Search

Previously our laboratory identified that poly-2'-deoxycytidine (dCn) strands of DNA with lengths greater than 12 nucleotides could adopt i-motif folds, while the pH-dependent stabilities follow a 4n - 1 repeat pattern with respect to chain length (J. Am. Chem. Soc., 2017, 139, 4682-4689). Herein, model i-motif folds in which loop configurations were forced by judiciously mutating dC to non-dC nucleotides allowed a structural model to be proposed to address this phenomenon. The model was developed by systematically studying two i-motifs with either an even or odd number of d(C·C)+ hemiprotonated base pairs in the core. First, a trend in the pH-dependent stability vs. loop nucleotide identity was observed: dC > dT ∼ dU ≫ dA ∼ dG. Next, loops comprised of dT nucleotides in the two different core base pair configurations were studied while systematically changing the loop lengths. We found that an i-motif with an even number of base pairs in the core with a single nucleotide in each of the three loops was the most stable, as well as an i-motif with an odd number of core base pairs having one nucleotide in the two exterior loops and three nucleotides in the central loop. A systematic increase in the central loop from 1-4 nucleotides for an odd number of base pairs in the i-motif core reproduced the 4n - 1 repeat pattern observed in the poly-dCn strands. Additional loop configurations were studied to further support the model. The results are discussed with respect to their biological relevance.

The oxidative addition of organic electrophilesinto electrochemically generated Co(I) complexes has beenwidely utilized as a strategy to produce carbon-centeredradicals when cobalt is ligated by apolydentate ligand.Changing to a bidentate ligand provides the opportunity toaccess discrete Co(III)−Cbonded complexes for alternativereactivity, but knowledge of how ligand and/or substratestructuresaffect catalytic steps is pivotal to reaction design andcatalyst optimization. In this vein, experimental studies thatcan determine the exact nature of elementary organometallicsteps remain limited, especially for single-electron oxidativeaddition pathways. Herein, we utilize cyclic voltammetrycombinedwith simulations to obtain kinetic and thermodynamicproperties of the two-step, halogen-atom abstraction mechanism, validated by analyzing kinetic isotope and substituenteffects. Complex Hammett relationships could be disentangled to allow understanding of individual effects onactivation energybarriers and equilibrium constants, and DFT-derived parameters used to build predictive statistical models for rates of newligand/substrate combinations.