Partitioning of neutral organic compounds to structural proteins

Protein-water partition coefficients (K(pw)) of neutral organic chemicals were measured using muscle proteins (from chicken, fish, and pig), collagen and gelatin. K(pw) values for these structural proteins were consistently lower than those of bovine serum albumin (BSA), indicating that the use of BSA as a model protein leads to an overestimation of K(pw) for structural proteins. Differences in K(pw) between chicken, fish, and pig muscle proteins were small. Across the structural proteins, K(pw) values were often in the order: muscle proteins > collagen ≥ gelatin. Differences in K(pw) between the structural proteins were relatively large (<2 log units) for nonpolar compounds, and much smaller or insignificant for polar compounds. There were correlations between log K(pw) of muscle proteins and log K(ow) (R(2) = 0.83-0.86, SD: 0.35-0.40, n = 45-46). The polyparameter linear free energy relationship (PP-LFER) models fit even better to the data (R(2) = 0.95, SD: 0.22). The good model fitting suggests that the reversible binding to muscle proteins can be considered to be nonspecific binding. There was an indication that some chemicals may sorb irreversibly to muscle proteins, which needs further research. We found that the partitioning to muscle protein is typically weaker than that to lipids, but that the protein partitioning of an H-bond donor compound can be as strong as the storage lipid partitioning.