Determination, by dynamic surface-tension analysis, of the molar mass of proteins denatured in guanidine thiocyanate.

A drop-based dynamic surface-tension detector (DSTD) has been used to study the dynamic surface tension behavior of proteins denatured in guanidine thiocyanate (GndSCN). The dynamic surface tension at the air-liquid interface is obtained by measuring the internal pressure of drops that grow and detach at a specified rate. In the method the sample of interest is injected and subsequently flows to the DSTD-sensing capillary tip. For this work, a novel DSTD calibration procedure utilizing two distinct mobile phases is applied. Here, the mobile phases are aqueous with different constituents, for example GndSCN and phosphate buffer, either added or omitted. The dual-mobile phase calibration procedure gives the analyst the capability of making protein measurements in a GndSCN-phosphate buffer mobile phase, while measuring a calibration standard in another mobile phase, such as water, in which the surface tension of the calibration standard is readily available. Results are presented with drop volumes of either 2 microL (i.e. 2-s drops) or 7 microL (i.e. 7-s drops) for proteins varying in molar mass from 12,000 to 330,000 g mol(-1). We demonstrate that the DSTD can be used to determine the molar mass of proteins denatured in GndSCN. The method applies a regime where the denatured protein is detected by surface-active properties, and selectivity with regard to molar mass is contained in the dynamic component of the DSTD signal. The dynamic surface pressure signals of the denatured proteins suggest that diffusion plays a large role in the kinetics of the surface activity. The limit of detection for the denatured proteins studied ranged from 3 mg L(-1) to 14 mg L(-1). The DSTD, coupled with the novel dual-mobile phase calibration procedure, can be used to investigate the fundamental properties of proteins. Insight into the behavior at the air-liquid interface for native and denatured proteins is achieved; this is a novel tool for studying protein denaturation, complementary to other common approaches such as spectroscopy and calorimetry. Furthermore, the reported method could be widely applied to the study of effects on the interfacial properties of proteins after a variety of chemical and physical modifications that are possible with the dual-mobile phase calibration procedure.