Lindner and Lemal showed that perfluorination of keto-enol systems significantly shifts the equilibrium toward the enol tautomer. Quantum mechanical calculations now reveal that the shift in equilibrium is the result of the stabilization of the enol tautomer by hyperconjugative π → σ*_CF interactions and the destabilization of the keto tautomer by the electron withdrawal induced by the neighboring fluorine atoms. The preference for the enol tautomer further increases in smaller perfluorinated cyclic keto-enol systems. This trend is in contrast to the nonfluorinated compounds, where the enol is strongly disfavored in the smaller rings. The fluoro effect overrides the effect of the ring size that controls the equilibria in nonfluorinated compounds. The increased overlap of the enol π bond with the σ*_CF orbitals of the allylic C-F bonds results in the increased preference for the enol tautomer in smaller perfluorinated keto-enol systems. We show here why the effect is much greater than in 3,3-difluorocyclooctyne.

Epithiodiketopiperazines (ETPs) are a structurally complex class of fungal natural products with potent anticancer activity. In ETPs, the diketopiperazine ring is spanned by a disulfide bond that is constrained in a high-energy eclipsed conformation. We employed computational, synthetic, and spectroscopic methods to investigate the physicochemical attributes of this atypical disulfide bond. We find that the disulfide bond is stabilized by two n→π* interactions, each with large energies (3-5 kcal/mol). The n→π* interactions in ETPs make disulfide reduction much more difficult, endowing stability in physiological environments in a manner that could impact their biological activity. These data reveal a previously unappreciated means to stabilize a disulfide bond and highlight the utility of the n→π* interaction in molecular design.

The Diels-Alder reactivity of 4,4-difluoro-3,5-diphenyl-4H-pyrazole was investigated experimentally and computationally with endo-bicyclo[6.1.0]non-4-yne. The computationally predicted rate enhancement from hyperconjugative antiaromaticity induced by fluorination of cyclopentadienes at the 5-position extends to five-membered heterocyclic dienes containing a saturated center. 4,4-Difluoro-4H-pyrazoles are new electron-deficient dienes with rapid reactivities toward strained alkynes.

The glycyl radical enzyme (GRE) superfamily utilizes a glycyl radical cofactor to catalyze difficult chemical reactions in a variety of anaerobic microbial metabolic pathways. Recently, a GRE, trans-4-hydroxy-L-proline (Hyp) dehydratase (HypD), was discovered that catalyzes the dehydration of Hyp to (S)-Δ1-pyrroline-5-carboxylic acid (P5C). This enzyme is abundant in the human gut microbiome and also present in prominent bacterial pathogens. However, we lack an understanding of how HypD performs its unusual chemistry. Here, we have solved the crystal structure of HypD from the pathogen Clostridioides difficile with Hyp bound in the active site. Biochemical studies have led to the identification of key catalytic residues and have provided insight into the radical mechanism of Hyp dehydration.

Wound healing is a complex process that requires the intervention of cytoactive factors. The one-time application of soluble factors to a wound bed does not maintain a steady, sufficient concentration. Here we investigated the benefits of anchoring a factor in a wound bed via a tether to endogenous collagen. We used a collagen-mimetic peptide (CMP) as a pylon. The CMP binds to damaged but not intact collagen and thus localizes a pendant cytoactive factor in the regions of a wound bed that require intervention. As a model factor, we chose substance P, a peptide of the tachykinin family that promotes wound healing. Using splinted wounds in db/db mice, we found that the one-time application of a CMP-substance P conjugate enhances wound healing compared to unconjugated substance P and other controls. Specifically, all 16 wounds treated with the conjugate closed more thoroughly and, did so with extensive re-epithelialization and mitigated inflammatory activity. These data validate a simple and general strategy for re-engineering wound beds by the integration of beneficial cytoactive factors. Copyright © 2014 John Wiley & Sons, Ltd.