<P> Hydrogen bonding plays an important role in determining the three - dimensional structures and the properties adopted by many synthetic and natural proteins . </P> <P> In these macromolecules, bonding between parts of the same macromolecule cause it to fold into a specific shape, which helps determine the molecule's physiological or biochemical role . For example, the double helical structure of DNA is due largely to hydrogen bonding between its base pairs (as well as pi stacking interactions), which link one complementary strand to the other and enable replication . </P> <P> In the secondary structure of proteins, hydrogen bonds form between the backbone oxygens and amide hydrogens . When the spacing of the amino acid residues participating in a hydrogen bond occurs regularly between positions i and i + 4, an alpha helix is formed . When the spacing is less, between positions i and i + 3, then a 3 helix is formed . When two strands are joined by hydrogen bonds involving alternating residues on each participating strand, a beta sheet is formed . Hydrogen bonds also play a part in forming the tertiary structure of protein through interaction of R - groups . (See also protein folding). </P> <P> The role of hydrogen bonds in protein folding has also been linked to osmolyte - induced protein stabilization . Protective osmolytes, such as trehalose and sorbitol, shift the protein folding equilibrium toward the folded state, in a concentration dependent manner . While the prevalent explanation for osmolyte action relies on excluded volume effects, that are entropic in nature, recent circular dichroism (CD) experiments have shown osmolyte to act through an enthalpic effect . The molecular mechanism for their role in protein stabilization is still not well established, though several mechanism have been proposed . Recently, computer molecular dynamics simulations suggested that osmolytes stabilize proteins by modifying the hydrogen bonds in the protein hydration layer . </P>

An example of hydrogen bond is the bond between