9728240 Ho The primary objective of the current research is to develop a testable thermodynamic model to describe macromolecular crystallization. This project will use a heptanucleotide crystal system that is unique, in that the thermodynamics of crystallization can be studied using a crystallographic titration assay. This experimental system is a series of heptanucleotides that form duplexes. The individual strands are distinct in the lattice, and can be accurately quantified crystallographically or chromatographically. By varying the ratio of the two strands in the crystallization solutions, we can determine the energies of interaction between duplexes during crystallization. Transitions between lattice types were observed from initial studies to be highly cooperative. Thus, the assay has led to estimates for the energies and cooperativity for interactions between the DNA duplexes that form the initial seed crystal. The proposed studies are to derive a model to accurately describe this crystallization titration, accurately determine the parameters for this model, then extend the model to be more generally applicable to other crystal systems. The models will be validated by monitoring the effects of concentrations on the strand compositions of the crystals. X-ray crystallography has contributed significantly to our understanding of macromolecular structure and function, particularly at the atomic level. However, the method is entirely dependent on the ability to grow diffraction quality single crystals. Although a number of recent tools have been developed to facilitate the crystallization of macromolecules, the actual process of crystallization remains poorly understood. The result of the proposed studies will be a thermodynamic model that should be useful as a tool for the rational design of crystallization studies. The model will be generally useful as a predictive tool for crystallizing nucleic acids and proteins. The model will incorporate measurable properties of mo lecules to predict the probabilities for seed formation versus crystal growth, and conditions that may favor these two processes.
