Binding thermodynamics of divalent metal ions to several biological buffers

Abstract

The interactions of divalent metal ions with several normal biological buffers, such as 2-(N-morpholino)ethane sulfonic acid (MES), 3-(N-morpholino)propane sulfonic acid (MOPS), 4-(2-hydroxyethyl)-1-piperazineethane sulfonic acid (HEPES), tris(hydroxymethyl)aminomethane (Tris) and N-[tris(hydroxymethyl)methyl]-2-aminoethanesulfonic acid (TES) have been investigated by using isothermal titration calorimetry (ITC) from the calorimetric perspective. MES at pH 6.0 does not form complexes with Zn2+, Cd2+, Ca2+, Co2+, Mg2+, Mn2+, and Ni2+, and shows imperceptible complexation ability to Cu2+ and Pb2+. MOPS and HEPES at pH 7.4 show no binding affinity to Ca2+, Co2+, Mg2+, Mn2+, and Ni2+, mild binding affinity towards Cu2+ and Pb2+, and weak binding affinity to Zn2+ and Cd2+. The metal complexation abilities of Tris are much stronger than MOPS or HEPES, especially towards Cu2+ and Pb2+. TES shows negligible binding affinity to Zn2+, Cd2+, Ca2+, Co2+, Mg2+, Mn2+, and Ni2+, and a moderate binding affinity towards Cu2+.