The models in Tables 8.2 and 8.3 were applied to data on the frequency dependence of 2D-NMR for solutions of the proteins lysozyme, bovine serum albumin (BSA), alkaline phosphatase, and hemocyanin. In all cases, models with at least two independent correlation times gave the best fit to the data. Some typical results are illustrated graphically in Figures 8.5 and 8.6. For globular proteins, the shortest correlation time (10~8 s) was identified with the rotational correlation time of the monomeric protein. A second correlation time (10 7 s) was attributed to aggregates of the proteins.
Extra sum of squares F tests confirmed that more than a single correlation time was required to fit the data with a 99% probability of best fit in all cases. Lysozyme, phosphatase, 10% BSA, and hemocyanin data required two correlation times (Table 8.3, k = 2), whereas three were required for 25% and 33% BSA (k = 3). The latter behavior is indicative of increased aggregation as protein concentration is increased. The Bj parameters in the model can be used to estimate the amount of hydration .
In the past, field dispersion data for protein solutions have been fitted by the Cole-Cole equation , However, this expression has little a priori validity. The correlation time found from the Cole-Cole analysis are overestimates compared to those found by analysis using the models in Table 8.3 ,
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