Alpha helix

He later joined other researchers (notably the American chemist Maurice Huggins) in proposing that: Although incorrect in their details, Astbury's models of these forms were correct in essence and correspond to modern elements of secondary structure, the α-helix and the β-strand (Astbury's nomenclature was kept), which were developed by Linus Pauling, Robert Corey and Herman Branson in 1951 (see below); that paper showed both right- and left-handed helices, although in 1960 the crystal structure of myoglobin[2] showed that the right-handed form is the common one.

[3] Neurath's paper and Astbury's data inspired H. S. Taylor,[4] Maurice Huggins[5] and Bragg and collaborators[6] to propose models of keratin that somewhat resemble the modern α-helix.

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.

Dunitz[9] describes how Pauling's first article on the theme in fact shows a left-handed helix, the enantiomer of the true structure.

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 (−70°, −35°).

[17] Helices observed in proteins can range from four to over forty residues long, but a typical helix contains about ten amino acids (about three turns).

In general, the backbone hydrogen bonds of α-helices are considered slightly weaker than those found in β-sheets, and are readily attacked by the ambient water molecules.

However, in more hydrophobic environments such as the plasma membrane, or in the presence of co-solvents such as trifluoroethanol (TFE), or isolated from solvent in the gas phase,[18] oligopeptides readily adopt stable α-helical structure.

Since the α-helix is defined by its hydrogen bonds and backbone conformation, the most detailed experimental evidence for α-helical structure comes from atomic-resolution X-ray crystallography such as the example shown at right.

It is clear that all the backbone carbonyl oxygens point downward (toward the C-terminus) but splay out slightly, and the H-bonds are approximately parallel to the helix axis.

Finally, cryo electron microscopy is now capable of discerning individual α-helices within a protein, although their assignment to residues is still an active area of research.

Such long, isolated helices can also be detected by other methods, such as dielectric relaxation, flow birefringence, and measurements of the diffusion constant.

In stricter terms, these methods detect only the characteristic prolate (long cigar-like) hydrodynamic shape of a helix, or its large dipole moment.

At the other extreme, glycine also tends to disrupt helices because its high conformational flexibility makes it entropically expensive to adopt the relatively constrained α-helical structure.

α-helices often occur with the N-terminal end bound by a negatively charged group, sometimes an amino acid side chain such as glutamate or aspartate, or sometimes a phosphate ion.

In general, the fifth and seventh residues (the e and g positions) have opposing charges and form a salt bridge stabilized by electrostatic interactions.

[33][34] Myoglobin and hemoglobin, the first two proteins whose structures were solved by X-ray crystallography, have very similar folds made up of about 70% α-helix, with the rest being non-repetitive regions, or "loops" that connect the helices.

This is because of the convenient structural fact that the diameter of an α-helix is about 12 Å (1.2 nm) including an average set of sidechains, about the same as the width of the major groove in B-form DNA, and also because coiled-coil (or leucine zipper) dimers of helices can readily position a pair of interaction surfaces to contact the sort of symmetrical repeat common in double-helical DNA.

At least five artists have made explicit reference to the α-helix in their work: Julie Newdoll in painting and Julian Voss-Andreae, Bathsheba Grossman, Byron Rubin, and Mike Tyka in sculpture.

San Francisco area artist Julie Newdoll,[43] who holds a degree in microbiology with a minor in art, has specialized in paintings inspired by microscopic images and molecules since 1990.

[43] This same metaphor is also echoed from the scientist's side: "β sheets do not show a stiff repetitious regularity but flow in graceful, twisting curves, and even the α-helix is regular more in the manner of a flower stem, whose branching nodes show the influence of environment, developmental history, and the evolution of each part to match its own idiosyncratic function.

Ribbon diagrams of α-helices are a prominent element in the laser-etched crystal sculptures of protein structures created by artist Bathsheba Grossman, such as those of insulin, hemoglobin, and DNA polymerase.

Tyka has been making sculptures of protein molecules since 2010 from copper and steel, including ubiquitin and a potassium channel tetramer.