The evaluation of type I and type II beta-turn mixtures. Circular dichroism, NMR and molecular dynamics studies

Circular dichroism (CD) and 1H-(1H)NOE spectra were obtained for Piv-Pro-Ser-NHCH3 (1), [Piv-(CH3)3-C-CO], Boc-Pro-Ser-NHCH3 (2) and Boc-Val-Ser-NHCH3 (3), to determine the solution conformation of these beta-turn models. In the crystal, 1 and 3 adopt an ideal type I beta-turn, while 2 is characterized by a semifolded backbone geometry incorporating a cis Boc-Pro tert-amide bond. The predominance of a beta-turn conformation in solution was suggested for models 1-3 on the basis of 1H-(1H)NOE data. In a nonpolar solvent the prevailing trans rotamer form (> 80%) of 2 has a beta-turn conformation according to heteronuclear NOE measurement. Positive 1H-(1H)NOEs were detected between the H alpha(Pro)/NH(Ser), H alpha(Ser)/NH(Ser) and NH(NHCH3)/HN(Ser) protons in the trans Boc-Pro rotamer form of 2 at -20 degrees in CDCl3. Similar positive homonuclear NOE enhancements were also observed on the appropriate proton signals in other models, such as Boc-Val-Ser-NHCH3 (3), Boc-Val-D-Ser-NHCH3 (4) and Boc-Pro-D-Ser-NHCH3 (5), in various solvents. The 1H-(1H)NOE experiments carried out in CD3CN clearly showed that besides the type I (or III) beta-turn structure, one of the main conformations of models 1-5 is close to the type II beta-turn backbone geometry in a nonpolar solvent. Unexpectedly, the conformational mixture of models 1-3 were characterized by class C (helix-like) CD spectra, although class C spectra are generally only correlated with the type I beta-turn conformation. These acyclic models are the first carefully investigated examples of -L-L- triamide systems, containing a significant amount of a type II beta-turn, as well as the type I beta-turn and, however, yielding a class C circular dichroism spectra. The CD spectra recorded for 3 and 4 in acetonitrile were 'calibrated' using the 1H-(1H)NOE data. Such a "calibration", as well as the semi-quantitative CD and NMR comprehensive analyses, demonstrated that class C, class B, as well as class C' CD spectra may be obtained from the linear combination of the same two-component spectra, with different conformational weights. Therefore, it is suggested that the extraction of the conformational components of such models, simply on the basis of their CD spectra, must be made with caution.