Calculations of 13C NMR chemical shifts and F–C coupling constants of ciprofloxacin

Ciprofloxacin is a widely used fluoroquinolone antibiotic. In this work, a comprehensive evaluation of MP2 and DFT with different functionals and basis sets was carried out to select the most suitable level of theory for the study of the NMR properties of ciprofloxacin. Their relative predictive capabilities were evaluated comparing the theoretically predicted and experimental spectral data. Our computational results indicated that in contrast to the solid state, the molecule of ciprofloxacin does not exist as a zwitterion in gaseous state. The results of the calculations of the chemical shifts most close to the experimental were obtained with B3LYP/aug-cc-pVDZ. The F–C coupling constants were calculated systematically with different DFT methods and several basis sets. In general, the calculations of the coupling constants with the BHandH computational method including the applied in this work 6-311++G**, EPRII, and EPRIII basis sets showed a good reproducibility of the experimental values of the coupling constants.     

Synthesis of the C1–C13 fragment of eribulin mesylate

Synthesis of the C1–C13 fragment of eribulin mesylate has been accomplished. It features a highly stereoselective construction of a trans-dihydropyran framework using three key reactions: (1) Sharpless epoxidation, (2) regioselective ring opening, and (3) olefin metathesis.     

Can polarization of triple bond in tolanes be deduced from 13C NMR shifts? Re-evaluation of factors affecting regiochemistry of the palladium-catalyzed hydrostannation of alkynes

Polarization of the triple bond in a series of differently substituted ortho- and para-tolanes has been studied by NMR and computational methods in order to examine if (13)C NMR data can be effectively used for the assessment of electronic polarization of a triple bond in diarylacetylenes. DFT calculations of both natural charges and NMR properties revealed that chemical shifts concur with effective charges on sp-carbon atoms in para-tolanes, whereas in ortho-analogues magnetic anisotropy complicates the analysis making (13)C NMR data inapplicable for ascribing triple bond polarization. The obtained information was used to reevaluate factors affecting the regiochemistry of the Pd-catalyzed hydrostannation of the triple bond in tolanes. Computational study on the polarization of triple bonds taken together with the experimental data on hydrostannation of various mono- and disubstituted tolanes bearing para- and ortho-substituents demonstrated that the regioselectivity of hydrostannation is governed by a combination of electronic and steric factors. In para-tolanes, the electronic effect prevails and alpha- and beta-vinylstannanes are obtained predominantly for substrates with electron-withdrawing and electron-donating groups, respectively. In the ortho-series, steric factors dominate over electronics and alpha-isomers are produced with high selectivity regardless of the substituents' nature. However, it was found that in disubstituted "push-pull" tolanes steric control of an ortho-group can be overruled by the very strong electronic effect of an electron-withdrawing substituent in para-position.     

A new determination of the structure of water at 25°C

The results of a new neutron diffraction experiment to measure the structure of water are presented. The data, measured at the McMaster Nuclear Reactor, are of a high quality and are analysed to yield the hydrogen-hydrogen pair correlation function using a subtraction procedure which has been used in previous experiments of this kind. This procedure circumvents the necessity of applying inelasticity corrections. The results are in good agreement with earlier work and serve to establish the general correctness of the subtraction procedure when used to determine hydrogen correlations. The data are further analysed to yield separate oxygen-hydrogen and oxygen-oxygen partial structure factors for liquid water. For the second part of the analysis an effective mass model of the dynamic scattering law is used, with the model parameter, the effective mass of the scattering particle, chosen by a least-squares fit to the measured differential cross sections. The final pair correlation functions are obtained using a maximum entropy analysis of the structure functions.     

The carbon-13 chemical shifts and the analysis of the relaxation times T1 and long range 13C−1H coupling constants of quinoline and of 1-(X-quinolyl)ethyl acetate derivatives

The ¹³C chemical shifts and the ¹³C−¹H coupling constants of quinoline (1-(X-quinolyl)ethyl acetate derivatives (where X=−CH(OAc)CH₃ substituted at positions 2,4,5–8) are reported. Substituent chemical shift (SCS) effects for the ethyl acetate group are additive at all positions. A substantial upfield shift of 4.5 and 4.8 ppm was observed at C-4 and C-5, arising from the peri interaction of 5- and 4-ethyl acetate substituents respectively. A vicinal (peri) ³J CCCH coupling constant of approximately 5 Hz is observed between both C₅−H₄ and C₄−H₅. Carbon-13 relaxation times (T₁) and nuclear Overhauser enhancements (η) have been measured for quinoline and its derivatives, and the contributions of dipolar, T₁^DD, and spin rotation, T₁^SR, relaxation have been determined. Intramolecular dipole-dipole interactions are found to provide by far the most important spin-lattice relaxation mechanism whenever protons are bound directly to the carbons under investigation. Non-protonated ring carbons are relaxed by both DD and SR mechanisms. Anisotropic motion has an easily observable effect on the DD contribution to T₁, and can form the basis for spectral assignments, as in 1-phenylethyl acetate. Long-range ¹³C−¹H coupling constants were observed both between ring carbons and between ring carbons with ring side-chain hydrogens. These results have been used for the structure determination of the title compounds.     

Toward the Total Synthesis of Bryostatin 11: Stereoselective Construction of the C13-Exocyclic Enoate in the C1–C16 Fragment

We have utilized a spiroketal template in an approach to the C1–C16 fragment of bryostatin. The stereoselective construction of an exocyclic enoate at C13 and insertion of a vinyl group on C15 were accomplished by using Peterson–Yamamoto olefination and copper-catalyzed addition of vinyl magnesium bromide, respectively.      

Diffusion coefficients of the system α-cyclodextrin-n-butylurea-water at 25°C

Diffusion coefficients were measured for the ternary system -cyclodextrin(I)-n-butylurea(2)-water at three average concentrations. The cross coefficient D₁₂ was found to be almost zero and D₂₁ large and negative. These results are in agreement with the presence of an inclusion complex whose mobility is close to that of the host cyclodextrin molecules. The values of the four experimental diffusion coefficients are used to compute a value of the inclusion constant which is in reasonably good agreement with the calorimetric value.     

13C Chemical shifts and 13C?15N coupling constants of some 15N-labelled bridgehead nitro compounds

Measurement of the ¹³C NMR spectra of the bridgehead nitro compounds 1a–5a has been performed. It is found that one-bond ¹³C ¹⁵N coupling is not necessarily a reflection of the degree of s character of the bridgehead carbon exocyclic bonding orbital. Although the magnitude of vicinal coupling to the bridgehead carbon increases, in accordance with the number of three-bond pathways available for transmission of spin information, coupling is actually reduced by opposing contributions from through-space intramolecular orbital interactions.     

Determination of stability constant of ternary surface complexes at liquid-solid interface in sea water by the rule of left-right shifts of S-shaped curve

A new method is suggested for determining the stability constant of ternary surface complexes (TSC) at liquid-solid interfaces in natural water. Its basic principle is based on the rule of left-right shift (RLRS) of S-shaped curve, from which the peak-type curve of E(%) with the concentration of organic matter is obtained. The peak-type curve is further used to obtain the (ksAM/kSM) values by using respectively the method of equality of E(%) when [H2A] = [H2A]0 (1) and the method of [H2A]max (2). From the known KSM,the stability constant KSAM of liquid-solid interfacial ternary surface complexes can be obtained.     

An Approach towards the Synthesis of the Spiroimine Fragment of 13-Desmethylspirolide C and Gymnodimine A

A general access to the spiroimine skeleton of gymnodimine and spirolides is described, relying on the construction of the cyclohexene fragment using an enantiocontrolled Diels–Alder reaction, the installation of the all-carbon quaternary stereocenter through a stereocontrolled alkylation or aldolisation and the elaboration of the lateral chains at C7 and C22 using Wittig–Horner olefinations. The spiroimine core of gymnodimine is made available through a 16-step linear sequence in a 21 % overall yield.     

Are accurate computations of the 13C′ shielding feasible at the DFT level of theory?

The goal of this study is twofold. First, to investigate the relative influence of the main structural factors affecting the computation of the ¹³C′ shielding, namely, the conformation of the residue itself and the next nearest‐neighbor effects. Second, to determine whether calculation of the ¹³C′ shielding at the density functional level of theory (DFT), with an accuracy similar to that of the ¹³C^α shielding, is feasible with the existing computational resources. The DFT calculations, carried out for a large number of possible conformations of the tripeptide Ac‐G XY ‐NMe, with different combinations of X and Y residues, enable us to conclude that the accurate computation of the ¹³C′ shielding for a given residue X depends on the: (i) (?,ψ) backbone torsional angles of X ; (ii) side‐chain conformation of X ; (iii) (?,ψ) torsional angles of Y ; and (iv) identity of residue Y . Consequently, DFT‐based quantum mechanical calculations of the ¹³C′ shielding, with all these factors taken into account, are two orders of magnitude more CPU demanding than the computation, with similar accuracy, of the ¹³C^α shielding. Despite not considering the effect of the possible hydrogen bond interaction of the carbonyl oxygen, this work contributes to our general understanding of the main structural factors affecting the accurate computation of the ¹³C′ shielding in proteins and may spur significant progress in effort to develop new validation methods for protein structures. © 2013 Wiley Periodicals, Inc.     

Determination of the geometry of acetoxyendiynes and acetoxyenynes by NMR heteronuclear 13C?1H scalar couplings and 13C NMR chemical shifts. Structural assignment of the oxylipin natural products peyssonenynes A and B

Solution NMR methods were used for the structural characterization of the acetoxyendiyne E/Z configuration of the marine natural products peyssonenynes A and B and their synthetic analogs derived from palmitic acid. The scarcity of protons in the proximity of the olefin precluded the determination of the double bond geometry using ¹H NMR methods that rely on proton–proton scalar couplings or experiments such as NOESY or ROESY. Long range ¹H? ¹³C heteronuclear scalar couplings, ⁿJ_CH, measured with the 2D excitation sculptured indirect detection experiment (EXSIDE) proved useful and highly reliable for the analysis of the enol acetate geometry. In addition, it was found that the chemical shift of some carbon atoms in the proximity of the olefin was also sensitive to the double bond configuration of these molecules providing an even simpler way to determine their geometry. This protocol showed its robustness by similar analysis of simpler silyl‐protected acetoxyenynes derived from fatty acids. These NMR experimental results and stereochemical predictions were rationalized by DFT calculations. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of pressure on the solubility of naphthalene in water at 25°C

Solubility of naphthalene in water was measured at 25°C and pressures up to 200 MPa. The solubility decreased with increasing pressure. From the pressure coefficient of the solubility, the volume change V accompanying the dissolution was estimated as 13.8±0.4 cm ³ -mol ^–1 . Further we estimated the volume change V _CH accompanying hydrophobic hydration as –0.1±0.6 cm ³ -mol ^–1 using the V value, the molar volume of crystalline naphthalene, and the partial molar volume of naphthalene in n-heptane. This V _CH is much larger (i.e., less negative) than that for hydrophobic hydration of alkyl-chain compounds and suggests that the hydration structure of naphthalene differs from that of alkyl-chain compounds.     

Challenges in measuring the δ13C of the soil surface CO2 efflux

The δ13C of the soil surface efflux of carbon dioxide (δ13CRS) has emerged as a powerful tool enabling investigation of a wide range of soil processes from characterising entire ecosystem respiration to detailed compound-specific isotope studies. δ13CRS can be used to trace assimilated carbon transfer below ground and to partition the overall surface efflux into heterotrophic and autotrophic components. Despite this wide range of applications no consensus currently exists on the most appropriate method of sampling this surface efflux of CO2 in order to measure δ13CRS. Here we consider and compare the methods which have been used, and examine the pitfalls. We also consider a number of analysis options, isotope ratio mass spectrometry (IRMS), tuneable diode laser spectroscopy (TDLS) and cavity ring-down laser spectroscopy (CRDS). δ13CRS is typically measured using chamber systems, which fall into three types: closed, open and dynamic. All are imperfect. Closed chambers often rely on Keeling plots to estimate δ13CRS, which may not be appropriate without free turbulent air mixing. Open chambers have the advantage of being able to maintain steady-state conditions but analytical errors may become limiting with low efflux rates. Dynamic chambers like open chambers are complex, and controlling pressure fluctuations caused by air movement is a key concern. Both open and dynamic chambers in conjunction with field portable TDLS and CRDS analysis systems have opened up the possibility of measuring δ13CRS in real time permitting new research opportunities and are on balance the most suited to this type of measurement.     

Thermodynamical study of the zinc selenate-cobalt selenate-water system at 25°C

Summary The solubility in the ZnSeO₄-CoSeO₄-H₂O system has been studied at 25°C. It has been established that the CoSeO₄·6H₂O monoclinic crystal structure is unstable and undergoes a change into a tetragonal structure due to the included zinc ions. Activity coefficients of the salt components in the saturated solutions and in the mixed crystal hexahydrates have been determined by the isopiestic method. G ^F of (Zn_xCo_1–x)SeO₄·6H₂O mixed crystals shows a negative deviation from ideality.

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Thermodynamische Untersuchung des Systems Zinkselenat-Kobaltselenat-Wasser bei 25°C

Zusammenfassung Die Löslichkeitsisotherme des Systems ZnSeO₄-CoSeO₄-H₂O wurde bei 25°C untersucht. Es wird festgestellt, daß das monokline CoSeO₄·6H₂O unstabil ist und in tetragonale Mischkristalle übergeht. Mittels der isopiestischen Methode wurden die Aktivitätskoeffizienten der Komponenten in den gesättigten Dreistofflösungen und Mischkristallen bestimmt. G ^F der Mischkristallbildung weist eine negative Abweichung vom idealen Zustand auf.

     

Correlation of the 13C13C spin—spin coupling constants with the stretching force constants of single and double carbon—carbon bonds

Linear correlations between the spin—spin carbon—carbon coupling constants and the carbon—carbon stretching force constants for single and double bonds have been found through analysis of the available literature data. The corresponding equations are K^s_CC = 0.0344 J^s_CC+3.25 and K^d_CC = 0.180 J^d_CC−3.25 for single and double bonds respectively.     

13C Nmr Spectra of Methyl Deoxyfluoro-β-D-Galactopyranosides and Their Per-O-Acetyl Derivatives

Abstract

Methyl 6-deoxy-6-fluoro-β-d-galactopyranoside has been obtained by treatment of methyl β-d-galactopyranoside with diethyl-aminosulfur trifluoride (DAST). Improvements over the existing syntheses of methyl 2, 3-di-O-benzyl-4-deoxy-4-fluoro-β-d-galacto-pyranoside from the corresponding 6-O-substituted 4-O-arylsul-fonyl-d-gluco derivatives are described. ¹³C NMR spectra of a series of methyl deoxyfluoro-β-d-galactopyranosides and their per-O-acetyl derivatives have been measured. The data obtained can be used as an aid for the interpretation of ¹³C NMR spectra of deoxyfluoro-β-d-galactopyranose-containing oligosaccharides and related substances.     

Sheet-like assemblies of charged amphiphilic α/β-peptides at the air-water interface

There is growing interest in the design of molecules that undergo predictable self-assembly. Bioinspired oligomers with well-defined conformational propensities are attractive from this perspective, since they can be constructed from diverse building blocks, and self-assembly can be directed by the identities and sequence of the subunits. Here we describe the structure of monolayers formed at the air-water interface by amphiphilic α/β-peptides with 1:1 alternation of α- and β-amino acid residues along the backbone. Two of the α/β-peptides, one a dianion and the other a dication, were used to determine differences between self-assemblies of the net negatively and positively charged oligomers. Two additional α/β-peptides, both zwitterionic, were designed to favor assembly in a 1:1 molar ratio mixture with parallel orientation of neighboring strands. Monolayers formed by these α/β-peptides at the air-water interface were characterized by surface pressure-area isotherms, grazing incidence X-ray diffraction (GIXD), atomic force microscopy and ATR-FTIR. GIXD data indicate that the α/β-peptide assemblies exhibited diffraction features similar to those of β-sheet-forming α-peptides. The diffraction data allowed the construction of a detailed model of an antiparallel α/β-peptide sheet with a unique pleated structure. One of the α/β-peptide assemblies displayed high stability, unparalleled among previously studied assemblies of α-peptides. ATR-FTIR data suggest that the 1:1 mixture of zwitterionic α/β-peptides assembled in a parallel arrangement resembling that of a typical parallel β-sheet secondary structure formed by α-peptides. This study establishes guidelines for design of amphiphilic α/β-peptides that assemble in a predictable manner at an air-water interface, with control of interstrand orientation through manipulation of Coulombic interactions along the backbone.