![]() This can occur in the presence of two consecutive proline residues, which create an angled kink in the polypeptide chain and bend it back upon itself. Contrarily, in the anti-parallel arrangement the hydrogen bonds are aligned directly opposite each other, making for stronger and more stable bonds.Ĭommonly, an anti-parallel beta-pleated sheet forms when a polypeptide chain sharply reverses direction. The parallel arrangement is less stable because the geometry of the individual amino acid molecules forces the hydrogen bonds to occur at an angle, making them longer and thus weaker. The "pleat" occurs because of the alternating planes of the peptide bonds between amino acids the aligned amino and carbonyl group of each opposite segment alternate their orientation from facing towards each other to facing opposite directions. In parallel arrangement, the C-terminus end and the N-terminus end are on the same sides for both segments. In anti-parallel arrangement, the C-terminus end of one segment is on the same side as the N-terminus end of the other segment. Parallel and anti-parallel arrangement is the direct consequence of the directionality of the polypeptide chain. This can happen in a parallel arrangement: psi-loop) segments of a polypeptide chain overlap one another and form a row of hydrogen bonds with each other. This structure occurs when two (or more, e.g. Stable alpha-helices typically end with a charged amino acid to neutralize the dipole moment. In addition, the lack of a hydrogen on Proline's nitrogen prevents it from participating in hydrogen bonding.Īnother factor affecting alpha-helix stability is the total dipole moment of the entire helix due to individual dipoles of the C=O groups involved in hydrogen bonding. Proline also destabilizes alpha-helices because of its irregular geometry its R-group bonds back to the nitrogen of the amide group, which causes steric hindrance. ![]() Amino acids whose R-groups are too large (tryptophan, tyrosine) or too small (glycine) destabilize alpha-helices. The structural integrity of an alpha-helix is in part dependent on correct steric configuration. This regular pattern gives the alpha-helix very definite features with regards to the thickness of the coil and the length of each complete turn along the helix axis. Such a hydrogen bond is formed exactly every 4 amino acid residues, and every complete turn of the helix is only 3.6 amino acid residues. This coil is held together by hydrogen bonds between the oxygen of C=O on top coil and the hydrogen of N-H on the bottom coil. Secondary Structure of a Polypeptide ChainĪn alpha-helix is a right-handed coil of amino-acid residues on a polypeptide chain, typically ranging between 4 and 40 residues. ![]()
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