Question

PolyAspartate, (Asp)n, forms regions of α-helices in aqueous solutions below pH 3.0, but above pH 5.0, polyAsp assumes an extended conformation.

a) Explain this observation.

b) At what pH would polyLysine, (Lys)n, be likely to form α-helix?

Answer 1

Hi, please note that alpha helices are stabilized by H-bonding between every fourth amino acid in the helix structure backbone. so in aqueous solution at low pH values the amino acids are protonated which confers stability to the alpha-helix structure. At higher pH values these amino acids are deprotonated and H-bonding is either minimal or not present and the alpha-helix structure is not stabilized so the peptide tends to form an extended conformation.

However the conditions which hold true for alpha-helix formation for polyaspartate, do not hold the same for polylysine. Since the side chain has a different functionality: It is basic in contrast to aspartic acid which has acidic side chain. So it occurs in the alpha-helix conformation at pH values greater than 10.6. You might also like to check out the following journal artice about polylysine.

http://www.ncbi.nlm.nih.gov/pubmed/1540655