Normalized Harmonic Map Heat Flow

For any closed Riemannian manifold N we propose the normalized harmonic map heat flow as a means to obtain nonconstant harmonic maps , m ≥ 3 . © 2019 Wiley Periodicals, Inc.     

One Way Traffic: Base-to-Backbone Hole Transfer in Nucleoside Phosphorodithioate

The directionality of the hole-transfer processes between DNA backbone and base was investigated by using phosphorodithioate [P(S⁻)=S] components. ESR spectroscopy in homogeneous frozen aqueous solutions and pulse radiolysis in aqueous solution at ambient temperature confirmed initial formation of G^.+-P(S⁻)=S. The ionization potential of G-P(S⁻)=S was calculated to be slightly lower than that of guanine in 5′-dGMP. Subsequent thermally activated hole transfer from G^.+ to P(S⁻)=S led to dithiyl radical (P-2S^.) formation on the μs timescale. In parallel, ESR spectroscopy, pulse radiolysis, and density functional theory (DFT) calculations confirmed P-2S^. formation in an abasic phosphorodithioate model compound. ESR investigations at low temperatures and higher G-P(S⁻)=S concentrations showed a bimolecular conversion of P-2S^. to the σ²-σ*¹-bonded dimer anion radical [-P-2S 2S-P-]⁻ [ΔG (150 K, DFT)=−7.2 kcal mol⁻¹]. However, [-P-2S 2S-P-]⁻ formation was not observed by pulse radiolysis [ΔG° (298 K, DFT)=−1.4 kcal mol⁻¹]. Neither P-2S^. nor [-P-2S 2S-P-]⁻ oxidized guanine base; only base-to-backbone hole transfer occurs in phosphorodithioate.     

Does Perdeuteration Increase the Polarity of Janus Face Cycloalkanes?

Stimulated by a suggestion of the late Professor Jack D. Dunitz, that perdeuterated Janus face cycloalkanes may be more polar than their unlabelled forms, the deuterated isotopologue of all cis-1,2,3,4,5,6-hexafluorocyclohexane ([²H₆]- 1a ) and all cis-1,2,3,4-tetrafluorocyclopentane ([²H₆]- 3a ) were prepared. Computation at the B3LYP−D3 level indicated that [²H₆]- 1a is not more polar than its protio form 1, however perdeuterated cyclopentane [²H₆]- 3a may indeed be more polar than 3 , although the magnitude is predicted to be small. None-the-less retention time analysis on a reverse phase GC/MS column of an add-mix of 3 and [²H₆]- 3a gave some indication that the per-deuterated isotopologue 3a was detected marginally before the unlabelled compound consistent with increased polarity associated with perdeuteration.     

Ruthenium-Catalyzed C−C Coupling of Terminal Alkynes with Primary Alcohols or Aldehydes: α,β-Acetylenic Ketones (Ynones) via Oxidative Alkynylation

The first metal-catalyzed oxidative alkynylations of primary alcohols or aldehydes to form α,β-acetylenic ketones (ynones) are described. Deuterium labelling studies corroborate a novel reaction mechanism in which alkyne hydroruthenation forms a transient vinylruthenium complex that deprotonates the terminal alkyne to form the active alkynylruthenium nucleophile.     

Enantioselective Rhodium-Catalyzed Pauson–Khand Reactions of 1,6-Chloroenynes with 1,1-Disubstituted Olefins

An enantioselective rhodium(I)-catalyzed Pauson–Khand reaction (PKR) using 1,6-chloroenynes that contain challenging 1,1-disubstituted olefins is described. In contrast to the previous studies with these types of substrates, which are only suitable for a single type of tether and alkyne substituent, the new approach results in a more expansive substrate scope, including carbon and heteroatom tethers with polar and non-polar substituents on the alkene. DFT calculations provide critical insight into the role of the halide, which pre-polarizes the alkyne to lower the barrier for metallacycle formation and provides the proper steric profile to promote a favorable enantiodetermining interaction between substrate and chiral diphosphine ligand. Hence, the chloroalkyne enables the efficient and enantioselective PKR with 1,6-enynes that contain challenging 1,1-disubstituted olefins, thereby representing a new paradigm for enantioselective reactions involving 1,6-enynes.     

Bottom-up Solution Synthesis of Graphene Nanoribbons with Precisely Engineered Nanopores

The incorporation of nanopores into graphene nanostructures has been demonstrated as an efficient tool in tuning their band gaps and electronic structures. However, precisely embedding the uniform nanopores into graphene nanoribbons (GNRs) at the atomic level remains underdeveloped especially for in-solution synthesis due to the lack of efficient synthetic strategies. Herein we report the first case of solution-synthesized porous GNR ( pGNR ) with a fully conjugated backbone via the efficient Scholl reaction of tailor-made polyphenylene precursor ( P1 ) bearing pre-installed hexagonal nanopores. The resultant pGNR features periodic subnanometer pores with a uniform diameter of 0.6 nm and an adjacent-pores-distance of 1.7 nm. To solidify our design strategy, two porous model compounds ( 1 a , 1 b ) containing the same pore size as the shortcuts of pGNR , are successfully synthesized. The chemical structure and photophysical properties of pGNR are investigated by various spectroscopic analyses. Notably, the embedded periodic nanopores largely reduce the π-conjugation degree and alleviate the inter-ribbon π–π interactions, compared to the nonporous GNRs with similar widths, affording pGNR with a notably enlarged band gap and enhanced liquid-phase processability.     

Time-Resolved Mn2+−NO and NO−NO Distance Measurements Reveal That Catalytic Asymmetry Regulates Alternating Access in an ABC Transporter**

ATP-binding cassette (ABC) transporters shuttle diverse substrates across biological membranes. Transport is often achieved through a transition between an inward-facing (IF) and an outward-facing (OF) conformation of the transmembrane domains (TMDs). Asymmetric nucleotide-binding sites (NBSs) are present among several ABC subfamilies and their functional role remains elusive. Here we addressed this question using concomitant NO−NO, Mn²⁺−NO, and Mn²⁺−Mn²⁺ pulsed electron–electron double-resonance spectroscopy of TmrAB in a time-resolved manner. This type-IV ABC transporter undergoes a reversible transition in the presence of ATP with a significantly faster forward transition. The impaired degenerate NBS stably binds Mn²⁺−ATP, and Mn²⁺ is preferentially released at the active consensus NBS. ATP hydrolysis at the consensus NBS considerably accelerates the reverse transition. Both NBSs fully open during each conformational cycle and the degenerate NBS may regulate the kinetics of this process.     

Catalytic Synthesis of N‐Unprotected Piperazines,Morpholines, and Thiomorpholines from Aldehydes and SnAP Reagents

Commercially available SnAP (stannyl amine protocol) reagents allow the transformation of aldehydes and ketones into a variety of N‐unprotected heterocycles. By identifying new ligands and reaction conditions, a robust catalytic variant that expands the substrate scope to previously inaccessible heteroaromatic substrates and new substitution patterns was realized. It also establishes the basis for a catalytic enantioselective process through the use of chiral ligands.