Karle Isabella L, Venkateshwarlu P, Ranganathan S
Laboratory for the Structure of Matter, Naval Research Laboratory, Washington, D.C. 20375-5341, USA.
Biopolymers. 2006;84(5):502-7. doi: 10.1002/bip.20535.
In the course of our work relating to the design of a bihelical structure (I) from diphenic anhydride by tethering with cystine di-OMe, stable, hard, and rigid crystals, mp 215-218 degrees C were isolated in low yields ( approximately 2%). The crystal structure established that it was a bis amide (II) arising from diphenic acid and cystine di-OMe [(II), C(22)H(22)N(2)O(6)S(2) (a = 9.897 (1) A, b = 12.210 (1) A, c = 18.192 (1) A, sp. gr. P2(1)2(1)2(1))]. An authentic sample of (II) was subsequently prepared in 47% yields by condensation of diphenic acid dichloride with cystine di-OMe. A most surprising feature of II was, despite its high density, rigidity, and hardness, it did not exhibit any normal hydrogen bonds. The nearest approximation to a "usual" hydrogen bond was the single NH...OC linkage that occurred between molecules along a twofold screw axis. In this linkage, N...O = 3.265 A and H...O = 2.43 A, values that are at least 10% longer than those usually observed in peptides. The rigidity of the crystals appears to depend upon many weak hydrogen bonds of the type CH...O, CH...pi, CH...S, and NH...S working in concert. Even these attractions have separations that are at the high end of the range of previously observed values, although some of the weak hydrogen bonds have been rarely reported and have poorly defined ranges. The attractive effect of each of these weak bonds may be enhanced by the occurrence of a number of them in a parallel fashion like rungs in a ladder.
在我们通过用二甲基胱氨酸连接由二苯二甲酸酐设计双螺旋结构（I）的工作过程中，分离出了熔点为215 - 218℃的稳定、坚硬且刚性的晶体，但产率较低（约2%）。晶体结构确定其为二苯二甲酸和二甲基胱氨酸形成的双酰胺（II）[（II），C₂₂H₂₂N₂O₆S₂（a = 9.897 (1) Å，b = 12.210 (1) Å，c = 18.192 (1) Å，空间群P2(1)2(1)2(1)）]。随后通过二苯二甲酰氯与二甲基胱氨酸缩合以47%的产率制备了（II）的纯品。II最令人惊讶的特征是，尽管它具有高密度、刚性和硬度，但并未表现出任何正常的氢键。最接近“通常”氢键的是沿着二重螺旋轴在分子间出现的单个NH...OC键。在这个键中，N...O = 3.265 Å，H...O = 2.43 Å，这些值比通常在肽中观察到的值至少长10%。晶体的刚性似乎取决于许多协同作用的CH...O、CH...π、CH...S和NH...S类型的弱氢键。即使这些吸引力的间距处于先前观察值范围的高端，尽管一些弱氢键很少被报道且范围界定不明确。这些弱键中的每一个的吸引作用可能会因它们像梯子的梯级一样以平行方式大量出现而增强。
