The amino acid residues in an helix have conformations with ps = -45deg to -50deg and ph= -60deg, and each helical turn includes 3.6 amino acid residues. The helical twist of the a helix found in all proteins is right-handed. Extended left-handed a helices are theoretically less stable and have not been observed in proteins. The a helix proved to be the predominant structure in a-keratins. Ref: Lehninger Principles of Biochemistry (7th Edition) Pg no 359 Ref: Lehninger&;s Principles of Biochemistry 6th Editoin - D L Nelson, Cox Lehninger - W H Freeman Page no 120. A final factor affecting the stability of an a helix in a polypeptide is the identity of the amino acid residues near the ends of the a-helical segment. A small electric dipole exists in each peptide bond . These dipoles are connected through the hydrogen bonds of the helix, resulting in a net dipole extending along the helix that increases with helix length . The our amino acid residues at each end of the helix do not paicipate fully in the helix hydrogen bonds. The paial positive and negative charges of the helix dipole actually reside on the peptide amino and carbonyl groups near the amino-terminal and carboxyl-terminal ends of the helix, respectively. For this reason, negatively charged amino acids are often found near the amino terminus of the helical segment, where they have a stabilizing interaction with the positive charge of the helix dipole; a positively charged amino acid at the aminoterminal end is destabilizing. The opposite is true at the carboxyl-terminal end of the helical segment. Ref: Lehninger&;s Principles of Biochemistry 6th Edition - D L Nelson, Cox Lehninger - W H Freeman Pg No 122