Thermal properties of human IgG.

Dynamic light scattering experiments have been performed to study the aggregation kinetics of human immunoglobulin G (IgG). Aggregation and irreversible cluster growth results when IgG solutions (2-15 mg/ml) are heated above 50 degrees C. The measured scattering intensity I and effective hydrodynamic radius (R) can be described consistently by a Smoluchowski aggregation process. The number of clusters ni(t) containing i monomers at time t are computed. The radius of an i cluster is assumed to be Ri = R0 i beta, where beta is the cluster exponent. This kinetic process results in the following characteristic power law behavior: (R)/R0 = (1 + gamma R (T, C, c)t) alpha R and (I)/I0 = (1 + gamma 1 (T, C, c)t) alpha I. Here R0 = 5.51 nm, is the monomer hydrodynamic radius, and I0 the scattered intensity from the monomer solution at temperature T and concn C. A fraction, c approximately 0.48 of the IgG monomers are heat stable up to 63 degrees C and do not participate in the aggregation process. The power-law behavior of mean value of R/R0 and mean value of I/I0 indicates scaling, and indeed a very satisfactory data collapse results from our data. The best non-linear fit of the power-law forms gives alpha R = 0.48 +/- 0.05, alpha I = 1.00 +/- 0.01 and beta = 0.39 +/- 0.04. We also find that the heat aggregation of IgG is an activated process. Fits of the experimental data Gibbs free energy for the activated complex delta G* = 13.8 +/- 0.1 kcal/mole at 56 degrees C. The temp dependence of the growth rates exhibits an Arrhenius behavior with an enthalpy of activation delta H* = 120 +/- 5 kcal/mole.