A. Dinola et al., INTERNAL BETA-TURN HYDRATION - CRYSTALLOGRAPHIC EVIDENCE AND MOLECULAR-DYNAMICS SIMULATION, Journal of physical chemistry, 99(23), 1995, pp. 9625-9631
The X-ray structure analysis of the monohydrate phase of the peptide F
or-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 a
nd causing significant modification of the secondary structure. The hy
drated beta-turn found in the crystal can be an interesting static mod
el 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 hydrat
ion, with a H-bonded water bridge and modification of the secondary st
ructure, differs from the most common external hydration, where the ex
ternally bound water does not cause modification of secondary structur
e. In order to verify whether the internally hydrated beta-turn could
be found even in solution, a molecular dynamics simulation has been pe
rformed. 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 s
ubstantially 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 heli
cal peptide segments found in oligopeptides and proteins.