Question

Which of these polyamino acids will form an α-helix?

		polyproline at pH 7.0
		polyaspartic acid at pH 7.0
		polylysine at pH 2.5
		polyaspartic acid at pH 2.5

Answer 1

ans: polyaspartic acid at pH 2.5.

polyaspartic acid and polyglutamic acid, which are highly negatively charged at pH 7.0, form only random structures because of strong charge repulsion between the R groups along the peptide chain
--at pH 1.5 to 2.5, however, where the side chains are protonated and thus uncharged, these latter species spontaneously form α-helical structures
-similarly, polylysine is a random coil at pH values below about 11, where repulsion of positive charges prevents helix formation
--at pH 12, where polylysine is a neutral peptide chain, it readily forms an α-helix