Nh2

Side-chain-helical dipole electrostatic interactions

Asparagine (Asn) (N-cap)

Helix dipole

Figure 11.1 (a) The a-helix forms because —NH and —C=O groups interact through hydrogen bonding pulling the backbone into a spiral, as shown in this diagram in a 9-amino peptide: (A) here only the central carbon atoms are shown for clarity; in (B) the nitrogen and carbons of the backbone are shown and in (C) the oxygen and hydrogen atoms have been added. All the —NH groups point in the same direction 'up' and the —C=O groups point 'down' to allow the formation of —C=O---H—N— bonds. In (D) the side-chains are added and point outwards from the a-helix. Hydrophobic interactions between some helix-promoting side-chains (the more hydrophobic chains, see Table 11.3) help to stabilize the helix. (Modified from reference 3, with permission).

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