Use of optical spectroscopic methods to study the thermodynamic stability of proteins
The biophysical characterization of globular proteins will almost always include some type of study of the unfolding of protein to obtain thermodynamic parameters. The basic idea is that a transition between a native and unfolded state, induced by temperature, pH, or denaturant concentration, can serve as a standard reaction for obtaining a thermodynamic measure of the stability of the native state. For example, the free energy change for the unfolding reaction can be used to compare the stability of a set of mutant forms of a protein (1-4). This type of analysis is based both on assumptions of the thermodynamic model for the unfolding process and on assumptions in the way the data are analysed; some of these assumptions and their limitations will be discussed below. There are a variety of methods that can be used to monitor an unfolding process. A common method is differential scanning calorimetry, DSC, which measures the variation in the specific heat of a protein-containing solution as a protein is thermally unfolded (5-7). DSC is a popular method for this purpose, but optical methods can also provide suitable information for tracking the unfolding of a protein The spectroscopic signals for the native and unfolded states of a protein can give some insight regarding the structure of the states, and often can provide advantages of economy, ease of measurement and amenability to a wide range of sample concentration. The optical spectroscopic methods that have been used most often for this purpose are absorption spectroscopy, circular dichroism and fluorescence, which will be discussed in this chapter. A key to each of these methods and their use in protein unfolding studies is that the signal is a mole fraction weighted average of the signals of each thermodynamic state. That is, the observed signal, S, can be expressed as . . . S = ∑XiSi . . . . . . 1 . . . where Xi is the mole fraction of species i and si is the intrinsic signal of species i. In order for a particular spectroscopic signal to be useful for tracking a N ↔ U transition of a protein, the signal must be sufficiently different for the N and U states.