Quantitative analysis of cyclic beta-turn models.

The beta-turn is a frequently found structural unit in the conformation of globular proteins. Although the circular dichroism (CD) spectra of the alpha-helix and beta-pleated sheet are well defined, there remains some ambiguity concerning the pure component CD spectra of the different types of beta-turns. Recently, it has been reported (Hollósi, M., Kövér, K.E., Holly, S., Radics, L., & Fasman, G.D., 1987, Biopolymers 26, 1527-1572; Perczel, A., Hollósi, M., Foxman, B.M., & Fasman, G.D., 1991a, J. Am. Chem. Soc. 113, 9772-9784) that some pseudohexapeptides (e.g., the cyclo[(delta)Ava-Gly-Pro-Aaa-Gly] where Aaa = Ser, Ser(OtBu), or Gly) in many solvents adopt a conformational mixture of type I and the type II beta-turns, although the X-ray-determined conformation was an ideal type I beta-turn. In addition to these pseudohexapeptides, conformational analysis was also carried out on three pseudotetrapeptides and three pseudooctapeptides. The target of the conformation analysis reported herein was to determine whether the ring stress of the above beta-turn models has an influence on their conformational properties. Quantitative nuclear Overhauser effect (NOE) measurements yielded interproton distances. The conformational average distances so obtained were interpreted utilizing molecular dynamics (MD) simulations to yield the conformational percentages. These conformational ratios were correlated with the conformational weights obtained by quantitative CD analysis of the same compounds. The pure component CD curves of type I and type II beta-turns were also obtained, using a recently developed algorithm (Perczel, A., Tusnády, G., Hollósi, M., & Fasman, G.D., 1991b, Protein Eng. 4(6), 669-679). For the first time the results of a CD deconvolution, based on the CD spectra of 14 beta-turn models, were assigned by quantitative NOE results. The NOE experiments confirmed the ratios of the component curves found for the two major beta-turns by CD analysis. These results can now be used to enhance the conformational determination of globular proteins on the basis of their CD spectra.