INTERNAL BETA-TURN HYDRATION - CRYSTALLOGRAPHIC EVIDENCE AND MOLECULAR-DYNAMICS SIMULATION

The X-ray structure analysis of the monohydrate phase of the peptide For-Met-Leu-Delta(2)Phe-Phe-OMe has revealed that the type-II beta-turn supported by the 4 --> 1 H-bond, expected for the backbone containing the alpha,beta-unsaturated residue Delta(2)Phe, has been modified by the water molecule. The water prevents the 4 --> 1 H-bond between Met CO and Phe NH groups, forming a H-bonded bridge between these groups and causing significant modification of the secondary structure. The hydrated beta-turn found in the crystal can be an interesting static model useful for the comprehension of the internal hydration mechanism of beta-turns, secondary structures largely present in globular proteins, but generally distributed only on their surface. The internal hydration, with a H-bonded water bridge and modification of the secondary structure, differs from the most common external hydration, where the externally bound water does not cause modification of secondary structure. In order to verify whether the internally hydrated beta-turn could be found even in solution, a molecular dynamics simulation has been performed. The results show that the two forms, the beta-turn with the 4 --> 1 H-bond and that with the H-bonded water bridge, do not differ substantially in their energy values. Moreover, no high-energy barrier prevents interconversion between the two forms. The internal beta-turn hydration presents strong analogies with the internally hydrated helical peptide segments found in oligopeptides and proteins.