In 1965, Professor Paul W. Chun received his Ph.D. in biochemistry from the University of Missouri for the characterization of interacting casein molecules. He joined the Department of Biochemistry and Molecular Biology in 1966. His research includes three decades of studies on interacting protein systems, using analytical ultracentrifugation, stop-flow measurements and scanning molecular sieve chromatography. He has demonstrated the universal applicability of the Planck-Benzinger thermal work function in the analysis of energetics for protein folding, self-associating protein systems, receptor-ligand interaction and protein-DNA interaction, as well as protein unfolding and DNA unwinding.
In a recent study applying the Planck-Benzinger methodology, the sequence-specific hydrophobic interactions of 35 dipeptide pairs were examined over a temperature range of 273-333 K, based on data reported by Nemethy and Scheraga in 1962 to investigate the origins of the negative Gibbs free energy change minimum. The hydrophobic interaction in these sequence-specific dipeptide pairs is highly similar in its thermodynamic behavior to that of other biological systems. The results indicated that the negative Gibbs free energy change minimum at a well-defined stable temperature, where the bound unavailable energy is 0, has its origin in the sequence-specific hydrophobic interactions, which are highly dependent on details of molecular structure. Each case confirms the existence of a thermodynamic molecular switch wherein a change of sign in leads to true negative minimum in the Gibbs free energy change of reaction, and hence a maximum in the related equilibrium constant.
Indeed, all interacting biological systems examined using the Planck-Benzinger methodology have shown such a thermodynamic switch at the molecular level, suggesting its existence may be universal.

Dr. Chun is continuing thermodynamics studies on complementation reactions in RNase S' and angiogenin. To investigate the role of Phe 8 and Met 13 in the stabilization of two S-peptides in their interaction with S-protein of ribonuclease S' system, he synthesized 13 S and 20 S peptides in which both Phe 8 and Met 13 amino acid residues have been replaced by other charged amino acids. The peptides-protein complementation reaction provides a way of delineating the effect of amino acid substitution on the stability of the complex in terms of location in the secondary structure. Studies on S-peptide-S-protein interaction with substitution at Met 13 indicated that substitutions at the beginning or middle of the -helix weaken the helix-dipole interaction of the -helix. Such amino acid substitutions are apparently much more disruptive to underlying structural stability than a substitution at any other position.