Variance always has squared units. For example, the variance of a set of weights estimated in kilograms will be given in kg squared.

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#Id: 8477

#Unit 1. Molecules and their Interaction Relevant to Biology

Each single strand of the b-sheet structure can be pictured as a twofold helix, that is, a helix with two residues per turn.

The arrangement of successive amide planes has a pleated appearance due to the tetrahedral nature of the Ca atom.

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#Unit 1. Molecules and their Interaction Relevant to Biology

Parallel sheets are typically large structures; those composed of fewer than five strands are rare. 

Antiparallel sheets may consist of as few as two strands.

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#Unit 1. Molecules and their Interaction Relevant to Biology

Parallel sheets characteristically distribute hydrophobic side chains on both sides of the sheet

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#Id: 8480

#Unit 1. Molecules and their Interaction Relevant to Biology

Antiparallel sheets are usually arranged with all their hydrophobic residues on one side of the sheet. 

This requires an alternation of hydrophilic and hydrophobic residues in the primary structure of peptides involved in antiparallel b-sheets because every other side chain projects to the same side of the sheet

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#Id: 8481

#Unit 1. Molecules and their Interaction Relevant to Biology

Turns are classified into several classes, like 

β-Turn  (I, II, I’, II’) 

α -Turn 

γ- Turn

π -Turn

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#Id: 8482

#Unit 1. Molecules and their Interaction Relevant to Biology