Effect of Functional Groups at N-Terminus on the Properties and Structures of Crystalline Nano-Cavities in Flexible Peptide Ni(II)-Macrocycles

To clarify the design strategy for achieving functionality in crystalline multi-component systems comprising cyclic complexes of flexible short peptides, crystalline Ni(II) cyclic complexes were synthesized using two novel dipeptides (2 and 3) possessing different functional groups at the N-terminus of dipeptide 1. X-ray single-crystal structural analysis revealed that 2 and 3 formed tetranuclear cyclic complexes ([Ni4L4]8+), which were also observed for 1. The crystalline packing structure of the cyclic complexes of dipeptide 2 was almost the same as that of the cyclic complexes of dipeptide 1, whereas that of 3 had a different packing structure. The cooperative opening/closing of the crystalline heterogeneous cavity, which demonstrates humidity-responsive cooperative binding in the cyclic complexes of 1, was not observed for the complexes of 2, plausibly because of the decrease in crystalline voids of the latter. In contrast, the amounts for water and alcohol vapor adsorbed at room temperature were almost the same for the open forms of the cyclic complexes of 1 and 2, which was supported by the expansion of the flexible cavities in the crystalline cyclic complexes of 2. The effect of functional groups at the N-terminus of peptides on the properties of the crystalline cavities in peptide Ni(II) macrocycles was investigated. These functional groups influence the packing structures and the mode of cooperative structural changes in the crystalline peptide Ni(II) macrocycle. In contrast, the amount of adsorbed water/alcohol vapor remained constant despite the decrease in the size of crystalline cavities.