Structure of a compact peptide from staphylococcal nuclease determined by circular dichroism and NMR spectroscopy

Compact regions in proteins are thought to correspond to domains. If this is true, the structure of a compact region excised from a protein should closely resemble the structure in the intact protein. To test this theory, a compact peptide corresponding to residues 129-142 of staphylococcal nuclease (Ac-EAQAKKEKLNIWS-NH2) was synthesized and its solution structure determined using circular dichroism (CD) and 2D NMR. In aqueous solution, the peptide exhibits CD spectra characteristic of a nascent helix. This nascent helical structure is stabilized by the addition of 2,2,2-trifluoroethanol. Under these conditions, the chemical shift indexes of the 1H alpha and 13C alpha resonances, temperature coefficients of amide protons, and NOE constraints are all consistent with the peptide's structure being a helix-turn. This structure is almost identical to that found in the intact protein.