Outer membrane of gram negative provides barrier to certain antibiotics, lysozyme, detergents, heavy metals, bile salts, etc.

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poly(Pro) helix are stabilized by water molecule hydrogen bonding

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Poly(Gly) chains also adopt secondary conformations. 

In the solid state poly(Gly) has been shown to adopt two regular conformations designated I and II. 

State I has an extended conformation like a β strand whilst state II has three resides per turn and is similar to poly(Pro).

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Beta sheets usually have their B strands either parallel or antiparallel

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β Sheets in proteins contain 2 to as many as 22 polypeptide strands, with an average of 6 strands. 

Each strand may contain up to 15 residues, the average being 6 residues.

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Repeat period is shorter for the parallel conformation (6.5 Å, vs. 7 Å for antiparallel)

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