Two stretches of the polypeptide chain may also adopt a planar structure. In this instance, the hydrogen-bonds responsible for the stability of the structure involve C=O and N-H from non-contiguous aminoacids :
. This is a parallel β-sheet: both aminoacids stretches have the same orientation (check in which sense the
N-C_α-C=O
block repeats itself in each sequence). Observe how the atoms involved in the hydrogen bridges are not in the same straight line: the hydrogen bods in a parallel β-sheet are therefore weaker than those in the α-helix.
In a parallel β-sheet, sidechains alternatedly poit upwards and downwards from the plane of the sheet. Spin the structure in order to fully appreciate it. See how one both faces of the sheet contain only hydrophobic residues (and therefore probably face the protein core)

.

Other interactive models:

Metabolic pathways:

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