A New Application for Radioimmunoassay Measurement of Thermodynamic Constants Carol N. Angstadt and Edward J. Barbieri Departments of Biological Chemistry and Pharmacology, Hahnemann Medical College and Hospital, Philadelphia, PA 19102 Grafton D. C h a s e Department of Chemistry, Philadelphia College of Pharmacy and Science, Philadelphia, PA 19104 Most undergraduate and graduate courses in physical chemistry devote a significant amount of time to a study of thermodynamics. All students of physical chemistry are familiar with the concepts of enthalpy (AH), entropy (AS), Gibbs free energy (AG), equilibrium constants and the relat i o n s b i ~of these to each other. Not all ~ h v s i c a lchemistry courses; however, expand the study of the&dynamics to thk student lahoratorv. Even those courses which include an experiment to measire equilibrium constants rarely extend to measurements of enthalpv . . and entropv . . as well. Freauentlv sue h experiment:. iirc m ~ i t t e dhenlune t h q either du.nol cun\.imientl\ into I he time prrild allotted ior the liihumtory or they require more of ankxpenditure for equipment than is possible. This paper describes a laboratory experiment in which an equilihrium radioimmunoassay (RIA) is used to estimate thermodynamic parameters such as equilibrium constants, AH, AS, and AG. The RIA is simple to perform; students without prior exposure to this technique have little difficulty with the basic concepts and procedures. This experiment is applicable to a two to three hour laboratory period, is not overly expensive to run, and introduces the student to a technique that has become increasingly important in the clinical chemistry and research laboratory. An equilihrium RIA is based on the competition between a radioactively-labeled ligand (P*) and an unlabeled ligand ( P ) for a limited number of binding sites on an antihody (Q), l.e.,
in
P* Free Ligand
PQ Bound Ligand
where K and K' are the equilibrium constants of the reactions. It is assumed that K and K' are equal, P and P* are chemically indistinguishable, and the ligand and antihody react in 1:l ratio, i.e., that the antihody binding sites are equivalent and independent and that the ligand is monovalent. In an equilibrium RIA there are several wavs of determinine the eauilihrium constant (K); the most commonly employed metiod utilizes a Scatchard s lot. In a Scatchard analvsis (1).the ratio 01 hound to free lig& at equilihriunt is I)l~;~ttrl aiain,t this wncentrariur of h
