Selective molecular recognition of arginine by anionic salt bridge formation with bis-phosphate crown ethers: implications for gas phase peptide acidity from adduct dissociation

Arginine forms a stable noncovalent anionic salt bridge complex with DP (a crown ether which contains two endocyclic dialkylhydrogenphosphate esters). Abundant adduct formation with DP is observed for complexes with arginine, YAKR, HPPGFSPFR, AAKRKAA, RR, RPPGFSPFR, RYLGYL, RGDS, and YGGFMRGL in electrospray ionization mass spectrometry (ESI-MS) experiments. DFT calculations predict a hydrogen bonded salt bridge structure with a protonated guanidinium flanked by two deprotonated phosphates to be the lowest energy structure. Dissociation of DP/peptide adducts reveals that, in general, the relative gas phase acidity of a peptide is dependent on peptide length, with longer peptides being more acidic. In particular, peptides that are six residues or more in length can stabilize the deprotonated C-terminus by extensive hydrogen bonding with the peptide backbone. Dissociation of DP/peptide complexes often yields the deprotonated peptide, allowing for the facile formation of anionic peptides that otherwise would be difficult to generate in high abundance. Although DP has a preference for binding to arginine residues in peptides, DP is also observed to form less abundant complexes with peptides containing multiple lysines. Lys-Xxx-Lys and Lys-Lys sequences form low abundance anionic adducts with DP. For example, KKKK exclusively forms a double adduct with one net negative charge on the complex.     

Rare example of nucleophilic substitution at vinylic carbon with inversion: mechanism of methyleneaziridine formation by sodium amide induced ring closure revisited

Intramolecular nucleophilic substitution of the C-Br bond of (E)- and (Z)-2-bromobut-2-enylamines by the pendant nitrogen atom leads to 2-ethyleneaziridines by way of stereochemical inversion at the vinylic carbon atom. The stereochemistry of the products is unambiguously established by X-ray crystallography performed on two derivatives. These cyclizations represent some of the first examples of substitution with inversion in unactivated vinylic substrates. In conjunction with additional deuterium-labeling experiments, the accepted mechanism for this reaction is shown to be flawed.     

Cross-linked DNA generated by "bis-click" reactions with bis-functional azides: site independent ligation of oligonucleotides via nucleobase alkynyl chains

Template-free cross-linking of single-stranded DNA bearing octadiynyl side chains at the 7-position of 8-aza-7-deazapurine moieties with bisfunctional azides is reported employing a Cu(I)-catalyzed azide-alkyne "bis-click" reaction. Bis-adducts were formed when the bis-azide:oligonucleotide ratio was 1:1; monofunctionalization occurred when the ratio was 15:1. Four-stranded DNA consisting of two cross-linked duplexes was obtained after hydridization. Cross-linked duplexes are as stable as individual duplexes when ligation was introduced at terminal positions; ligation at a central position led to a slight duplex destabilization.     

冠醚配合物热力学性质的研究 X: 用NMR研究15-冠-5-衍生物与钠离子的配位反应

用^2^3Na NMR研究了15-冠-5(15C5), 2,3-苯并-15-冠-5(B-15C5), 2,3-苯并-6-甲基-15-冠-5(BCl-15C5), 2,3-苯并-8,12-二甲基-15-冠-5(BC2-15C5)在丙酮中与Na^+的配位反应, 由此探索甲基的影响, 实验结果表明, 当在金属盐溶液中加入冠醚, 配体为15C5时, 体系表观化学位移移至低场, 配体为B-15C5, BC1-15C5, BC2-15C5时, 体系在表观化学位移移至高场, 后者与苯并-18-冠-6及其甲基衍生物不同, 这是由于苯环在空间排布不同所致, 用Newton-Raphson法和描坑法(Pit-mapping)拟合实验数据, 发现BC1-15C5和BC2-15C5除形成NaL^+和NaL2^+外还有不稳定的Na3L2^3^+形成。本文对形成过程和生成NaL^+反应的平衡常数lgK11与甲基数的关系亦作了讨论, lgK11与甲基数目n近似成直线关系, 可用下式表示: lgK11=3.50-0.80n, n=0,1,2。     

Bis[(benzo-18-crown-6)-4,5-ylmethyl]-1,3,4,6-diphenylglycoluril: The first representative of a new class of bis(crown ethers) and the synthesis and crystal structure of its complex with sodium picrate

A statistical Monte Carlo computer simulation has shown that the use of a diphenylglycoluril fragment as a rigid linker should give bis(crown ethers) with identically oriented benzocrown rings located closely in space. The syntheses of the first representative of this series, bis[(benzo-18-crown-6)-4,5-ylmethyl]-1,3,4,6-diphenylglycoluril and its complex with sodium picrate have been described. The crystal structure of the complex has been determined by X-ray diffraction: space group $P\bar 1$ , a = 13.976(3) Å, b = 16.723(3) Å, c = 20.578(4) Å, α = 98.74(2)°, β = 106.19(3)°, γ = 114.07(3)°. The complex represents a new type of coordination polymer formed by a combination of coordination and hydrogen bonds and stacking, which are favored by the U-shaped conformation of diphenylglycoluril.     

Thermochemical behaviour of crown ethers in the mixtures of water with organic solvents: Part VII. Enthalpy of solution of 15-crown-5 and benzo-15-crown-5 in the mixtures of water with acetone at 298.15 K

Enthalpy of solution of crown ethers (15-crown-5 and benzo-15-crown-5) in water-acetone mixtures have been measured within the whole range of mole fraction at 298.15 K. The obtained data have been compared with those of the solution enthalpy of both crown ethers in the mixtures of water with dimethyl sulfoxide. The replacement of SO group with CO in the molecule of the organic solvent brings about an increase in the exothermic effect of the solution of 15-crown-5 and benzo-15-crown-5 ethers, especially in the mixtures with a medium water content. The observed effect is connected with the preferential solvation of the molecules of both crown ethers by acetone molecules in the water-acetone mixtures. The process of preferential solvation of 15-crown-5 and benzo-15-crown-5 ethers does not take place in the water-dimethyl sulfoxide mixture.     

The effect of carbonyl carbon atom replacement in acetone molecule (ACN) by sulfur atom (DMSO): Part II. Thermodynamic functions of complex formation of crown ethers with Na<Superscript>+</Superscript> in mixed solvents

The thermodynamic functions of complex formation of benzo-15-crown-5 ether (B15C5) and sodium cation (Na⁺) in acetone–water mixtures at 298.15 K have been calculated. The equilibrium constants of B15C5/Na⁺ complex formation have been determined by conductivity measurements. The enthalpic effect of complex formation has been measured by the calorimetric method. The complexes are enthalpy-stabilized but entropy-destabilized in acetone–water mixtures. The effects of hydrophobic hydration, preferential solvation of B15C5 by a molecule of water and acetone, respectively and the solvation of Na⁺ on the complex formation processes have been discussed. The calculated thermodynamic functions of B15C5/Na⁺ complex formation and the effect of benzene ring on the complex formation have been compared with analogous data obtained in dimethylsulfoxide–water mixtures. The effect of carbonyl atom replacement in acetone molecule by sulphur atom (DMSO molecule) on the thermodynamic functions of complex formation has been analysed.     

Synthesis of the mixed lithium-potassium-(bis)magnesium N-metallated/N,C-dimetallated amide [Li2K2Mg4[But(Me3Si)N]4[But[Me2(H2C)Si]N]4]: an inverse crown molecule with an atomless cavity

Unlike previous members of the inverse crown family, which are heterobimetallic and have cationic rings surrounding anionic cores, the title compound is heterotrimetallic and its "guest" anion is intramolecularly stitched into the complex fused-ring structure of its cationic "host".     

Chemo-, stereo- and regioselective hydrogenolysis of carbohydrate benzylidene acetals. Synthesis of benzyl ethers of benzyl α-d-, methyl β-D-mannopyranosides and benzyl α-D-rhamnopyranoside by ring cleavage of benzylidene derivatives with the LiAlH4-AlCl3 reagent

Treatment of benzyl α-(1) and methyl β-d-mannopyranoside (2) with α,α-dimethoxytoluene gave the exo and endo isomers (3,5 and 4,6) of the dibenzylidene derivatives of 1 and 2. Hydrogenolysis of the exo isomers (3 and 5) with a molar equivalent of AlH₂Cl gave the 3-0-benzyl-4,6-0-benzylidene derivatives (7 and 21), whereas the endo isomers (4 and 6) gave the 2-0-benzyl-4,6-0-benzylidene compounds (8 and 22). The 2-0-allyl ether 9 of 7, the 3-0-allyl derivative (10) of 8 and compounds 21 and 22 were treated with an additional molar equivalent of AlH₂Cl at reflux and the products were the 4-0-benzyl-6-hydroxyl derivatives (11, 12, 23 and 24), whereas in the case of 22 the 6-0-benzyl-4-hydroxyl isomer (25) was also isolated. By deallylation of 11 and 12, 3,4-(13) and 2,4-di-0-benzyl (14) ethers of 1 were prepared. Tosylation of 11 and 12, and subsequent reduction of the products (15 and 16) made possible the preparation of the partially protected benzyl α-d-rhamnopyranoside derivatives (17–20). The structures of the compounds synthesized were characterized by ¹H and ¹³C NMR spectroscopic investigation and by chemical methods.