芳酰胺折叠体分子内N—H…OMe氢键强度评估

Evaluation on the Stability of the Intramolecular N—H…OMe Hydrogen Bonds of Aromatic Amide Foldamers

为了评估分子内N—H…OMe氢键诱导的芳酰胺折叠体分子内氢键的稳定性,我们从相应间-苯二胺和间-苯二甲酸前体出发构筑了3个三、五和七聚体芳酰胺折叠体,并合成了3个并入这些折叠体片段的基于十六烷二胺的酰胺聚合物.1H NMR,定量芳酰胺氢-氘交换和晶体结构研究揭示,折叠体中间区域的氢键最弱,而处于骨架两端的氢键最为稳定.氢-氘交换实验测定出了不同酰胺氢发生这一过程的半衰期,最大差别约为8倍.对并入折叠体片段的聚合物的单分子力谱(SMFS)研究揭示,折叠体片段内不同区域分子内氢键的稳定性与单个折叠体分子内相应位置氢键的稳定性一致.通过SMFS实验,我们还测定出了不同氢键的绝对力值.结果显示,并入到聚合物中的短的三聚体折叠体具有最强的分子内氢键,而五聚体和七聚体折叠体的部分分子内氢键较弱,其力值出现在较低位置.

To evaluate the relative stability of different intramolecular N--H-OMe hydrogen bonds of aromatic am- ide-based foldamers, 3-, 5-, and 7-met aromatic amide foldamers F-3, F-5 and F-7, which possess one, two, and three differ- ent amide units, have been constructed from benzene-l,3-diamine and isophthalic acid derivatives. 1H NMR experiments in CDCI2CDC12 and DMSO-d6 showed that the hydrogen bonds formed in the central area of the foldamer backbones are least stable, whereas the hydrogen bonds formed at the two ends are most stable. 1H NMR hydrogen-denterium exchange experi- ments for F-3, F-5 and F-7 in CDCI2CDC12-CD3OD （19 ： 1, V/V） and DMSO-d6-CD3OD （19 1, V/V） were performed. In the former less polar solvent mixture, the half-life values of the process, corresponding to amides from the central area to the end areas, were determined to be 140 h for F-3, 71.8 and 405 h for F-5, and 36.3, 216 and 314 h for F-7, respectively. In the latter more polar solvent mixture, the related values were evaluated to be 97.1 h for F-3, 69.0 and 300 h for F-5, and 13.5, 38.3 and 57.5 h for F-7, respectively. These quantitative results are consistent with the above 1H NMR observation. To further assess the strength of the intramolecular hydrogen bonds, the three folded aromatic amide segments have also been incorpo- rated into the main chains of dodecane-l,12-diamine-defived amide polymers to afford macromolecules P-3, P-5 and P-7. The degree of polymerization of the macromolecules was determined by GPC to be 22, 14 and 13, respectively. Force-extension curves obtained from single molecular force spectroscopy （SMFS） revealed that, in tetrachloroethane, all the three macromolecules exhibited saw-tooth force peaks, which had been attributed to the step-by-step breaking of the in- tramolecular hydrogen bonds of the foldarner segments. P-3 exhibited 4 peaks at ca. 83, 121, 181 and 236 pN, P-5 displayed 7 peaks at ca. 20, 44, 73, 101, 130, 171 and 278 pN, and P-7 generated 8 peaks at ca. 31, 43, 50, 60, 90, 152, 173 and 221 pN. The increasing number of the force peaks observed from P-3 to P-5 and P-7 was ascribed to the increasing number of the intramolecular hydrogen bonds. It was proposed that the peaks at lower forces corresponded to the less stable hydrogen bonds, whereas those observed at higher forces were produced by the breaking of the more stable ones. The fact that the first peaks of P-3 was higher than that of P-5 and P-7 indicated that the intramolecular hydrogen bonds of P-3 were pronouncedly more stable than some of the intramolecular hydrogen bonds of P-5 and P-7, which is consistent with the above 1H NMR and hydrogen-denterium exchange observations. Similar results were also observed for P-5 and P-7 in hexadecane, whereas P-3 did not generate measurable force peaks possibly due to the strong absorption of its short, but more planar foldamer segments to the surface of the slide. Simulated stretching curves of the three macromolecutes were also consistent with the SMFS re- sults.