What does circular dichroism measure in proteins?

The measurement of circular dichroism (CD) in proteins primarily involves analyzing the differences in absorption of left and right circularly polarized light. This technique is extremely valuable for understanding the secondary structure of proteins since different structural elements (such as alpha-helices and beta-sheets) have distinct CD spectra.

The basis of circular dichroism lies in the fact that chiral molecules, like proteins, can interact differently with circularly polarized light. Left and right circularly polarized light consists of waves rotating in opposite directions, and despite being constituted of the same wavelengths, proteins can absorb these two types differently based on their structural conformation. By measuring the extent of this differential absorption, researchers can infer vital information about the protein's secondary structure and folding. This characteristic is not only essential for protein characterization but also for examining conformational changes induced by various factors such as ligand binding, temperature variations, or changes in pH.

The other options either do not accurately describe the technique or focus on aspects unrelated to circular dichroism's primary purpose in protein studies. For example, measuring the angle of rotational change in light is more associated with optical activity but does not encompass the specific functionality of CD in terms of left vs. right circularly polarized light absorption.