Feasibility of circular dichroism to study protein structure at extreme concentrations.

Circular dichroism (CD) is widely used for protein structure and interaction analyses. Here, we have studied the structure of four proteins at widely different concentrations ranging from 250 to 0.001 mg/ml using CD. These four proteins showed different concentration dependences of secondary and tertiary structures. Bovine gamma globulin showed near UV CD too weak to detect concentration dependence, while showing no concentration dependence of secondary structure. Lysozyme showed concentration and time dependent changes in secondary structure below 0.002 mg/ml, while showing no dependence of tertiary structure above 0.02 mg/ml. Chymotrypsin showed small, but significant, changes in both secondary and tertiary structures. Bovine serum albumin showed changes in secondary structure at pH 3.0 at low protein concentrations. In conclusion, we demonstrate feasibility of CD analysis to study protein structure at widely different protein concentrations.