Unlike rigid colloidal particles, proteins have only limited conformational stability and any perturbation, such as adsorption to a surface, may involve energies comparable to the cohesive energy of the molecule and hence may engender conformational changes. Suppose that the enthalpy of protein adhesion to a surface equals or exceeds the enthalpy of the intramolecular contacts in the native protein conformation. Then unfolding (denaturation), which implies a gain of entropy, since many denatured conformations are possible, inevitably implies a loss of free energy, which drives the process towards adsorption with denaturation (Fernández and
6 If the particle-free solution (PFS) is introduced rapidly, then the entire diffusion boundary region will be rapidly swept clear, i. e. c(z) = 0 for all z > 0. On the other hand, if the PFS is introduced slowly, then c(z) = 0 for only z > ó initially.
Ramsden 2001). In general, it can only be prevented if the surface interacts less with the protein than the protein interacts with itself, i. e. if the molecule would lose less enthalpy upon binding than upon folding into its native conformation.
In considering the energy balance, it should be borne in mind that many surfaces engender changes in the solvent structure, especially of extensively hydrogen bonded solvents such as water (Wiggins 2002), and these changes may perturb the protein even before it arrives at the surface.
Evidence for conformational rearrangement, including simple reorientation, can be obtained from a series of adsorption measurements carried out at different bulk concentrations. If the usual method of analysis (e. g. Ramsden 1993a) yields decreasing a with increasing cb, rearrangement can be inferred, and its characteristic time ts = 1/ks can be obtained by equating it to the characteristic time for adsorption, Ta = 1/(Jcb0a), at the bulk concentration corresponding to the mid-point between the limiting lower and higher areas, where J is the protein flux to the empty surface normalized to unit adsorbent area and unit bulk concentration. A more sophisticated approach involves simultaneously fitting the adsorption data to equations explicitly taking rearrangement into account (Van Tassel et al. 1998):
where subscripts a and ft refer respectively to the lower and higher areas, and W is a function analogous to 0 giving the probability that space to rearrange (taking up more space) is available.
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