CONFORMATIONAL-ANALYSIS OF PSEUDOCYCLIC HEXAPEPTIDES BASED ON QUANTITATIVE CIRCULAR-DICHROISM (CD), NOE, AND X-RAY DATA - THE PURE CD SPECTRA OF TYPE-I AND TYPE-II BETA-TURNS

The beta-turn (type I and type II) is the third frequently found structural unit involved in the 3D structure of globular proteins, in addition to the two major secondary structural elements, the alpha-helix and the beta-pleated sheet. Despite several theoretical and empirical efforts, the circular dichroism (CD) spectra of the pure type I and type II beta-turns are still not established beyond doubt. Even though considerable information is shown about the CD spectra of the alpha-helix and beta-sheet, the lack of knowledge concerning the beta-turn pure component spectra renders the estimation of global secondary structure of proteins rather inaccurate. To obtain more precise information about beta-turn conformations, cyclo[Gly-Pro-Ser(O(t)Bu)-Gly-(delta)Ava] (2) and cyclo[Gly-Pro-Ser(OH)-Gly-(delta)Ava] (3) were synthesized, and their CD spectra were studied. The object of the conformational analysis reported herein was to determine the crystalline state geometry of 2 as well as the solution conformations of 2 and 3 by NMR. In the crystal, 2 was found to adopt an 'ideal' type I beta-turn, encompassing the -Pro-Ser- dipeptide. Quantitative nuclear Overhauser effect measurements yielded interproton distances and the conformational ratios of 2 and 3. NOE distances so obtained were compared to values determined by X-ray diffraction, while solution conformational ratios were compared with the results of a quantitative CD interpretation. Using a recently developed algorithm, the conformational deconvolution of the measured CD spectra yielded the pure component CD curves of type I and type II beta-turns. The NOE data confirmed the ratios of the CD component curves of the two major beta-turn types. These results will facilitate the conformational determination of globular proteins on the basis of their CD spectra.