Editing Alpha helix (section)

== Experimental determination ==
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. Protein structures from [[protein NMR|NMR spectroscopy]] also show helices well, with characteristic observations of [[nuclear Overhauser effect]] (NOE) couplings between atoms on adjacent helical turns. In some cases, the individual hydrogen bonds can be observed directly as a small scalar coupling in NMR.

There are several lower-resolution methods for assigning general helical structure. The [[protein NMR|NMR]] [[chemical shift]]s (in particular of the C<sup>α</sup>, C<sup>β</sup> and C′) and [[residual dipolar coupling]]s are often characteristic of helices. The far-UV (170–250&nbsp;nm) [[circular dichroism]] spectrum of helices is also idiosyncratic, exhibiting a pronounced double minimum at around 208 and 222&nbsp;nm. [[Infrared]] spectroscopy is rarely used, since the α-helical spectrum resembles that of a [[random coil]] (although these might be discerned by, e.g., [[hydrogen-deuterium exchange]]). 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.

Long homopolymers of amino acids often form helices if soluble. 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 spheroid|prolate]] (long cigar-like) hydrodynamic shape of a helix, or its large [[Molecular dipole moment|dipole moment]].