A thermal unfolding study of the 45-residue α-helical domain UBA(2) using circular dichroism is presented. The protein is highly thermostable and exhibits a clear cold unfolding transition with the onset near 290 K without denaturant. Cold denaturation in proteins is rarely observed in general and is quite unique among small helical protein domains. The cold unfolding was further investigated in urea solutions, and a simple thermodynamic model was used to fit all thermal and urea unfolding data. The resulting thermodynamic parameters are compared to those of other small protein domains. Possible origins of the unusual cold unfolding of UBA(2) are discussed.

papilloma model. In contrast, the nonnative state showed only a fraction of the proteolytic activity of the native form. This study demonstrates that hexafluoroisopropanol can induce a conformational change in stem bromelain to a form with potentially useful therapeutic properties different from those of the native protein. 2308-Pos Board B78Cold Denaturation in a Small Protein Domain Small, stable protein domains have become increasingly important as models for protein folding. However, one of the general thermodynamic characteristics of protein structures -cold denaturation -has not been observed for such model domains. We have investigated the thermal unfolding of a small 45 residue a-helical UBA domain using CD and fluorescence spectroscopy. In addition to a relatively high thermal stability (T m~3 30K), we have also detected unfolding at cold temperatures, whose onset begins around 285K in the absence of denaturant. To further probe the cold denaturation, urea was used to destabilize the protein and therefore shift the onset of the cold denaturation to higher temperatures. All experimental data could be explained using a simple thermodynamic model, which assumes linear dependence of the unfolding free energy (DG) on the denaturant concentration. The model yields a large positive heat capacity change upon unfolding, which is traditionally associated with solvent exposure of hydrophobic groups. This small UBA domain therefore provides a valuable model for studying the still controversial phenomenon of cold denaturation and for understanding folding of larger proteins, which exhibit cold denaturation behavior. 2309-Pos Board B79Perturbing the Central Hydrophobic Cluster (CHC) of Ab(10-35) by Incorporation of Fluorinated Phenylalanine Derivatives Anwesha Bhattacharya, Ishita Mukerji. Wesleyan University, Middletown, CT, USA. One of the putative causes of Alzheimer's disease involves aggregation of misfolded amyloid b (Ab), a 39-42 residue polypeptide chain, and its subsequent deposition as amyloid plaques. The aggregation process proceeds via a nucleated polymerization mechanism where disordered peptide monomers interact with each other through hydrophobic interactions and rapidly extend and aggregate to eventually form larger fibrils with a highly ordered cross-strand b-sheet structure. It has also been suggested that the aromatic amino acid residues, tyrosine Y10 and phenylalanines (F19 and F20) in the central hydrophobic cluster (CHC) of the peptide play an important role in fibril assembly. In particular, F19 and F20 are suspected to be the drivers of the aggregation mechanism because of their hydrophobicity and aromaticity. In this context perturbation of the CHC through the introduction of non-natural (fluorinated) amino acids is expected to affect the aggregation process. Fluorinated amino acids in particular demonstrate distinct properties dictated by the presence of highly electronegative and hydrophobic fluorine atoms. However such fluorination is known to potentially eliminate the favorable interactio...