No abstract

No abstract

DNA gyrase catalyzes ATP-dependent negative supercoiling of DNA in a strand passage mechanism. A double-stranded segment of DNA, the T-segment, is passed through the gap in a transiently cleaved G-segment by coordinated closing and opening of three protein interfaces in gyrase. T-segment capture is thought to be guided by the C-terminal domains of the GyrA subunit of gyrase that wrap DNA around their perimeter and cause a DNA-crossing with a positive handedness. We show here that the C-terminal domains are in a downward-facing orientation in the absence of DNA, but swing up and rotate away from the gyrase body when DNA binds. The upward movement of the C-terminal domains is an early event in the catalytic cycle of gyrase that is triggered by binding of a G-segment, and first contacts of the DNA with the C-terminal domains, and contributes to T-segment capture and subsequent strand passage.

A systematic survey of zeolites Y containing tris(2,2‘-bipyridine)ruthenium(II) with various levels of loading was undertaken. Almost pure Ru(bpy)3 2+ was obtained whenever the loading was less than approximately one complex per two supercages. At higher loading an increasing amount of byproducts corresponding formally to Ru(bpy) n (NH3)6-2n 2+, n < 3, was found. This dependence of the yield of the “ship-in-a-bottle” synthesis of Ru(bpy)3 2+ in zeolite Y on the ruthenium exchange degree was interpreted in terms of transport problems and in terms of sterical fitting between host zeolite Y supercages and guest Ru(bpy)3 2+. For high loadings, we observed a tendency of Ru(bpy)3 2+ to accumulate toward the surface of the zeolite microcrystals instead of a random distribution in the bulk. The consequence of this was to slow down the bpy diffusion during the in situ complexation process and finally to prevent the reaction from being completed at high loading levels.

No abstract