Capillary circular dichroism

Automated High-Throughput Capillary Circular Dichroism and Intrinsic Fluorescence Spectroscopy for Rapid Determination of Protein Structure

Assessing the physical stability of proteins is one of the most important challenges in the development, manufacture, and formulation of biotherapeutics. Here, we describe a method for combining and automating circular dichroism and intrinsic protein fluorescence spectroscopy. By robotically injecting samples from a 96-well plate into an optically compliant capillary flow cell, complementary information about the secondary and tertiary structural state of a protein can be collected in an unattended manner from considerably reduced volumes of sample compared to conventional techniques. We demonstrate the accuracy and reproducibility of this method. Furthermore, we show how structural screening can be used to monitor unfolding of proteins in two case studies using (i) a chaotropic denaturant (urea) and (ii) low-pH buffers used for monoclonal antibody (mAb) purification during Protein A chromatography.

Circular and linear dichroism spectroscopy for the study of protein-ligand interactions

Circular dichroism (CD) is the difference in absorption of left and right circularly polarized light, usually by a solution containing the molecules of interest. A non-zero signal for solutions is only measured for chiral molecules such as proteins whose mirror image is not superposable on the original molecule. A CD spectrum provides information about the bonds and structures responsible for the chirality. When a small molecule (or ligand) binds to a protein, it acquires an induced CD (ICD) spectrum through chiral perturbation to its structure or electron rearrangements (transitions). The wavelengths of this ICD are determined by the ligand's own absorption spectrum, and the intensity of the ICD spectrum is determined by the strength and geometry of its interaction with the protein. Thus, ICD can be used to probe the binding of ligands to proteins. This chapter contains an outline of how to perform protein CD and ICD experiments, together with some of the issues relating to experimental design and implementation. Addition of a quarter wave plate to a CD spectropolarimeter converts it to a linear dichroism (LD) spectrometer. When protein samples are aligned either in flow (as for fibers or membrane proteins in liposomes) or on surfaces the orientations of ligands with respect to the protein backbone or other subunits can be determined.