Thermodynamic Exploration of Eosin−Lysozyme Binding

Abstract

We developed a modular pair of experiments for use in the undergraduate physical chemistry and biochemistry laboratories. Both experiments examine the thermodynamics of the binding of a small molecule, eosin Y, to the protein lysozyme. The assay for binding is the quenching of lysozyme fluorescence by eosin through resonant energy transfer. In both experiments, students measure fluorescence quenching at constant lysozyme concentration as a function of added eosin and determine the dissociation constant, KD, and ΔG° for binding. In the physical chemistry experiment, students repeat the fluorescence measurements at several temperatures to determine the temperature dependence of ΔG°, and therefore ΔH° and ΔS° as well. Typical student results yield a KD of (28.3 ± 7.5) μmol L−1 and a binding ΔG° of (−26.04 ± 0.70) kJ mol−1 at 25 °C. ΔH° and ΔS° of binding are found to be (−4.1 ± 2.5) kJ mol−1 and (73.9 ± 8.7) J mol−1 K−1, respectively. This pair of laboratories represents an interdisciplinary module in which students in different courses examine the same system but from different perspectives and with different emphases.