The Extended Chain Structures

As was first pointed out by Pauling and Corey, an important structural principle is that within proteins the maximum possible number of hydrogen bonds involving the C=O and N–H groups of the peptide chain should be formed. One simple way to do this is to line up fully extended chains (φ = ψ = 180°) and to form hydrogen bonds between them. Such a structure exists for polyglycine and resembles that. Notice that on the left side of this figure, the adjacent chains run in opposite directions; hence, the term antiparallel structure. The antiparallel arrangement not only gives the best hydrogen bond formation between chains but also permits a single chain to fold back on itself giving a compact hairpin loop.

The extended chain β pleated sheet structures. (A) Stereoscopic drawing without atomic symbols. (B) Drawing with atomic symbols. At the left is the antiparallel structure. The 0.70 nm spacing is slightly decreased from the fully extended length. The amino acid side chains (R) extend alternately above and below the plane of the “accordion pleated” sheet. The pairs of linear hydrogen bonds between the chains impart great strength to the structure. The chain can fold back on itself using a “β turn” perpendicular to the plane of the pleated sheet. The parallel chain structure (right side) is similar but with a less favorable hydrogen bonding arrangement. Arrows indicate chain directions.