<P> Two key developments in the modeling of the modern α - helix were: the correct bond geometry, thanks to the crystal structure determinations of amino acids and peptides and Pauling's prediction of planar peptide bonds; and his relinquishing of the assumption of an integral number of residues per turn of the helix . The pivotal moment came in the early spring of 1948, when Pauling caught a cold and went to bed . Being bored, he drew a polypeptide chain of roughly correct dimensions on a strip of paper and folded it into a helix, being careful to maintain the planar peptide bonds . After a few attempts, he produced a model with physically plausible hydrogen bonds . Pauling then worked with Corey and Branson to confirm his model before publication . In 1954, Pauling was awarded his first Nobel Prize "for his research into the nature of the chemical bond and its application to the elucidation of the structure of complex substances" (such as proteins), prominently including the structure of the α - helix . </P> <P> The amino acids in an α - helix are arranged in a right - handed helical structure where each amino acid residue corresponds to a 100 ° turn in the helix (i.e., the helix has 3.6 residues per turn), and a translation of 1.5 Å (0.15 nm) along the helical axis . Dunitz describes how Pauling's first article on the theme in fact shows a left - handed helix, the enantiomer of the true structure . Short pieces of left - handed helix sometimes occur with a large content of achiral glycine amino acids, but are unfavorable for the other normal, biological L - amino acids . The pitch of the alpha - helix (the vertical distance between consecutive turns of the helix) is 5.4 Å (0.54 nm), which is the product of 1.5 and 3.6 . What is most important is that the N-H group of an amino acid forms a hydrogen bond with the C =O group of the amino acid four residues earlier; this repeated i + 4 → i hydrogen bonding is the most prominent characteristic of an α - helix . Official international nomenclature (3) specifies two ways of defining α - helices, rule 6.2 in terms of repeating φ, ψ torsion angles (see below) and rule 6.3 in terms of the combined pattern of pitch and hydrogen bonding . The α - helices can be identified in protein structure using several computational methods, one of which being DSSP (Dictionary of Protein Secondary Structure). </P> <P> Similar structures include the 3 helix (i + 3 → i hydrogen bonding) and the π - helix (i + 5 → i hydrogen bonding). The α - helix can be described as a 3.6 helix, since the i + 4 spacing adds three more atoms to the H - bonded loop compared to the tighter 3 helix, and on average, 3.6 amino acids are involved in one ring of α - helix . The subscripts refer to the number of atoms (including the hydrogen) in the closed loop formed by the hydrogen bond . </P> <P> Residues in α - helices typically adopt backbone (φ, ψ) dihedral angles around (− 60 °, − 45 °), as shown in the image at right . In more general terms, they adopt dihedral angles such that the ψ dihedral angle of one residue and the φ dihedral angle of the next residue sum to roughly − 105 ° . As a consequence, α - helical dihedral angles, in general, fall on a diagonal stripe on the Ramachandran diagram (of slope − 1), ranging from (− 90 °, − 15 °) to (− 35 °, − 70 °). For comparison, the sum of the dihedral angles for a 3 helix is roughly − 75 °, whereas that for the π - helix is roughly − 130 ° . The general formula for the rotation angle Ω per residue of any polypeptide helix with trans isomers is given by the equation </P>

Which of the following amino acids will terminate an alpha helical structure in globular proteins