Our aim is to detect and measure specific interactions involving side chains that affect the stability of an isolated Alpha-helix in aqueous solution. The interactions can be of two types: (1) interactions between neighboring residues, and (2) interactions between charged residues and the Alpha-helix dipole. As a starting point, the C-peptide (residues 1-13) of ribonuclease A is known to show 30% helix formation at 0 degrees C, pH 5, whereas the Zimm-Bragg equation and host-guest data predict that no 13-residue peptide can show measurable Alpha-helix formation in water, regardless of amino acid sequence or temperature. Consequently, specific side chain interactions must be important for C-peptide helix stability. Our approach is to use chemically synthesized analogs of C-peptide. We have found that two charged groups, Glu2- and His12+, at either end of the helix play a critical role in stabilizing the C-peptide helix. We have also found that Glu9 can be replaced without loss of helix stability, and therefore that a possible Glu9-...His12+ salt bridge is not important. We will use the same approach to test for a Glu2-...Arg10+ salt bridge. Tests are in progress to detect possible helix-stabilizing interactions involving Glu2-, His12+ and the Alpha-helix dipole. We will also test for neighbor-dependent interactions between side chains in pairs of specific residues.
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