, Chicago, Illinois 60637

1. Heat capacities from 11–305°K have been measured for crystalline bovine zinc insulin containing 4% water and for crystalline chymotrypsinogen A containing 10.7% water. Both proteins were then studied in the anhydrous state. Heat capacities, entropies, (H° - H₀°)/T, and -(G° - H₀°)/ T are tabulated as functions of temperature for the anhydrous materials. For the hydrated samples only heat capacities and entropies are tabulated.

2. No elevated heat capacities for the hydrated samples were noted in the region 250–275°K; i.e. there was no evidence of the fusion of ice. Values of the apparent molal heat capacity of water in the proteins are calculated.

3.The average value for the entropy of formation of a peptide bond is 9.3 cal deg^-1 per bond for insulin and 9.0 cal deg^-1 per bond for chymotrypsinogen A which is close to the value observed in the formation of the bond in dipeptides.

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