Abstract
The ß-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 -helix and the ß-pleated sheet. Despite several theoretical and empirical efforts, the circular dichroism (CD) spectra of the pure type I and type II ß-turns are still not established beyond doubt. Even though considerable information is known about the CD spectra of the -helix and ß-sheet, the lack of knowledge concerning the ß-tum pure component spectra renders the estimation of global secondary structure of proteins rather inaccurate. To obtain more precise information about ß-turn conformations, cyclo[Gly-Pro-Ser(0'Bu)-Gly-(6)Ava] (2) and cyclo[Gly- Pro-Ser(OH)-Gly-(S)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 ß-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 determdined 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 the ratios of the CD component curves of the two major ß-tum types.
of globular proteins on the basis of their CD spectra