Application of induced optical activity to the study of protein denaturation

Terahertz circular polarization sensing for protein denaturation based on a twisted dual-layer metasurface

Protein denaturation has very important research value in nutrition, biomedicine, and the food industry, which is caused by the changes in the molecular structure of the protein. Since the collective vibrational and torsional modes of protein molecules are within the terahertz (THz) frequency range, THz spectroscopy can characterize the protein denaturation with several advantages of non-contact, label-free, real-time, and non-destructive. Therefore, we proposed a reflective THz time-domain polarization spectroscopy sensing method, and use a flexible twisted dual-layer metasurface film as a sensor to realize the thermal denaturation sensing, concentration sensing, and types identification of protein aqueous solutions. The experiment tested three proteins (bovine serum albumin, whey protein, and ovalbumin), and the results show that: for the thermal denaturation sensing, its detection sensitivity can reach 6.30 dB/% and the detection accuracy is 0.77%; for the concentration sensing, the detection sensitivity and detection accuracy reach 52.9 dB·mL/g and 3.6·10^-5 g/mL, respectively; in addition, different protein types can be distinguished by the difference of the circular polarization spectra.

Circular dichroism and fluorescence studies on the binding of ligands to the alpha subunit of tryptophan synthase

Binding to the alpha subunit of tryptophan synthase induces extrinsic Cotton effects in the substrates indole (IND), indoleglycerol phosphate (IGP), and D-glyceraldehyde-3-P (D-GAP) and in the inhibitor indolepropanol phosphate (IPP). These effects disappear when the enzyme is denatured in guanidinium chloride. The induced circular dichroism (CD) was used to determine the dissociation constant and the number of binding sites for IPP. The dissociation constant so determined is equal to 48 muM and is in good agreement with the value of 48 muM obtained by equilibrium dialysis. From the temperature dependence of the dissociation constant, a value of -2.8 kcal/mol for the binding enthalpy was obtained. The determination of dissociation constants by means of extrinsic Cotton effects is shown to be quite feasible. CD competition experiments with glycerol phosphate (GP) suggest that IPP binds bifunctionally to the enzyme: via its indole part and its phosphate group. Indolepropanol, which lacks the phosphate group, does not show an extrinsic Cotton effect. Since the induced CD is strongly dependent on the binding geometry, the close similarity between the induced spectra in IPP and IGP is additional evidence that IPP is a good substrate analog. Binding to the enzyme results in a blue shift of the IPP fluorescence emission maximum. The dissociation constant determined by fluorescence titration equals 46 muM and agrees well with the values determined by the other two methods. Previous biochemical and fast kinetic studies suggested the existence of multiple conformational states for the enzyme and of ligand-induced conformational changes. No evidence was found in the far-uv CD spectra for conformational changes upon binding of IND and D-GAP. For IPP a very small effect was observed.