Understanding Microscopic Interactions in Binding Reactions: The pH Titration of EDTA

Abstract

Students beginning the study of biochemistry or biophysics at the undergraduate or even early graduate level are often overwhelmed by the complexity of the systems and the nomenclature. By comparison, chemical systems appear simple, as students can more easily relate to introductory chemistry courses, where the molecules are smaller and bind yet smaller ions. This allows students to write the structure of the entire molecule on a piece of paper and see exactly to which functional groups an ion, such as a proton (H+) in the simplest case, binds. Yet, concepts that are fundamental in biochemical macromolecules, namely proteins, can perfectly well be taught at the undergraduate or beginning graduate level, and probably be more easily understood, by using simpler, familiar chemical examples. The concept of interacting binding sites, which is at the root of cooperativity in protein binding reactions and conformational changes, is already present in simple molecules, such as ethylenediaminetetraacetic acid (EDTA). In this article, we show how to teach these topics by using the idea of the partition function, rather than a formal algebraic approach, to treat the binding of protons to EDTA. Profound concepts, such as that of interacting sites, appear naturally in a small molecule, where the origin can be easily ascribed, in this case, mainly to electrostatic interactions. Equipped with this understanding and this approach, students will be able to tackle more complicated biochemical systems, in which the molecules are larger, but the concepts are the same.