FAR-UV CIRCULAR-DICHROISM REVEALS A CONFORMATIONAL SWITCH IN A PEPTIDE FRAGMENT FROM THE BETA-SHEET OF HEN LYSOZYME

The conformation of a 20-residue synthetic peptide corresponding to th e antiparallel triple-stranded beta-sheet in hen egg white lysozyme (r esidues 41-60) has been studied by circular dichroism (CD) and size-ex clusion chromatography. In aqueous solution the conformation of the pe ptide is strongly pH dependent. At pH values below 4.0 and 25 OC, the far-UV CD spectrum of the peptide resembles that expected for a predom inantly P-sheet structure (low-pH form), while at pH values exceeding 4.0 the spectrum changes to that of a predominantly unstructured confo rmation (high-pH form). The far-UV CD spectrum of the high-pH form (pH 6.8) is not affected by changes in the concentration of the peptide a nd by changes in temperature and ionic strength. By contrast, the far- UV CD spectrum of the low-pH form (pH 2.0) is concentration and temper ature dependent but is not affected by ionic strength. Size-exclusion chromatography has demonstrated that population of the low-pH state is associated with the formation of intermolecular trimers and higher ol igomers and that the monomeric form of the peptide at low pH is predom inantly random in nature. Significant helical structure was induced in the high-pH form of the peptide by both trifluoroethanol (TFE) and me thanol; by contrast, the conformation of the low-pH form of the peptid e was not changed with concentrations of methanol up to 50% (v/v), alt hough in the presence of TFE a state displaying significant helical ch aracter was induced. During the transition an intermediate which also displays significant P-sheet character but which has a distinct CD spe ctrum is populated. The relevance of this study to the folding pathway of the intact protein is discussed.