Non-covalent assembly of proton donors and p-benzoquinone anions for co-electrocatalytic reduction of dioxygen

The two-electron and two-proton p-hydroquinone/p-benzoquinone (H₂Q/BQ) redox couple has mechanistic parallels to the function of ubiquinone in the electron transport chain. This proton-dependent redox behavior has shown applicability in catalytic aerobic oxidation reactions, redox flow batteries, and co-electrocatalytic oxygen reduction. Under nominally aprotic conditions in non-aqueous solvents, BQ can be reduced by up to two electrons in separate electrochemically reversible reactions. With weak acids (AH) at high concentrations, potential inversion can occur due to favorable hydrogen-bonding interactions with the intermediate monoanion [BQ(AH)_m]˙⁻. The solvation shell created by these interactions can mediate a second one-electron reduction coupled to proton transfer at more positive potentials ([BQ(AH)_m]˙⁻ + nAH + e⁻ ⇌ [HQ(AH)_(m+n)−1(A)]²⁻), resulting in an overall two electron reduction at a single potential at intermediate acid concentrations. Here we show that hydrogen-bonded adducts of reduced quinones and the proton donor 2,2,2-trifluoroethanol (TFEOH) can mediate the transfer of electrons to a Mn-based complex during the electrocatalytic reduction of dioxygen (O₂). The Mn electrocatalyst is selective for H₂O₂ with only TFEOH and O₂ present, however, with BQ present under sufficient concentrations of TFEOH, an electrogenerated [H₂Q(AH)₃(A)₂]²⁻ adduct (where AH = TFEOH) alters product selectivity to 96(±0.5)% H₂O in a co-electrocatalytic fashion. These results suggest that hydrogen-bonded quinone anions can function in an analogous co-electrocatalytic manner to H₂Q.

Non-covalent interactions between reduced p-benzoquinone species and weak acids stabilize intermediates which can switch dioxygen reduction selectivity from H₂O₂ to H₂O for a molecular Mn catalyst.     

Moderate and advanced intramolecular proton transfer in urea-anion hydrogen-bonded complexes

The study of the interactions of the three urea-based receptors AH, BH(+) and CH(2+) with a variety of anions, in MeCN, has made it possible to verify the current view that hydrogen bonding is frozen proton transfer from the donor (the urea N-H fragment in this case) to the acceptor (the anion X(-)). The poorly acidic, neutral receptor AH establishes two equivalent hydrogen bonds N-H···X(-), with all anions, including CH(3)COO(-) and F(-), in which moderate proton transfer from N-H to the anion takes place. The strongly acidic, dicationic receptor CH(2+) forms, with most anions, complexes in which two inequivalent hydrogen bonds are present: one involving moderate proton transfer (N-H···X(-)) and one in which advanced proton transfer has taken place, described as N(-)···H-X. The degree of proton advancement is directly related to the basic tendencies of the anion. The cationic receptor BH(+) of intermediate acidic properties only forms complexes with two inequivalent hydrogen bonds (moderate+advanced proton transfer) with CH(3)COO(-) and F(-), and complexes with two equivalent hydrogen bonds (moderate proton transfer) with all the other anions. Moreover, [B···HF] and [C···HF](+), on addition of a second F(-) ion, lose the bound HF molecule to give HF(2)(-). Release of CH(3)COOH, with the formation of [CH(3)COOH···CH(3)COO](-), also takes place with the [B···CH(3)COOH] complex in the presence of a large excess of anion.     

On the formation of radical dications of protonated amino acids in a "microsolution" of water or acetonitrile and their reactivity towards the solvent

In high-energy collisions (50 keV) between O2 and protonated amino acids AH+, radical dications AH2+* are formed for A = Phe, His, Met, Tyr, and Trp. When solvated by water or acetonitrile (S), AH2+*(S)1,2 are formed for A = Arg, His, Met, Tyr, and Trp. The stability of the hydrogen-deficient AH2+* in the "microsolution" depends on the energetics of the electron transfer reaction AH2+* +S --> AH++S+*, the hydrogen abstraction reaction AH2+*+S --> AH2(2+)+[S-H]*, and the proton transfer reaction AH2+* + S --> A+*+SH+. Using B3LYP/ 6-311+G(2d,p)//B3LYP/6-31+G(d) model chemistry, we describe these three reactions in detail for A=Tyr and find that the first two reactions are unfavorable whereas the third one is favorable. However, energy is required for the formation of Tyr+* and SH+ from TyrH2+*(S) to overcome the Coulomb barrier, which renders the complex observable with a life-time larger than 5 micros. The ionization energy, IE, of TyrH+ is calculated to be 11.1 eV in agreement with an experimental measurement of 10.1+/-2.1 eV ([IE(CH3CN)+IE(Tyr)]/ 2); hydration further lowers the IE by 0.3 eV [IE(TyrH+(H2O) = 10.8 eV, calculated]. We estimate the ionization energies of TrpH+, HisH+, and MetH+ to be 10.1+/-2.1 eV, 12.4+/-0.2 eV, and 12.4+/-0.2 eV, and that of PheH+ to be larger than 12.6 eV.     

Configuration-Odor Relationships in 5β-Ambrox

The four possible A/B cis-fused diastereoisomers of Ambrox® have been synthesized and their configurations and conformations established by X-ray and NMR analysis. Only 5β-ambrox (= 1,2,3a,4,5,5β,6,7,8,9,9a,9bα-dodecahydro-3aβ,6,6,9aβ-tetramethylnaphtho[2,1-b]furan; 5 ) has an odor quality comparable to Ambrox®. The 1,3-synperiplanar/diaxial conformation of the substituents at C(8) ( = C(3a)) and C(10) (= C(9a)) has thus been confirmed to be a compulsory structure element for the particular odor.     

The mechanism of the self-initiated thermal polymerization of styrene. Theoretical solution of a classic problem

The Mayo and Flory mechanisms for the self-initiation of styrene polymerization were explored with B3LYP and BPW91 density functional calculations. The Diels-Alder dimer (AH) is the key intermediate, and the lowest energy pathway for AH formation is a stepwise mechanism via a gauche/sickle (*M2*Gs) or gauche/U-shaped (*M2*Gu) diradical. Ring closure of the 1,4-diradical to diphenylcyclobutane (DCB) is predicted to have a lower barrier than ring closure to AH. Dynamic effects are likely to play an important role in determining the rate of AH versus DCB formation. Hydrogen transfer from AH to styrene to generate two monoradical species is predicted to be a reasonable process that initiates monoradical polymerization.     

Bending and kinking in helical polymers

Molecular dynamics simulations provide a powerful tool to understand the mechanics of helical polymers. We report on all‐atom simulation of strong bending and buckling of α‐helix (AH) filaments by compression loading. We find that the filament buckling proceeds by an abrupt transition from the extended to folded states involving the dramatic drop in the helix span. At the buckling threshold the uniform filament curvature is broken and a sharp kink arises in the AH contour. At high loads the doubly kinked filaments may appear. The bending reversibility up to the point of the kink emergence is confirmed. Simulations identify significant limitations of the popular worm‐like chain model in representing the strongly curved filaments. The critical curvatures at the emergence of the doubly kinked structures in helical minicircles of AH and dsDNA are calculated. It is argued that an inclination for helix kinking, and the associated dual filament stiffness, represents a unique attribute of the compression mechanics in helical polymers. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1345–1357     

Struktur/wirkung-Beziehungen bei Geruchsstoffen mit dem Bicyclo[2.2.2.]octan-Gerüst

Structure-Activity Relationships of Odorants with a Bicyclo[2.2.2]octane System The synthesis of the olfactory interesting homonojigikualcohol 2 and of its epimer 10 leads to the keto alcohols 6a, b and 7a, b as intermediates. The latter have OH and C?O groups (an AH/B system) fixed at the rigid bicyclic nucleus at different distances (2.8 and 4.7 Å) and confirm Ohloff's rule for odoriferous properties which is based on this distance.     

Effects of polyphenol removal methods on the in vitro exsheathment inhibitory activity of Lysiloma latisiliquum extracts against Haemonchus contortus larvae

It has been suggested that the anthelmintic (AH) activity of tannin-rich forages is related to their tannin content. However, reports on tropical fodders such as Lysiloma latisiliquum describe the same AH activity after the addition of tannin-blocking agents, suggesting that the activity either depends on the method of tannin blocking/removal or is due to other type of secondary metabolites. This study compared both the effect of the drying process of the plant material and the effect of different polyphenol removal methods on the AH activity of L. latisiliquum acetone-water extracts when tested against Haemonchus contortus. The results showed that the extraction of oven-dried (OD) leaves of L. latisiliquum yielded a CT-free extract with high AH activity. However, polyphenol-free fractions showed similar or lower AH activity levels as of those original OD extract. HPLC analysis confirmed that common polyphenolic metabolites are not responsible for the AH activity of L. latisiliquum.     

Complexation of divalent metal ions with diols in the presence of anion auxiliary ligands: zinc‐induced oxidation of ethylene glycol to glycolaldehyde by consecutive hydride ion and proton shifts

Ternary complexes of the type AH???M²⁺???L^– (AH = diol, including diethylene and triethylene glycol, M = Ca, Mn, Fe, Co, Ni, Cu and Zn and auxiliary anion ligand L^– = CH₃COO^–, HCOO^– and Cl^–) have been generated in the gas phase by MALDI and ESI, and their dissociation characteristics have been obtained. Use of the auxiliary ligands enables the complexation of AH with the divalent metal ion without AH becoming deprotonated, although A^–???M²⁺ is often also generated in the ion source or after MS/MS. For M = Ca, dissociation occurs to AH + M²⁺???L^– and/or to A^–???M²⁺ + LH, the latter being produced from the H‐shifted isomer A^–???M²⁺???LH. For a given ligand L^–, the intensity ratio of these processes can be interpreted (barring reverse energy barriers) in terms of the quantity PA(A^–) – Ca_aff(A^–), where PA is the proton affinity and Ca_aff is the calcium ion affinity. Deuterium labeling shows that the complex ion HOCH₂CH₂OH???Zn²⁺???^–OOCCH₃, in addition to losing acetic acid (60 Da), also eliminates glycolaldehyde (HOCH₂CH=O, also 60 Da); it is proposed that these reactions commence with a hydride ion shift to produce the ion–dipole complex HOCH₂CHOH⁺??? HZnOOCCH₃, which then undergoes proton transfer and dissociation to HOCH₂CH=O + HZn⁺???O = C(OH)CH₃. In this reaction, ethylene glycol is oxidized by consecutive hydride ion and proton shifts. A minor process leads to loss of the isomeric species HOCH=CHOH. Copyright © 2012 John Wiley & Sons, Ltd.     

Determination of transition temperatures and crystalline content of linear,high-molecular-weight polyethylene by proton NMR spectroscopy

Decomposition of the proton NMR spectra of linear, high-molecular-weight polyethylene into a Gaussian function and a Lorentzian function is discussed in terms of the Wilson-Pake two-component method for an inhomogeneous system. The results of a Goldman-Shen experiment indicate that spin diffusion does not strongly couple the two regions. The temperature dependence of the Lorentzian (mobile) fraction is fitted to a function derived from the relation of English and Zoller. The critical temperatures give estimates of thermal transitions: T_α = 328 (± 5) K and T_γ = 163 (± 4) K. The fraction of Lorentzian spins excited by the γ relaxation correlates with the amorphous content of the sample as determined by differential scanning calorimetry and x-ray diffraction.     

Vibrational relaxation and νs(AH) absorption band profiles of weak hydrogen-bonded complexes in liquids

The ν _>(AH) IR absorption band profile of a weak hydrogen-bonded complex AH … B embedded in liquid is calculated within the two-mode (ν _>(AH)) and ν_AB coupling scheme. The generalized Langevin equation is invoked to describe the dynamics of the low-frequency (harmonic) ν_AB oscillator influenced by the random force from surrounding molecules. The main goal is to take account of not very short correlation times τ_c of this force, which is likely to be the case in real liquids. The non- zero τ_c value results in an appearance of a purely dissipative mode when A and B approach each other. This is shown to be responsible for the ν _>(AH) bandshaping in the hydrogen-bonded species exhibiting ν _>(AH) band halfwidths smaller than or of the order of the ν_AB vibration frequency. As for the ν_AB absorption band profile, it is formed by the damped oscillatory A→ ← B motional mode. The relationships connecting the characteristics of the ν _>(AH) and ν_AB bandshapes are obtained. It is shown that such entities as the mean-squared A … B length dispersion in solution, the mean-squared fluctuating force acting on the ν_AB oscillator and its correlation time τ_c can be estimated from the joint analysis of the experimental ν _>(AH) and ν_AB band profiles. Comparison with the experimental data for phenol(OD)-pyridine in CCl₄ yields τ_c = (O.6–1.0)x10^?2 s.     

Do condensed tannins from Lysiloma latisiliquum have a role in its anthelmintic activity?

We would like to comment on a recent paper by Hernandez-Bolio et al. (2017, Nat. Prod. Res. DOI: 10.1080/14786419.2017.1317774) where an extract free of condensed tannin (CT) was evaluated and concluded that Lysiloma latisiliquum CT does not have a role in its anthelmintic (AH) activity. This letter discusses the AH potential of L. latisiliquum based on previously published evidence of a partial role of CT using different AH assays and Haemonchus contortus isolates. Because the experimental design employed in the above-mentioned paper did not allow a clear-cut conclusion regarding the role of CT, this suggests that a specific experiment must be designed to confirm or reject the involvement of CT fraction(s) on L. latisiliquum AH activity.     

Conformational Analysis and Selection of Odor‐Active Conformers: Synthesis of Molecules Designed for the Lily‐of‐the‐Valley(Muguet)‐Type Odor

The selection of odor‐active conformers and the construction of a model for a targeted odor type, i.e., for the lily‐of‐the‐valley odor, were examined. The disagreement of the odors of 1,3,4,5‐tetrahydro‐2‐benzoxepin derivative 1 and 3‐[4‐(tert‐butyl)phenyl]‐2‐methylpropanal ( 2 ) is discussed in terms of their stable conformers. The conformer active for the lily‐of‐the‐valley odor was investigated by conformational analyses of several related compounds. Based on the integrated model consisting of the assumed active conformers (Fig. 5), compounds anticipated to possess the lily‐of‐the‐valley odor were designed and synthesized. The odor of synthetic 7‐(tert‐butyl)‐1,2,4,5‐tetrahydro‐3H‐benzocyclohepten‐3‐one ( 8 ) and 3‐[4‐(tert‐butyl)phenyl]cyclopentanone ( 13 ) were evaluated by perfumers to have a floral odor and to recall the lily‐of‐the‐valley and lilac odors, respectively. Our methodology to design new odoriferous compounds, based on conformational analysis, selection of odor‐active conformers, and construction of a model, proved to be satisfactory.     

Metal complexes with macrocyclic ligands. Part XXXIV. Geometrical and conformational changes during the on/off reaction of the side chain in the CuII complex of 11-(3-aminopropyl)-1,4,7,11-tetraazacyclotetradecane

Solution studies of the Cu²⁺ complex with 11-(3-aminopropyl)-1,4,7,11-tetraazacyclotetradecane(L) indicate that, depending on the pH and on the age of the solution, different species are present. Dissolving the solid [CuL](ClO₄)₂ in slightly acidic solution gives the protonated complex AH , characterized by an absorption maximum at 574 nm, by a relatively fast proton-induced dissociation kinetics and by the typical colour change in basic solution to give the deprolonated form A with coordinated side chain. AH slowly interconverts in acidic solution to a new species BH , which has an absorption maximum at 547 nm, and which is kineticaily more stable against acid dissociation and shows no coordination of the amino group of the side chain. In alkaline solution, however, the deprotonated form B deliver A in a base induced reaction. The X-ray diffraction studies of A and BH allow to determine the geometry of the metal ion and the configuration of the macrocycle. In A , the Cu²⁺ is pentacoordinated by the five N-atoms of the ligand and the macrocycle is in the RRSR configuration, whereas in BH the Cu²⁺ is octahedrally coordinated by the four N-atoms of the macrocycle and two axial perchlorate O-atoms with the macrocycle in the RRRS configuration. The amino group of the side chain is protonated and not coordinated. Thus, the on/off equilibrium of the side chain not only changes the geometry of the metal ion, as is generally found, but also alters the macrocyclic moiety.     

Geminate proton recombination at the surface of SDS and CTAC micelles probed with a micelle-anchored anthocyanin

The functionalized flavylium salt 6-hexyl-7-hydroxy-4-methyflavylium chloride (HHMF) was employed to probe some of the fundamental features of proton transfer reactions at the surface of anionic sodium dodecyl sulfate (SDS) and cationic hexadecyltrimethylammonium chloride (CTAC) micelles. In contrast to most ordinary flavylium salts, HHMF is insoluble in water, but readily incorporates into SDS and CTAC micelles. In the ground state, the rate constant for deprotonation of the acid form (AH+) of HHMF decreases 100-fold upon going from CTAC (kd = 3.0 x 10(6) s(-1)) to SDS (kd = 1.4 x 10(4) s(-1)), consistent with the presence of an activation barrier for proton transfer in the ground state and reflecting, respectively, stabilization or destabilization of the AH+ cation by the micelle. Reprotonation of A is diffusion-controlled in both micelles (kp(SDS) = (2.1 x 10(11))[H+]aq s(-1) and kp(CTAC) = (3.7 x 10(8))[H+]aq s(-1)), the difference reflecting the rate of proton entry into the micelles. In the excited singlet state, the rate constants for deprotonation of the AH+* form of HHMF are similar in the two micelles (2.4 x 10(10) s(-1)), consistent with activationless proton transfer. Reprotonation of the excited A is dominated by fast geminate recombination of the photogenerated (A*-H+) pair at the micelle surface (k(rec)(SDS) = 6.1 x 10(9) s(-1) and k(rec)(CTAC) = 3.4 x 10(10) s(-1)) and the net efficiencies of geminate recombination are quite similar in SDS (0.89) and CTAC (0.86).     

Synthesis and Properties of Conformationally Constrained Analogues of Floral‐Type Odorants

The twelve bridged analogues 8 – 19 of floral‐type odorants related to cyclamenaldehyde ( 1 ) were synthesized (Schemes 1–5) to investigate the relationship between the structural and conformational features of these compounds and their odor properties. Comparison of the data from sensory evaluation and molecular modeling suggests that the side chain of both the unconstrained and the constrained active analogues is not extended (anti) but rather folded (gauche) in the ‘bioactive’ conformation. However, it is mainly the nature of the substituents at the α position of the aldehyde function that critically influences the odor quality and strength. These studies provide new information that should aid ongoing efforts to develop models of odorant? receptor interactions.     

Rotating frame proton spin-lattice relaxation measurements of poly(ethylene oxides)

Measurements of T_1ρ as a function of temperature have been made on three polyethylene oxides (PEO) with molecular weights of 550, 6000, and 2.8 × 10⁶ in order to try to elucidate various problems arising in the interpretation of previous T₁ measurements on PEO. In contrast to the T₁ measurements, the T_1ρ measurements show discontinuities at the melting or softening points of the respective polymers concerned, and also show nonexponential magnetization decay in the case of PEO 6000 and 2.8 × 10⁶, which is presumbly due to the existence of “mobile” and “crystalline” regions, in qualitative agreement with NMR studies and x-ray measurements. Motional correlation frequencies and activation energies have been derived where possible by using the BPP theory adapted to the rotating frame and also the strong collision Slichter-Ailion theory. There is reasonable correlation with other data on motional frequencies where it is available, although the overall situation for the so-called α transition in PEO 6000 and 2.8 × 10⁶ is still not clear. It is suggested that spin-diffusion is an important mechanism for all three polymers; for PEO 550 because it contains CH₃ endgroups which act as sinks, and for the other two polymers where the mobile fraction performs the same function.     

基于异噁唑酮类份菁染料的激发态分子内质子转移的研究

运用量子化学理论计算方法研究了3-甲基-4-(1H-吲哚-3-次甲基)-异噁唑-5-酮(A)及其衍生物份菁染料的激发态分子内质子转移性质.研究表明:在基态3种染料AH(R=H),AO(R=—O(H3))和AP(R=—O(H2Ph))只存在酮式构型,在激发态AH与AP存在酮式和烯醇式2种构型,而AO存在酮式、烯醇式和仲胺式3种构型.红外光谱表明化合物从基态跃迁到激发态存在分子内的氢键增强作用,势能曲线显示激发态的质子转移为放热反应且能垒较低,通过分析电子光谱得到具有较大斯托克位移的激发态分子内质子转移的荧光发射峰,前线分子轨道理论计算进一步说明了其质子转移的发生过程.     

Rotating frame proton spin-lattice relaxation measurements of poly (ethylene oxides)

Measurements of T_lρ as a function of temperature have been made on two polyethylene oxides (PEO) with molecular masses of 5,000 and 30,000. The T_1ρ measurements show biexponential behavior of the relaxation function in the temperature range from 170 K to 350 K. The intensities of the components of the relaxation function are constant over this temperature range in agreement with the crystallinities of the samples. The two relaxation times can be associated with the crystalline and amorphous component; the relaxation time minima describe the α relaxation in the crystalline regions of PEO and the glass transition in amorphous PEO.