Lecture #5: Proteins II: Structure/Function I. Overview A. General Principles Handout #14B. Four main levels of protein structure (fig 3-16) II. Secondary Structure A. The peptide bond is rigid and planar (fig 4-2) 1. Resonance hybrid has partial double bond character 2. No rotation occurs about the peptide bond 3. All peptide bonds are in a trans configuration. Exceptions to this can occur when proline is involved 4. Rotation about the N-Cαand Cα-C bonds (φ andψ) is limited by steric overlap between the carbonyl O and the α-amino H. Significance: These properties limit the number of conformations that a protein can assume.
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