Homooligomers of substituted prolines and β-prolines: syntheses and secondary structure investigation
作者:Cécile Caumes、Nicolas Delsuc、Redouane Beni Azza、Isabelle Correia、Fabrice Chemla、Franck Ferreira、Ludovic Carlier、Alejandro Perez Luna、Roba Moumné、Olivier Lequin、Philippe Karoyan
DOI:10.1039/c3nj00127j
日期:——
Homooligomers of enantiomerically pure (2S,3R)-3-methyl-proline, (3R,4R)-4-methyl-β-proline and (3R,4S)-3,4-dimethyl-β-proline were synthesized and studied using circular dichroism (CD) in water, methanol and propanol and using NMR in water. Changes in the far-UV CD spectrum were observed from dimers to hexamers, but little change was observed from hexamers to octa- or nonamers, both in water and methanol. CD and NMR data allowed us to conclude that oligomers of 3-substituted prolines with more than six residues adopt a characteristic PPII secondary structure both in water and aliphatic alcohols. Oligomers of (3R,4R)-4-methyl-β-proline bear the same CD signature as non-substituted β-proline oligomers, suggesting that substitution at position 3 is not sufficient to reduce conformational heterogeneity in β-proline oligomers. In the case of 3,4-disubstituted-β-proline oligomers, an atypical signature with an extra negative band at around 225 nm was observed, together with a concentration dependent CD spectrum indicating association properties. Nevertheless, NMR studies of 13C labelled oligomers of 3,4-disubstituted-β-prolines revealed a complex mixture of cisâtrans conformers even for longer oligomers.
合成了对映纯(2S,3R)-3-甲基-脯氨酸、(3R,4R)-4-甲基-β-脯氨酸和(3R,4S)-3,4-二甲基-β-脯氨酸的同型寡聚体,并在水中、甲醇和丙醇中使用圆二色谱(CD)以及在水中使用NMR进行了研究。从二聚体到六聚体,远紫外CD光谱发生了变化,但从六聚体到八聚体或九聚体,无论在水中还是甲醇中,变化都很小。CD和NMR数据使我们得出结论,含有超过六个残基的3-取代脯氨酸的寡聚体在水中和脂肪族醇中都采取特征性的PPII二级结构。(3R,4R)-4-甲基-β-脯氨酸的寡聚体具有与非取代β-脯氨酸寡聚体相同的CD特征,表明在第3位的取代并不足以减少β-脯氨酸寡聚体的构象异质性。对于3,4-二取代β-脯氨酸的寡聚体,观察到一个非典型的特征,在约225 nm处有一个额外的负带,并伴有一个浓度依赖性的CD光谱,表明有缔合性质。然而,对13C标记的3,4-二取代β-脯氨酸寡聚体的NMR研究表明,即使是较长的寡聚体,也存在复杂的顺反异构体混合物。