[article in Biopolymers 17,817-836 (1978)] Previously reported' standard pressure coefficients (Go) and standard partial specific volumes of the solvated component (i&) for egg albumin (EA), pooled human y-immunoglobulin (IgG), and bovine serum mercaptalbumin (BMA) are in error. These errors are due to the incorrect use of the formula2 to calculate the quantity ( r f -r:). The effect of using the correct expression for (r: -r:) is to substantially decrease in magnitude Ap-especially a t high angular velocities (w2). This effect substantially changes the slopes of the Ap-vs-PO plots for the aforementioned proteins. The new values for Us,o for EA at pHs 10.14 and 7.14 and IgG at pH 7.00 are 0.766,0.762, and 0.772 ml/g, respectively. The new values of $O are 35.9 f 1.3,34.6 f 1.1, and 9.6 f 5.1 X atm-*, respectively. Note that Go is positive for IgG in contrast to the negative value reported previously, but is not significantly different from zero at p = 0.05. The Ap-vs-PO plot for BMA at pH 5.34 using the correct expression for ( r f -r;) retains its curvilinear characteristics. No values of Uz,O and $0 are reported for BMA because of this curvilinear character and the lack of data at low pressures. The discussions regarding the physical significance of these values for Us,O and $O in the previous report' should be reevaluated accordingly.

The compositional buoyant densities, ρ 00;, of human γ‐immunoglobulin, bovine serum mercaptalbumin, and egg albumin have been measured in CsCl solutions in the analytical ultracentrifuge as a function or pressure. Standard pressure coefficients, ψ0, and standard partial specific volumes of the solvated proteins, υ 0italicS,0, have been computed from these data. The ψ0 values obtained are strikingly different from each other and from the only other pressure coefficients which have been measured, those values obtained for nucleic acids and nucleoproteins. The ψ value for γ‐immunoglobulin is negative, the first nonpositive value obtained, and suggests an unusual internal structure for this protein. The pressure coefficient of mercaptalbumin is not constant. A second‐order relation is derived and utilized to interpret these data. The slope of the ρ 00(P) plot for egg albumin was constant and negative and yielded values of ψ0 which are about 20% as large as those reported for DNA. Evaluation of published isopiestic data for egg albumin in CsCl solutions provided the dependence of preferential hydration on water activity. This quantity, (dΓ′/da 00) as well as α, were found to be negative. The values of ψ0 and α were used to compute the effective density gradient from which the correct molecular weight of egg albumin was obtained. The apparent specific volume of egg albumin in a buoyant CsCl solution was measured using the Mettler‐Paar densimeter.

The buoyant titrations of poly-L-lysine and poly-L-histidine in CsCI, RbCI, CsBr, RbBr and KBr were measured. Large differences in buoyant densities measured at low pH were observed for both polymers. Densities in the alkali chloride solutions are lower than for the bromides and the buoyant density increases as the size of the cation decreases in the halide series. All buoyant densities converge to a common value for each polymer at high pH. These data are interpreted in terms of salt-pair formation at low pH and preferential hydrations were computed both for this charged species at low pH and for the neutral polymers at high pH. The resulting data were correlated with the water activity in each buoyant solution. The variation of hydration with water activity for the charged species is found to be similar to that previously reported for bovine serum mercaptalbumin while the hydrations of the neutral polymers are found to be nearly independent of water activity. Approximate observed ionization constants were determined for each polymer in four salt solutions.