SynopsisSerum albumin undergoes a conformational change a t pH 4, known as the N-F transition. In the customary LinderstrZm-Lang treatment of hydrogen ion titration, the carboxyl groups in serum albumin either have an abnormally low pK, or are buried in charged form, and the LinderstrZm-Lang charging parameter w decreases dramatically at the N-F isomerization. In the present paper partition functions are derived and distribution functions are calculated for a model permitting salt bonding between the positively and negatively charged sites on a macromolecule. The N-form has an abnormally high salt bonding constant whereas that of the F-form corresponds to that of small ions. The result obtained is consistent with a "normal" intrinsic pK of the carboxyl groups of serum albumin without burying of any charges and with an unchanged W.Further, it is shown that the "abnormal salt bonding" of serum album'n can explain its unusual ability to bind anions. Theoretical binding curves are calculated and compared with literature data of the C1-binding of serum albumin. The relation of the present model to other models of hydrogen ion and anion binding to proteins is discussed. Some additional consequences of the present model are pointed out; a transition in the alkaline range, analogous to the acid transition, seems probable. Literature data support the existence of such a transition but do not allow detailed calculations a t present.A general, thermodynamic treatment of the interactions between small ligands and macromolecules is outlined. Important points are the choice of the statistical-mechanical ensemble and considerations of the fluctuations about the mean binding, if (i)there are not only a ligand-locus interaction but also interligand interactions (in particular intdigand attraction), or (ii) there is a conformational change in themacromolecule depending on the ligand binding. In these cases, the binding isotherms obtained from thermodynamically closed systems (canonical ensemble)) may erroneously indicate a distribution about a single probability maximum, i.e., the statistical mean binding N, and fluctuations about this value. The description of a phase change in a bound phase or a change in the "internal" self-interactions of a macromolecule requires a binding equation permitting distributions about two maxima, i.e., (i) N1* < iV ("thin" phase) and Nz* > iV (L'condensed" phase) or (ii) two macromolecular conformations P', and P", having occupancy numbers iV, and ITz, respectively. The N-F transition is an example illustrating the relation between the complete distribution functions and the two-state approximation. 2197