Diffusion, generally anomalies

The typical symptoms of untreated individuals with PKU are the manifestation of the neurotoxic effect of phenylalanine on the central nervous system. A morphological change in the brain in patients with PKU affects both white and gray matter. Microcephaly, where the brain mass can be 80 % of that of a healthy individual, is characteristic feature for many untreated PKU patients [37]. This symptom is caused by myelin structure anomalies that result in a loss of myelin volume, disturbances in cortical neuronal development, diffuse cortical atrophy, and general abnormalities in protein synthesis [37, 39, 40]. 

While trends are generally consistent with expectations, the higher value of D for Polymer JR 400 in virgin hair than bleached hair is inconsistent. Furthermore, derived values of D for the diffusion of Polymer JR 125 into bleached hair showed a strong dependence on concentration (7). Some anomalies also exist in the desorption kinetics as discussed below. For these reasons the simple diffusion model can only be considered prelim-inaiy. Undoubtedly, effects associated with the structure of hair, as discussed in the conclusion of this chapter, are involved. 

The regions of thermodynamic, dynamic and structural anomalies display the same hierarchy as in the monomeric case. In general, the range of pressures and temperatures occupied by these anomalous regions are larger than those observed in the monomeric case. The differences are more pronoimced for the regions of diffusion and structural anomalies and not so large for the thermodynamic anomalies. 

It is a remarkable and indeed essential feature of the method that boundary anomalies which may seriously interfere in boundary electrophoresis appear to influence zone electrophoresis to a much smaller degree. A detailed investigation of this in zone electrophoresis has not yet been made, but it appears likely that two factors are of importance in this respect. First the concentration of the migrating substance in each zone is generally lower than in boundary experiments. Second, as the front and the rear of each zone are much closer than in U-tube experiments, diffusion of electrolytes may tend to even out to some degree the local differences in the composition of the medium which are the cause of these anomalies. In any case, as has already been mentioned, zone electrophoresis is applicable to a number of substances of comparatively low molecular weight which are difficult or impossible to study in boundary electrophoresis because of pronounced boundary anomalies (e.g., amino acids, peptides, and nucleotides). 

The diffusion behaviour in a rubbery state is generally well described by Pick s laws with constant or concentration-dependent diffusivity. The rate of diffusion is much lower than the rate of relaxation and the polymer chains adjust so quickly to the presence of the liquid that they do not cause diffusion anomalies. The amount of liquid transferred at time t, C, as a fraction of the total amount of liquid transferred at infinite time, is very often expressed as a function of time by the relationship (Equation 11.1) ... 

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