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Zeolitic imidazolate frameworks (ZIFs) with intrinsic photoactivity represent an excellent scaffold for a single-site photocatalytic center. However, their application in visible-light-driven photocatalysis is hampered by their light-harvesting ability due to limited coverage in the realm of the solar spectrum and relatively low extinction coefficient. In this work, we report the first example of the enhanced light-harvesting property of ZIF-67 via energy transfer from a molecular photosensitizer, that is, the RuN3 complex, sensitized on the surface of ZIF-67. Using transient absorption spectroscopy, we show that energy transfer occurs from the excited RuN3 to ZIF-67 with ∼86.9% efficiency. More importantly, this RuN3/ZIF-67 hybrid system exhibits significantly enhanced photocatalytic activity for H2 production from water, which can be attributed to efficient energy transfer from RuN3.

Ras is a small monomeric GTPase acting as molecular switch in multiple cellular processes. The N-terminal G domain of Ras binds GTP or GDP accompanied by a magnesium ion, which is strictly required for GTPase activity and performs a structural role. Another ion-binding site on the opposite face of the G domain has been recently observed to specifically associate with calcium acetate in the crystal [Buhrman, G., et al. (2010) Proc. Natl. Aacd. Sci. U.S.A. 107, 4931-4936]. In this article, we report thermodynamic measurements of the affinity and specificity of the remote ion-binding site in H-Ras as observed in solution. Using (15)N-(1)H nuclear magnetic resonance spectroscopy, we determined that, in contrast to the crystalline state, the remote site in solution is specific for a divalent cation, binding both calcium and magnesium with anions playing a minimal role. The affinity of the remote site for divalent cations is in the low millimolar range and remarkably different for GDP- and GppNHp-bound forms of the G domain, indicating that the GTP-binding pocket and the remote site are allosterically coupled through the distance of more than 25 Å. Considering that the remote site is oriented toward the membrane surface in vivo, we hypothesize that its cognate biological ligand might be a positively charged group extending from a lipid or an integral membrane protein.

Through characterization of the solvent isotope effect on protein dynamics, we have examined determinants of the rate limitation to enzyme catalysis. A global conformational change in Ribonuclease A limits the overall rate of catalytic turnover. Here we show that this motion is sensitive to solvent deuterium content; the isotope effect is 2.2, a value equivalent to the isotope effect on the catalytic rate constant. We further demonstrate that the protein motion possesses a linear proton inventory plot, indicating that a single proton is transferred in the transition state. These results provide compelling evidence for close coupling between enzyme dynamics and function and demonstrate that characterization of the transition state for protein motion in atomic detail is experimentally accessible.