Comparison of tryptophan interactions to free and grafted BSA protein

The binding of D- and L-tryptophan molecules to bovine serum albumin (BSA) protein has been studied using liquid chromatography and ultrafiltration in the pH range from 7 to 11. A hydrophobic interaction between tryptophan an d BSA has been observed at pH 7.0 on BSA grafted chromatographic column. Ho wever, this interaction is negligible at higher pH for which the interactio n to the stereospecific site was predominant. For both grafted and free pro teins, the complexation mechanism was a competitive binding of D- and L-ena ntiomers on a single site. The apparent complexation constants for both D- and L-tryptophan show a maximum in the pH range 9-10. The variations of the apparent complexation constants versus pH were the result of the protonati on of both the amino acid and a single site of the protein assuming that th e complexation occurs between the zwitter-ionic amino acid form and the unp rotonated BSA site. The apparent pK(BSA) is slightly shifted from 8.3 for g rafted BSA protein to 9.4 for free BSA protein. This shift is presumably as a result of the different protein conformation. (C) 2000 Elsevier Science B.V. All rights reserved.