Characterizing the Cooperativity in H-Bonded Amino Structures
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- 作者：Tanja van Mourik ; Andrew J. Dingley
- 刊名：Journal of Physical Chemistry A
- 出版年：2007
- 出版时间：November 8, 2007
- 年：2007
- 卷：111
- 期：44
- 页码：11350 - 11358
- 全文大小：587K
- 年卷期：v.111,no.44(November 8, 2007)
- ISSN：1520-5215
文摘
Density functional theory calculations were used to examine the effect of H-bond cooperativity on the magnitudeof the NMR chemical shifts and spin-spin coupling constants in a C4h-symmetric G-quartet and in structuresconsisting of six cyanamide monomers. These included two ring structures (a planar C6h-symmetric structureand a nonplanar S6-symmetric structure) and two linear chain structures (a fully optimized planar Cs-symmetricchain and a planar chain structure where all intra- and intermolecular parameters were constrained to beidentical). The NMR parameters were computed for the G-quartet and cyanamide structures, as well as forshorter fragments derived from these assemblies without reoptimization. In the ring structures and the chainwith identical monomers, the intra- and intermolecular geometries of the cyanamides were identical, therebyallowing the study of cooperative effects in the absence of geometry changes. The magnitude of the 1JNHcoupling, 1H and 15N chemical shifts of the H-bonding amino N-H group, and the h2JNN H-bond couplingincreased, whereas the size of the 1JNH coupling of the non-H-bonded amino N-H bonds of the first aminogroup in the chain, which are roughly perpendicular to the H-bonding network, decreased in magnitude whenH-bonding monomers were progressively added to extending ring or chain structures. These effects are attributedto electron redistribution induced by the presence of the nearby H-bonding guanine or cyanamide molecules.