BCHM 310 Chapter Notes - Chapter 4.2a: Alpha Helix, Beta Turn, Peptide
31 Oct 2017
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A regular secondary structure occurs when each dihedral angle, phi and psi, remains the same/nearly the same throughout the segment. types of secondary structures. Ngstr m, is equal to 0. 1 nm ; used universally by structural biologists to describe atomic distances it is ~the length of a typical c-h bond. Not all polypeptides can form a stable alpha helix. Which conformation a polypeptide sequence becomes depends on the r groups and how the r group affects the capacity of joining main-chain atoms to take up characteristic phi and psi angles. Alanine shows the greatest tendency to form helices. Amino acid side chains can stabilize or destabilize the alpha-helical structure. For example, if a polypeptide chain has a long block of glu residues, this segment of the chain will not form an helix at ph 7. 0. The negatively charged carboxyl groups of adjacent glu residues repel each other so strongly that they prevent formation of the helix.
