Shieh H S, Ghisla S, Hanson L K, Ludwig M L, Nordman C E
Biochemistry. 1981 Aug 4;20(16):4766-74. doi: 10.1021/bi00519a037.
The molecular complex lumiflavin-2-aminobenzoic acid monohydrate (C13H12N4O2.C7H7NO2.H2O) crystallizes from from aqueous solution as red triclinic prisms. The space group is P1 with cell dimensions a = 9.660 A, b = 14.866 A, c = 7.045 A, alpha = 95.44 degrees , beta = 95.86 degrees, and gamma = 105.66 degrees . The crystal structure was solved by direct methods and refined by block-diagonal least-squares procedures to an R value of 0.050 on the basis of 1338 observed reflections. The structure is composed of stacks of alternating lumiflavin adn un-ionized (neutral) 2-aminobenzoic acid molecules. Two different modes of stacking interaction are observed. In one, 2-aminobenzoic acid overlaps all three of the isoalloxazine rings, at a mean distance of 3.36 A; in the other, 2-aminobenzoic acid interacts distance of 3.36 A; in the other, 2-aminobenzoic acid interacts with the pyrazine and dimethylbenzene moieties, at a distance of 3.42 A. Perpendicular to the stacking direction, the molecules form a continuous sheet. Each flavin is hydrogen bonded via O(2) and NH(3) to two symmetrically related aminobenzoates; the water of crystallization forms three hydrogen bonds, bridging two flavins, via O(4) and N(5), and one aminobenzoic acid. The red color of the crystals results from a charge-transfer transition involving stacked flavin and 2-aminobenzoic acid. The red color of the crystals results from a charge-transfer transition involving stacked flavin and 2-aminobenzoic acid molecules. Measurements of the polarized optical absorption spectra of crystals show that the transition moment direction for the long wavelength absorbance (beyond 530 nm) contains an out-of-plane component which can only arise from a charge-transfer interaction. Since the amino N does not make exceptionally close interactions with isoalloxazine atoms in either stacking mode (minimum interatomic distance 3.52 A), the charge transfer is presumed to involve pi orbitals of the 2-aminobenzoic acid donor.
分子复合物核黄素 - 2 - 氨基苯甲酸一水合物（C₁₃H₁₂N₄O₂·C₇H₇NO₂·H₂O）从水溶液中结晶形成红色三斜棱柱体。空间群为P1，晶胞参数a = 9.660 Å，b = 14.866 Å，c = 7.045 Å，α = 95.44°，β = 95.86°，γ = 105.66°。晶体结构通过直接法解析，并采用块对角最小二乘法进行精修，基于1338个观测反射，R值为0.050。该结构由交替排列的核黄素和未电离（中性）的2 - 氨基苯甲酸分子堆叠而成。观察到两种不同的堆叠相互作用模式。一种模式下，2 - 氨基苯甲酸与所有三个异咯嗪环重叠，平均距离为3.36 Å；另一种模式下，2 - 氨基苯甲酸与吡嗪和二甲苯部分相互作用，距离为3.42 Å。垂直于堆叠方向，分子形成连续的片层。每个黄素通过O(2)和NH(3)与两个对称相关的氨基苯甲酸盐形成氢键；结晶水通过O(4)和N(5)形成三个氢键，桥连两个黄素和一个氨基苯甲酸。晶体的红色源于涉及堆叠的黄素和2 - 氨基苯甲酸的电荷转移跃迁。晶体的红色源于涉及堆叠的黄素和2 - 氨基苯甲酸分子的电荷转移跃迁。晶体的偏振光吸收光谱测量表明，长波长吸收（超过530 nm）的跃迁矩方向包含一个面外分量，这只能由电荷转移相互作用产生。由于在任何一种堆叠模式下氨基N与异咯嗪原子之间都没有特别紧密的相互作用（最小原子间距离为3.52 Å），因此推测电荷转移涉及2 - 氨基苯甲酸供体的π轨道。
