Characterizing the cooperativity in H-bonded amino structures.

Density functional theory calculations were used to examine the effect of H-bond cooperativity on the magnitude of the NMR chemical shifts and spin-spin coupling constants in a C4h-symmetric G-quartet and in structures consisting of six cyanamide monomers. These included two ring structures (a planar C6h-symmetric structure and a nonplanar S6-symmetric structure) and two linear chain structures (a fully optimized planar Cs-symmetric chain and a planar chain structure where all intra- and intermolecular parameters were constrained to be identical). The NMR parameters were computed for the G-quartet and cyanamide structures, as well as for shorter fragments derived from these assemblies without reoptimization. In the ring structures and the chain with identical monomers, the intra- and intermolecular geometries of the cyanamides were identical, thereby allowing the study of cooperative effects in the absence of geometry changes. The magnitude of the |1JNH| coupling, 1H and 15N chemical shifts of the H-bonding amino N-H group, and the |h2JNN| H-bond coupling increased, whereas the size of the |1JNH| coupling of the non-H-bonded amino N-H bonds of the first amino group in the chain, which are roughly perpendicular to the H-bonding network, decreased in magnitude when H-bonding monomers were progressively added to extending ring or chain structures. These effects are attributed to electron redistribution induced by the presence of the nearby H-bonding guanine or cyanamide molecules.