Inter-strand hydrogen bonding

The (3-strand sequences are stretched out conformations of these polypeptide sections and are typically stabilized by inter-strand hydrogen bonds between keto (C = 0) oxygens and peptide bond NHs, the strands being arrayed in an antiparallel fashion. This type of secondary structure is favoured by amino acid residues with small R groups (such as Gly, Ala and Ser) that minimize steric overlap between chains. Thus a well-known protein having this type of secondary structure is silk fibroin that has a high proportion of repeated sequences involving Gly, Ala and Ser and an extensive antiparallel (3-pleated sheet structure. The macroscopic properties of silk fibroin (flexibility but lack of stretchability) reflect this type of secondary structure at the molecular level. 

The natural crystal is made up from meta-stable Cellulose I with all the cellulose strands parallel and no inter-sheet hydrogen bonding. The reason for this meta-stable crystalline form is that the conversion of glucose into cellulose takes place on the surface of an enzyme having about 30 active sites. As a consequence polymer molecules grow together in the same direction, producing a parallel faces crystal. 

When re-crystallized (for example, from base or CS2), cellulose I gives the thermodynamically more stable Cellulose II structure with an antiparallel arrangement of the strands and some inter-sheet hydrogen-bonding. Cellulose II contains two different types of anhydroglucose (A and B) with different backbone structures the chains consisting of -A-A- or -B-B-repeat units. Cellulose III is formed from cellulose mercerized in ammonia and is similar to cellulose II but with the chains parallel, as in cellulose la and cellulose Ip. 

Chart 3, Diagrammatic illustrations of (A and B) single- and double-stranded helical chains of leucine-containing polyphenylacetylene le stabilized by intra-and inter-chain hydrogen bonds and (C) theoretically possible conformations of chain segments of a substituted poly(phenylacetylene). 

Fic. 10.—Parallel packing arrangement of 6-fold, A-amylose (8) molecules, (a) A stereo side view of less than 2 turns of a pair of double helices 10.62 A (=al2) apart. The two strands in each helix are distinguished by open and filled bonds, and the helix axis is also drawn, for convenience. Note that atom 0-6 mediates both intra- and inter-double helix hydrogen bonds. 

Berger, I., Egli, M., and Rich, A. (1996) Inter-strand C-H- O hydrogen bonds stabilizing four-stranded intercalated molecules stereoelectronic effects of 04 in cytosine-rich DNA, Proc. Nat. Acad. Sci, USA 93, 12116-12121. 

---