The role of carbon-donor hydrogen bonds in stabilizing tryptophan conformations

Although most side-chain torsion angles correspond to low-energy rotameric positions, deviations occur with significant frequency.  One striking example arises in Trp residues, which have an important role in stabilizing protein structures because of their size and mixed hydrophobic/hydrophilic character.  Ten percent of Trp side-chains have unexplained conformations with χ2 near 0° instead of the expected 90°.  The current work is a structural and energetic anal. of these conformations.  It is shown that many Trp residues with these orientations are stabilized by three-center carbon-donor hydrogen bonds of the form C-H≡X≡H-C, where X is a polar hydrogen-bond acceptor in the environment of the side-chain.  The bridging hydrogen bonds occur both within the Trp side-chain and between the side-chain and the local protein back-bone.  Free energy maps of an isolated Trp residue in an explicit water environment show a min. corresponding to the off-rotamer peak obsd. in the crystallog. data.  Bridging carbon-donor hydrogen bonds are also shown to stabilize onrotamer Trp conformations, and similar bridging hydrogen bonds also stabilize some off-rotamer Asp conformations.  The present results suggest a previously unrecognized role for three-center carbon-donor hydrogen bonds in protein structures and support the view that the off-rotamer Trp side-chain orientations are real rather than artifacts of crystallog. refinements.  Certain of the off-rotamer Trp conformations appear to have a functional role.