Density functional theory calculations of the structure and the 15N and 13C chemical shifts of methyl bacteriopheophorbide a and bacteriochlorophyll a
Density functional theory calculations of the structure and the 15N and 13C chemical shifts of methyl bacteriopheophorbide a and bacteriochlorophyll a
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DOI：
10.1021/jp9800970
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发表时间：
1998-03-12
影响因子：
3.3
通讯作者：
Facelli, JC
中科院分区：
文献类型：
--
作者：
Facelli, JC
使用D95和D95**基组对细菌脱镁叶绿酸甲酯(MBPheo-a)和细菌叶绿素a(BCHL-a)进行了从头算(DFT)化学位移计算和几何构型优化。结果表明，在MBPheo-a和BCHL-a等大系统中计算化学位移是可能的，并且计算的化学位移对计算中使用的几何形状非常敏感。用至少优化了质子位置的分子几何计算化学位移，得到的结果比使用X射线实验结构的结果更符合实验值。C-13位移的结果是如此高的质量，它们可以用来验证相距超过5ppm的核磁共振的指认。本文给出的计算结果强烈建议对MBPheo-a和BCHL-a中吡咯环碳的C-13共振谱的几个指认进行修正。
Ab initio (DFT) chemical shifts calculations and geometry optimizations were performed in methyl bacteriopheophorbide a (MBPheo-a) using the D95 and D95** basis sets and in bacteriochlorophyll a (BChl-a) using the LanL2DZ basis set. The results show that it is possible to calculate chemical shifts in large systems, such as MBPheo-a and BChl-a, and that the calculated chemical shifts are quite sensitive to the geometries used in their calculation. Calculations of the chemical shifts using molecular geometries in which at least the positions of the protons have been optimized produce results in much better agreement with the experimental values than those using the X-ray experimental structure. The results for C-13 shifts are of such quality that they can be used to verify the assignments of NMR resonances separated by more than 5 ppm. The calculations presented here strongly suggest that several assignments of the C-13 resonances of the pyrrolic ring carbons in MBPheo-a and BChl-a should be revised.