Department of Chemistry and Biochemistry, Oberlin College , 119 Woodland Street, Oberlin, Ohio 44074, United States.
J Phys Chem A. 2014 Apr 10;118(14):2618-28. doi: 10.1021/jp411220y. Epub 2014 Mar 31.
Porous crystalline dipeptides have gained recent attention for their potential as gas-storage materials. Within this large class is a group of dipeptides containing alanine, valine, and isoleucine with very similar crystal structures. We report the (13)C (carbonyl and Cα) and (15)N (amine and amide) solid-state NMR isotropic chemical shifts in a series of seven such isostructural porous dipeptides as well as shift tensor data for the carbonyl and amide sites. Using their known crystal structures and aided by ab initio quantum chemical calculations for the resonance assignments, we elucidate trends relating local structure, hydrogen-bonding patterns, and chemical shift. We find good correlation between the backbone dihedral angles and the Cα1 and Cα2 shifts. For the C1 shift tensor, the δ11 value shifts downfield as the hydrogen-bond distance increases, δ22 shifts upfield, and δ33 shows little variation. The C2 shift tensor shows no appreciable correlation with structural parameters. For the N2 tensor, δ11 shows little dependence on the hydrogen-bond length, whereas δ22 and δ33 both show a decrease in shielding as the hydrogen bond shortens. Our analysis teases apart some, but not all, structural contributors to the observed differences the solid-state NMR chemical shifts.
多孔结晶二肽因其作为储气材料的潜力而受到关注。在这一大类中二肽中，有一组包含丙氨酸、缬氨酸和异亮氨酸的二肽，它们具有非常相似的晶体结构。我们报告了一系列七种结构相同的多孔二肽的 (13)C（羰基和 Cα）和 (15)N（胺和酰胺）固态 NMR 各向同性化学位移，以及羰基和酰胺位的位移张量数据。利用已知的晶体结构，并通过共振分配的从头算量子化学计算进行辅助，我们阐明了与局部结构、氢键模式和化学位移相关的趋势。我们发现，主链二面角与 Cα1 和 Cα2 位移之间存在很好的相关性。对于 C1 位移张量，δ11 值随着氢键距离的增加而向高场移动，δ22 向低场移动，而 δ33 变化不大。C2 位移张量与结构参数没有明显的相关性。对于 N2 张量，δ11 几乎不受氢键长度的影响，而 δ22 和 δ33 的屏蔽都随着氢键的缩短而减小。我们的分析梳理了一些，但不是全部，对观察到的固态 NMR 化学位移差异的结构贡献。
