Thermodynamic and conformational studies on an immunoglobulin light chain which reversibly precipitates at low temperatures

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Abstract
A .lambda. chain, isolated from an immunoglobulin G molecule, was found to reversibly precipitate at low temperatures. This cryoprecipitation was a function of pH, ionic strength, protein concentration and time as well as temperature. The .lambda. chain underwent a cooperative conformational change as the temperature was lowered from 26 to 0.degree. C as judged by UV difference spectroscopy and circular dichroism. Normal .lambda. chains showed no conformational change. By difference spectroscopy it was possible to calculate the equilibrium constant governing the conformational change. The change was strongly exothermic (.DELTA.H .apprx. -80 kcal mol-1) and accompanied by a large decrease in entropy (.DELTA.S .apprx. -280 eu). The midpoint of the transition was dependent on the initial protein concentration, suggesting that only the noncovalent dimer of the .lambda. chain exhibited the conformational change. The existence of a monomer-dimer equilibrium (KA .apprx. 4 .times. 105 M-1) was confirmed by sedimentation velocity. No conformational change was observed by circular dichroism at concentrations where greater than 95% of .lambda. chain was in the form of a monomer. Although high ionic strength inhibited the cryoprecipitation, it had no effect on the conformational change. Stabilization of the dimer by forming an interchain disulfide bond between 2 monomers abolished the conformational change and cryoprecipitation. A fragment corresponding to the constant region was isolated from peptic and tryptic digests of the .lambda. chain. This fragment neither cryoprecipitated nor showed temperature dependence conformational changes. It proved impossible to isolate a fragment corresponding to the variable region. Qualitative and quantitative models are presented to account for the behavior of the .lambda. chain at low temperatures.

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