Isotherm with Conformational Changes

We now generalize the model treated in section 2.10, but this time the polymers and the ligands are in the liquid phase. Again, the simplest way to obtain the generalized adsorption isotherm as well as other thermodynamic quantities of the system is to view the adsorption process as a chemical reaction, exactly as in the previous section, i.e., 

Formally, all the equations of the previous example apply here. The general result (8.4.10) is the same, with the reinterpretation of the coupling work W Si w) and W Sq w) in terms of the coupling work of the two conformations. Also, the binding energy U is reinterpreted as an average in the same way as in section 2.10. 

Again we assume that the polymer can be in one of two conformational states in equilibrium 

Normally, these two states are characterized by their Helmholtz (or Gibbs) energy levels (see section 2.4.4) which include many quantum-mechanical energy levels. However, as in the case of section 2.10, in order to stress the new appearance of Helmholtz (or Gibbs) 

This should be compared with (2.10.9). In the general case, Qh and the PFs of the empty L and H, could be different in their rotational-vibrational PFs (but not the momentum PF). Here we have assumed, as in section 2,10, that the two states L and H are characterized only by their energy levels El and Eh- The new factor that appears in (8.4.23) is the indirect work, here the Gibbs energy change, for the conversion of an empty L into an empty H. 

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The application of total internal reflection fluorescence spectroscopy (TIRF) by this laboratory to the study of protein adsorption at solid-liquid interfaces is reviewed. TIRF has been used to determine adsorption isotherms and adsorption rates from single-and multi-component protein solutions. Initial adsorption rates of BSA can be explained qualitatively by the properties of the adsorbing surface. Most recently, a TIRF study using monoclonal antibodies to probe the conformation of adsorbed sperm whale myoglobin (Mb) elucidated two aspects of the Mb adsorption process 1) Mb adsorbs in a non-random manner. 2) Conformational changes of adsorbed Mb, if they occur, are minor and confined to local regions of the molecule. Fluorescence energy transfer and proteolytic enzyme techniques, when coupled with TIRF, can characterize, respectively, the conformation and orientation of adsorbed Mb. 

Several applications of isothermal titration calorimetry to biochemical systems have been published recently. Franghanel et al. [240] proposed a new calorimetric method to investigate slow conformational changes in proteins and peptides, Tsamaloukas et al. [241] studied the interactions of cholesterol with unsaturated phospholipids and D Amico et al. [242] determined the binding enthalpies of a-amylases to different substrates. 

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