Beryllium binding at neutral pH: The importance of the motif

Beryllium speciation at physiological conditions is critical to understanding chronic beryllium disease (CBD). The MHC-class II receptor alleles that have been linked to CBD have more than six carboxylates in a short 20 amino acid segment of the binding pocket and it has been suggested that beryllium may bind within the MHC-class II receptor via the carboxylates. Previous reports also show that citric acid binds beryllium significantly stronger than similar carboxylate ligands such as tartaric acid and is one of the few ligands that can compete with hydrolysis to solubilize beryllium across the entire pH range at molar concentrations. We have characterized the binding of Be to citric acid and shown using a combination of NMR, mass spectrometry and ligand competition studies that Be2L and Be4L2 species dominate. A linkage with the bridging oxygen coming from the aliphatic alcohol is critical to the stability of the complex. We show through competition experiments that the most stable arrangement has one Be in a five-member ring and the other Be in a six-member ring. The unusual deprotonation of an aliphatic alcohol (pKa = 18) at neutral pH has significant ramifications on the potential interactions of Be with biological ligands such as carbohydrates and Ser and Thr residues.