Far-UV Circular Dichroism Reveals a Conformational Switch in a Peptide Fragment from the .beta.-Sheet of Hen Lysozyme

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Abstract
The conformation of a 20-residue synthetic peptide corresponding to the antiparallel triple-stranded beta-sheet in hen egg white lysozyme (residues 41-60) has been studied by circular dichroism (CD) and size-exclusion chromatography. In aqueous solution the conformation of the peptide is strongly pH dependent. At pH values below 4.0 and 25 degrees C, the far-UV CD spectrum of the peptide resembles that expected for a predominantly beta-sheet structured (low-pH form), while at pH values exceeding 4.0 the spectrum changes to that of a predominantly unstructured conformation (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 and 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 temperature dependent but is not affected by ionic strength. Size-exclusion chromatography trimers and higher oligomers and that the monomeric form of the peptide at low pH is predominantly random in nature. Significant helical structure was induced in the high-pH form of the peptide by both trifluoroethanol (TFE) and methanol; by contrast, the conformation of the low-pH form of the peptide was not changed with concentrations of methanol up to 50% (v/v), although in the presence of TFE a state displaying significant helical character was induced. During the transition an intermediate which also displays significant beta-sheet character but which has a distinct CD spectrum is populated. The relevance of this study to the folding pathway of the intact protein is discussed.

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