Thermodynamic Values Related to the Association of L-tryptophan Analogues to Human Serum Albumin.

L-Tryptophan and some of its analogues were previously observed to bind predominately at one site on human serum albumin (1). The unique nature of this site which favors the binding of L-tryptophan with an association constant 100 times greater than n-tryptophan made it seem worthwhile to investigate the entropy, enthalpy, and free energy changes accompanying the association at this site. In addition to L-tryptophan, the other analogues used for this investigation were one without the ammonium group (3-indolepropionate), one without the carboxylate group (tryptamine), one with the charge removed from the ammonium group (acetyl-L-tryptophan), and one with neither the ammonium or carboxylate groups (skatole).