Possible pH variations during crystallization of biological macromolecules by the vapor diffusion method have not been taken into account in most experiments so far reported. The present study demonstrates that when ammonium sulfate is used as the precipitant, pH changes occur due to ammonia transfer following ammonium/ammonia equilibrium. The pH in a crystallization droplet is shown to be controlled by that of the reservoir. The theory of the effect is given and the consequences of pH variations during crystallization are discussed in terms of reproducibility of experiments. An application, the crystallization of concanavalin A induced by pH variation, is presented.

During crystallization of biological macromolecules by the vapor diffusion method, with ammonium sulfate as the precipitant, pH variations were shown to occur in the crystallization droplet [Mikol, Rodeau & Giegé, (1989). J. Appl. Cryst. 22, 155–161]. Using pH-sensitive electrodes, the kinetics of these pH changes have now been experimentally studied as a function of the volume (10 to 50 μl) and initial pH (6.0 or 8.0) of the droplet, with a reservoir at pH 7.0, at 278, 286 and 293 K. A theoretical analysis is given and a model for estimating these kinetics under given crystallization conditions is proposed, illustrated by an application to the time course of concanavalin A supersaturation.

CD38 is a type II transmembrane glycoprotein with both ADP-rybosyl cyclase and glycohydrolase activities. CD38 is highly expressed at the surface of malignant plasma cells of multiple myeloma. SAR650984 is a humanized IgG1 antibody targeting CD38 in early clinical developement that is acting through several potential mechanisms including ADCC, CDC and pro-apoptotic activity. Here we report further preclinical characterization of SAR650984 with a high resolution structure of Fab-SAR650984 in complex with CD38 allowing an epitope mapping. The crystal structure of SAR650984-Fab/huCD38 complex shows that SAR650984 neither blocks the access nor alters the configuration of the ADPRC catalytic site of CD38 although in vitro assays have demonstrated that SAR650984 behaves as a strong inhibitor of the ADPRC activity of CD38. These results suggest that SAR650984 is likely an allosteric antagonist of CD38 that alters the dynamics of enzyme upon binding.