Perczel A, Hollósi M, Sándor P, Fasman G D
Graduate Department of Biochemistry, Brandeis University, Waltham, Massachusetts.
Int J Pept Protein Res. 1993 Mar;41(3):223-36. doi: 10.1111/j.1399-3011.1993.tb00330.x.
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.
获得了对甲苯磺酰基 - 脯氨酸 - 丝氨酸 - 甲胺（1）、[对甲苯磺酰基 - （CH₃）₃ - C - CO]、叔丁氧羰基 - 脯氨酸 - 丝氨酸 - 甲胺（2）和叔丁氧羰基 - 缬氨酸 - 丝氨酸 - 甲胺（3）的圆二色性（CD）和¹H - （¹H）核Overhauser效应（NOE）谱，以确定这些β - 转角模型的溶液构象。在晶体中，1和3采用理想的I型β - 转角，而2的特征是具有包含顺式叔丁氧羰基 - 脯氨酸叔酰胺键的半折叠主链几何结构。基于¹H - （¹H）NOE数据，表明模型1 - 3在溶液中以β - 转角构象为主。在非极性溶剂中，根据异核NOE测量，2中占主导的反式旋转异构体形式（> 80%）具有β - 转角构象。在CDCl₃中，在 - 20℃下，2的反式叔丁氧羰基 - 脯氨酸旋转异构体形式的Hα（Pro）/NH（Ser）、Hα（Ser）/NH（Ser）和NH（NHCH₃）/HN（Ser）质子之间检测到正的¹H - （¹H）NOE。在其他模型中，如叔丁氧羰基 - 缬氨酸 - 丝氨酸 - 甲胺（3）、叔丁氧羰基 - 缬氨酸 - D - 丝氨酸 - 甲胺（4）和叔丁氧羰基 - 脯氨酸 - D - 丝氨酸 - 甲胺（5），在各种溶剂中的适当质子信号上也观察到类似的正同核NOE增强。在CD₃CN中进行的¹H - （¹H）NOE实验清楚地表明，除了I型（或III型）β - 转角结构外，模型1 - 5的主要构象之一在非极性溶剂中接近II型β - 转角主链几何结构。出乎意料的是，模型1 - 3的构象混合物以C类（螺旋状）CD谱为特征，尽管C类谱通常仅与I型β - 转角构象相关。这些无环模型是首次仔细研究的 -L - L - 三酰胺系统的例子，包含大量的II型β - 转角以及I型β - 转角，然而却产生C类圆二色性光谱。使用¹H - （¹H）NOE数据对在乙腈中记录的3和4的CD谱进行了“校准”。这样的“校准”以及半定量的CD和NMR综合分析表明，C类、B类以及C'类CD谱可以从具有不同构象权重的相同两组分谱的线性组合中获得。因此，建议仅基于其CD谱来提取此类模型的构象成分时必须谨慎。
