On the distribution of the bulk-solvent correction parameters

The distribution of the bulk-solvent correction parameters (B_sol, k_sol) (as determined with an exponential scaling algorithm based on Babinet's principle) for 219 crystal structures deposited in the Protein Data Bank is presented. The distribution shows that (i) the range of values observed is far wider than the usually cited parameter range, (ii) the observed k_sol values do not correlate with their assumed physical meaning and (iii) the two parameters are not independent and a reasonable agreement with the experimental data can be obtained through the application of a simple exponential function. These findings are interpreted in terms of the inability of the currently used algorithms to uncouple the values of the two parameters during macromolecular refinement.