Conformational investigation of the cofactor (6R,1'R,2'S-)-5,6,7,8-tetrahydrobiopterin.

(6R,1'R,2'S)-5,6,7,8-Tetrahydrobiopterin is an essential cofactor for several enzymes. Different theoretical models (molecular mechanics, semiempirical quantum chemical calculations) investigating its stereostructure have yielded diverging answers. To clarify these issues, combined molecular mechanical and ab initio quantum chemical calculations were performed, investigating both the axial and the equatorial orientation of the dihydroxypropyl side-chain. After geometry optimization, the resulting most stable structures were subjected to systematic variation of two side-chain torsional angles in order to study the conformational flexibility. The axial side-chain orientation is slightly more stable than the equatorial form. Two weak intramolecular hydrogen bonds contribute to stabilization of the axial conformer, while in the equatorial conformer only one hydrogen bond is detected. An 8 ps molecular dynamical simulation at 310 K suggests that, at realistic temperatures, the molecule is flexible enough to undergo internal motions (rotations, vibrations), rendering questionable the biological significance of mere conformational properties.