<P> There are several lower - resolution methods for assigning general helical structure . The NMR chemical shifts (in particular of the C, C and C ′) and residual dipolar couplings are often characteristic of helices . The far - UV (170--250 nm) circular dichroism spectrum of helices is also idiosyncratic, exhibiting a pronounced double minimum at around 208 and 222 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 . </P> <P> 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 (long cigar - like) hydrodynamic shape of a helix, or its large dipole moment . </P> <P> Different amino - acid sequences have different propensities for forming α - helical structure . Methionine, alanine, leucine, glutamate, and lysine uncharged ("MALEK" in the amino - acid 1 - letter codes) all have especially high helix - forming propensities, whereas proline and glycine have poor helix - forming propensities . Proline either breaks or kinks a helix, both because it cannot donate an amide hydrogen bond (having no amide hydrogen), and also because its sidechain interferes sterically with the backbone of the preceding turn--inside a helix, this forces a bend of about 30 ° in the helix's axis . However, proline is often seen as the first residue of a helix, it is presumed due to its structural rigidity . 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 . </P> <P> Estimated differences in free energy, Δ (ΔG), estimated in kcal / mol per residue in an α - helical configuration, relative to alanine arbitrarily set as zero . Higher numbers (more positive free energies) are less favoured . Significant deviations from these average numbers are possible, depending on the identities of the neighbouring residues . </P>

Where are side chain groups situated in an α-helix
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