RESUMO
A long and highly flexible internally functionalized dipyridyl ligand α,α'-p-xylylenebis(1-(4-pyridylmethylene)-piper-4-azine), L, has been employed in the synthesis of a series of coordination polymer materials with Co(II), Cd(II), and Ag(I) ions. In poly-[Cd(L)(TPA)] 1 and poly-[Co(L)(IPA)], 2, (TPA = terephthalate, IPA = isophthalate) the ligand adopts a similar linear conformation to that seen in the structure of the unbound molecule and provides a long (2.6 nm) metal-metal bridging distance. Due to the mismatch of edge lengths with that provided by the carboxylate coligands, geometric distortions from the regular dia and (4,4) network geometries for 1 and 2, respectively, are observed. In poly-[Ag2(CF3SO3)2(L)], 3, the ligand coordinates through both pyridine groups and two of the four piperazine nitrogen donors, forming a high-connectivity 2-dimensional network. The compound poly-[Ag2(L)](BF4)2·2MeCN, 4, a porous 3-dimensional cds network, undergoes a fascinating and rapid single-crystal-to-single-crystal rearrangement on exchange of the acetonitrile guests for water in ambient air, forming a nonporous hydrated network poly-[Ag2(L)](BF4)2·2H2O, 5, in which the well-ordered guest water molecules mediate the rearrangement of the tetrafluoroborate anions and the framework itself through hydrogen bonding. The dynamics of the system are examined in greater detail through the preparation of a kinetic product, the dioxane-solvated species poly-[Ag2(L)](BF4)2·2C4H8O2, 6, which undergoes a slow conversion to 5 over the course of approximately 16 h, a transition which can be monitored in real time. The reverse transformation can also be observed on immersing the hydrate 5 in dioxane. The structural features and physical properties of each of the materials can be rationalized based on the flexible and multifunctional nature of the ligand molecule, as well as the coordination behavior of the chosen metal ions.
RESUMO
Ran is a small ras-related GTPase that controls the nucleocytoplasmic exchange of macromolecules across the nuclear envelope. It binds to chromatin early during nuclear formation and has important roles during the eukaryotic cell cycle, where it regulates mitotic spindle assembly, nuclear envelope formation and cell cycle checkpoint control. Like other GTPases, Ran relies on the cycling between GTP-bound and GDP-bound conformations to interact with effector proteins and regulate these processes. In nucleocytoplasmic transport, Ran shuttles across the nuclear envelope through nuclear pores. It is concentrated in the nucleus by an active import mechanism where it generates a high concentration of RanGTP by nucleotide exchange. It controls the assembly and disassembly of a range of complexes that are formed between Ran-binding proteins and cellular cargo to maintain rapid nuclear transport. Ran also has been identified as an essential protein in nuclear envelope formation in eukaryotes. This mechanism is dependent on importin-ß, which regulates the assembly of further complexes important in this process, such as Nup107-Nup160. A strong body of evidence is emerging implicating Ran as a key protein in the metastatic progression of cancer. Ran is overexpressed in a range of tumors, such as breast and renal, and these perturbed levels are associated with local invasion, metastasis and reduced patient survival. Furthermore, tumors with oncogenic KRAS or PIK3CA mutations are addicted to Ran expression, which yields exciting future therapeutic opportunities.