The nuclear spin relaxation induced by a freely diffusing paramagnetic center provides a direct measure of intermolecular accessibility. A number of factors are involved in a quantitative interpretation of relaxation data including excluded volume effects, solvation differences, and the details of the electron spin relaxation in the paramagnetic center. In the case where the electron relaxation time is short compared with correlation times describing the electron-nuclear coupling, the nuclear spin relaxation rates may be related to the effective local concentration of the paramagnetic center at different locations about the solute of interest. The local concentrations may in turn be related to differences in the local free energies of interaction between the diffusing paramagnet and the cosolute. We demonstrate this approach for the case of ribonuclease A and deduce surface free energy differences for a large number of protein proton sites. We find that the oxygen accessibility is poorly represented by hard-sphere models such as computed solvent or steric accessibility. There is a distribution of local intermolecular interactions with a width of the order of RT that dominates the report of the intermolecular exploration of the protein by this simple solute.
Directly correlating lifetime to coating composition by using standardized, artificial exposures, or even natural exposure, is often very difficult. However, significant progress can be made by breaking down the problem into smaller questions, which can be separately addressed. If one understands the physical parameters that affect end-use properties, then one can also group, and thus correlate, properties according to whether they depend on processes at the surface or in the bulk of a coating, or whether they depend on defects. A scheme is presented that shows how one can use knowledge from analytical physical or chemical materials science in a statistical model related to the ''chemical paradigm.'' Simple physical models that use this information, about the initial state of the coating and its rate of degradation, can be used to compare the performance of coatings and estimate, simply, service lifetime depending on the property of interest, in the environment of interest. One can see that different properties are sensitive in different ways to the degradation process and decay with a different rate. Thus, although properties may be determined by the same degradation process, and location within a coating, they do not correlate directly. These approaches show how to organize our knowledge of degradation processes, and environments, and be able to make some testable predictions on how coating properties deteriorate.
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