Long-range Correlation Experiments Research Articles

Investigation of the Complexation Activity of 2,4-Dithiouracil with Au(III) and Cu(II) and Biological Activity of the Newly Formed Complexes

The goal of this study is to synthesize, determine the structure, and examine the antimicrobial properties of novel Cu(II) and Au(III) complexes of 2,4-dithiouracil and its derivatives. These complexes were obtained by mixing aqueous solutions of the corresponding metal salts with the ligand dissolved in DMSO and aqueous NaOH, using a metal-to-ligand ratio of 1:4:2. The structures of the new compounds were analyzed by melting point determination, microwave plasma atomic emission spectrometry (MP-AES) for Cu and Au, inductively coupled plasma optical emission spectrometry (ICP-OES) for S, attenuated total reflection (ATR), solution and solid-state NMR, and Raman spectroscopy. The data for 2,4-dithiouracil obtained from the 1H NMR, 13C NMR, distortionless enhancement by polarization transfer spectrum (DEPT-135), proton–proton homonuclear correlation spectrum (1H-1H COSY), long-range 1H-13C heteronuclear multiple bond correlation experiment (HMBC), and heteronuclear single quantum coherence spectra (HSQC) aided the interpretation of the NMR data for the gold and copper complexes. Furthermore, the antimicrobial effect of the free ligands and their complexes was assessed against Gram-positive and Gram-negative bacteria, as well as yeasts.

Computer Assisted Structure Elucidation (CASE): Current and future perspectives.

The first efforts for the development of methods for Computer-Assisted Structure Elucidation (CASE) were published more than 50years ago. CASE expert systems based on one-dimensional (1D) and two-dimensional (2D) Nuclear Magnetic Resonance (NMR) data have matured considerably by now. The structures of a great number of complex natural products have been elucidated and/or revised using such programs. In this article, we discuss the most likely directions in which CASE will evolve. We act on the premise that a synergistic interaction exists between CASE, new NMR experiments, and methods of computational chemistry, which are continuously being improved. The new developments in NMR experiments (long-range correlation experiments, pure-shift methods, coupling constants measurement and prediction, residual dipolar couplings [RDCs]), and residual chemical shift anisotropies [RCSAs], evolution of density functional theory (DFT), and machine learning algorithms will have an influence on CASE systems and vice versa. This is true also for new techniques for chemical analysis (Atomic Force Microscopy [AFM], "crystalline sponge" X-ray analysis, and micro-Electron Diffraction [micro-ED]), which will be used in combination with expert systems. We foresee that CASE will be utilized widely and become a routine tool for NMR spectroscopists and analysts in academic and industrial laboratories. We believe that the "golden age" of CASE is still in the future.

Simplifying LR-HSQC spectra using a triple-quantum filter: The LR-HTQC experiment.

Long-range heteronuclear single quantum correlation (LR-HSQC) experiments may be applied for detecting long-range correlations but suffer from two disadvantages, common to all heteronuclear long-range correlation experiments: (i) The information density in LR-HSQC spectra may be too high to be used directly without "filtering out" shorter range correlations, and (ii) often, substantial differences in intensity among cross peaks exist, potentially hampering the visualization of weak, often crucial cross peaks. In this contribution, we propose a modified LR-HSQC experiment, the LR-HTQC experiment (Long-Range Heteronuclear Triple Quantum Correlation) that partially solves the problems aforementioned. We show theoretically and experimentally that the LR-HTQC experiment removes the intense cross peaks of CH spin pairs, substantially reduces the medium intensity of cross peaks originating from CHH' spin systems, whereas the typically weak intensity of cross peaks of CHH'H″ and C(H)n, n > 3 spin systems is less affected. Consequently, the LR-HTQC experiment affords simplified long-range heteronuclear shift correlation spectra and scales down large intensity differences among different types of cross peaks, although a certain general reduction of signal intensities has to be accepted.

Why is HMBC superior to LR-HSQC? Influence of homonuclear couplings JHH' on the intensity of long-range correlations.

Long-range heteronuclear single quantum correlation (LR-HSQC) experiments may be applied as an alternative to heteronuclear multiple-bond correlation (HMBC) experiments for detecting long-range correlations but has never enjoyed popularity for that purpose. To the best of our knowledge, the exact reasons have not yet been fully established. For both experiments, it is widely accepted that the evolution of proton-proton homonuclear couplings JHH' during the polarization transfer delays Δ leads to significant losses, and that the intensity of the observable coherence is zero when JHH' matches the condition Δ=0.5/JHH' . Here, we analyze the influence of JHH' on the intensity of long-range correlations in HMBC and LR-HSQC spectra. We show that for both experiments long-range correlations will not be canceled because of homonuclear couplings JHH' . Our theoretical and experimental results definitely establish and validate the superiority of HMBC-based experiments among the family of heteronuclear long-range correlation experiments: (a) the overall cross peak's intensity is higher, and (b) in LR-HSQC experiments, the intensity of the long-range cross peaks is additionally influenced in an unwanted way by the magnitude and number of passive homonuclear proton-proton couplings JHH' .

29Si–1H IMPACT HMBC: a suitable tool for analyzing silylated derivatives

A modified version of the IMPACT heteronuclear multiple bond correlation (HMBC) has allowed the characterization of an organosilane and a tetrasilylated yttrium complex. With the help of this sequence, an average gain in sensitivity close to 2 has been obtained compared with the standard HMBC experiment for disilanes as well as for yttrium complexes containing silylated ligands. This modified version of this long-range correlation experiment opens the way for following kinetics in the range of a fraction of a minute and to study by NMR low concentrated samples and low abundant nuclei.

Revision of the Structure of Escholidine

The structure of the quaternary tetrahydroprotoberberine alkaloid escholidine is revised on the basis of 2D NMR spectroscopy and X-ray diffraction. In contrast to the originally reported constitution, escholidine bears an -OH group at C-9 and an -OCH3 group at C-10. The 1H and 13C NMR data and long-range 1H-13C and NOE interactions of escholidine are compared with those of thalifendine and tetrahydroberberrubine. The 15N NMR data of escholidine obtained by using long-range 1H-15N correlation experiments at natural abundance are also reported.

Complete1H and13C NMR spectral assignment ofN-aralkylsulfonamides,N-sulfonyl-1,2,3,4-tetrahydroisoquinolines andN-sulfonyl-2,3,4,5-tetrahydro-1H-2-benzazepines. Conformational analysis ofN-[((3′,4′-dichlorophenyl)methyl)sulfonyl]-3-methyl-2,3,4,5-tetrahydro-1H-2-benzazepin

The complete and unambiguous assignment of the 1H NMR and 13C NMR spectra of 26 N-aralkylsulfonamides, N-sulfonyl-1,2,3,4-tetrahydroisoquinolines and N-sulfonylbenz[c]azepines was performed on the basis of APT, DEPT, homonuclear (gs-COSY) and 1H-detected heteronuclear one-bond (gs-HMQC) and long-range (gs-HMBC) correlation experiments. The methylated 2,3,4,5-tetrahydro-1H-2-benzazepine derivative 26 adopts a chair conformation as determined by 1H-1H coupling analysis and gamma-gauche effects. This is supported by a single-crystal X-ray structure analysis.

Multinuclear (1H, 13C and 15N) Magnetic Resonance Spectroscopy and Substituent Effects on N-Phenoxyethylanilines

1H, 13C and 15N NMR spectra of twenty substituted N-phenoxyethylanilines 1-20 were completely and unambiguously assigned using a combination of both homo- and heteronuclear (gs-COSY), 1H detected heteronuclear one-bond (gs-HMQC) and long-range (gs- HMBC) gradient-selected correlation experiments. Correlations between chemical shifts and substituent constants are analyzed separately for both phenyl rings using variable substituents para to the fixed substituent -OCH2CH2NHC6H5 (series I) and -NHCH2CH2OC6H5 (series II), respectively. The correlation coefficient for chemical shifts vs. a linear combination of inductive and resonance substituent constants is high and improves when only the six values, corresponding to each para-monosubstituted series, were used. For nitrogen chemical shifts excellent linear dependences were obtained. The results show that the ethylene chain is not able to transmit the substituent effect from one aromatic ring to the other.

Sequence determination and resonance assignments of an Azomonas siderophore using 13C natural abundance 13C–1H HNCA experiment

The determination of the amino acid sequence of small peptides as siderophores is commonly performed using long-range HMBC correlation experiments. However, this type of experiment can lead to ambiguities in the assignment resulting in erroneous primary sequence determination. In this paper, we propose to use an HNCA type experiment on a 15 N-labelled sample in order to avoid these ambiguities. This experiment was successfully used to determine the primary sequence of azoverdin, a siderophore produced by Azomonas . La détermination de séquences primaires de petits peptides comme les sidérophores bactériens s’appuie en général sur des spectres RMN de corrélation carbone–proton à longue distance de type HMBC. Cependant, ce type d’expérience ne permet pas de résoudre toutes les ambiguïtés d’attribution et peut conduire à des erreurs de séquence primaire. Dans cet article, nous proposons d’utiliser une expérience de type HNCA sur un échantillon enrichi à l’azote 15 afin de lever ces ambiguïtés. Cette expérience a été appliquée avec succès pour déterminer la séquence primaire de l’azoverdine, un sidérophore d’ Azomonas .

Use of Long-Range C-H Heteronuclear Multiple Bond Connectivity in the Assignment of the 13 C NMR Spectra of Complex Organic Molecules

Use of Long-Range C-H Heteronuclear Multiple Bond Connectivity in the Assignment of the 13 C NMR Spectra of Complex Organic Molecules

31P-1H correlation and resonance assignments along the DNA backbone: three-dimensional implementation of heteronuclear long-range correlation experiment

A three-dimensional heteronuclear NMR technique to correlate 31P spins with 1H along the backbone of DNA is described. The approach is based on a 31P–1H heteronuclear long-range correlation (HELCO) experiment in its refocused form. The utility of this technique is demonstrated by the sequence-specific resonance assignment of 31P and 1H spins along the backbone of a mismatched base-paired dodecanucleotide, d-GGTACIAGTACC. Copyright © 2000 John Wiley & Sons, Ltd.

Sensitivity Improvement and New Acquisition Scheme of Heteronuclear Active-Coupling-Pattern-Tilting Spectroscopy

A simplified phase-cycling scheme for heteronuclear active-coupling-pattern tilting (ACT) spectroscopy is presented. It is demonstrated that the theoretically expected twofold sensitivity gain over earlier implementations can be experimentally realized. A further intensity increase by a factor of 2 is obtained with standard sensitivity-enhancement pulse-sequence elements. The HSQC–HECADE sequence presented is designed for an accurate determination of heteronuclear one-bond and, with subsequent I-spin isotropic mixing, long-range coupling constants. As an exemplary application, the determination of the 3JN,Hβ coupling constants in a peptide at natural isotope abundance is demonstrated. Additionally, a new polarization-transfer step for the long-range HSQC–HECADE experiment is proposed which avoids a fixed delay tuned to a specific coupling-constant value. Thus, the long-range correlation experiment does not require prior knowledge of the coupling constants to be measured and yields more uniform cross-peak intensity for a broad range of active coupling constants.

Substituent Influences on the Keto-Enol Tautomerism in 1-(2-Hydroxyphenyl)-3-α-and -β-naphthylpropane-1,3-diones Monitored by 1H and 13C NMR Spectroscopy

Synthesis of 18 1-(2-hydroxyphenyl)-3-α- and -β-naphthylpropan-1,3-diones is described. Their 1H and 13C NMR spectra were completely and unambiguously assigned using a combination of both homo- and heteronuclear (gs-COSY ), 1H -detected heteronuclear one-bond (gs-HMQC ) and long-range (gs-HMBC) correlation experiments. Substituent and conformation effects on the tautomeric equilibria were identified and interpreted in terms of steric and electronic contributions.

1H and13C NMR chemical shift assignment of some 3H-naphtho[2,1-b]pyrans

The 1H and 13C NMR resonances of four 3H -naphtho[2,1-b]pyrans were completely and unambiguously assigned by a combination of homonuclear (gs-COSY), 1H-detected heteronuclear one-bond (gs-HMQC) and long-range (gs-HMBC) gradient-selected correlation experiments. Copyright © 1999 John Wiley & Sons, Ltd.

1H and13C NMR chemical shifts of some thienyl-substituted chromenes

The complete assignment of the 1H and 13C NMR spectra of eight thienyl-substituted chromenes was achieved by the concerted application of homonuclear (gs-COSY), 1H-detected heteronuclear one-bond (gs-HMQC) and long-range (gs-HMBC) gradient-selected correlation experiments. © 1998 John Wiley & Sons, Ltd.

1H and13C chemical shifts of some bicyclo[4.3.0]- and bicyclo[6.3.0]-γ-lactones and α-carbomethoxy-γ-lactones

The 1H and 13C NMR resonances of ten bicyclo[4.3.0]- and bicyclo[6.3.0]-γ-lactones and α-carbomethoxy-γ-lactones were completely and unambiguously assigned by a combination of homonuclear (gs-COSY), 1H-detected heteronuclear one-bond (gs-HMQC) and long-range (gs-HMBC) gradient-selected correlation experiments. © 1998 John Wiley & Sons, Ltd.

Amariinic acid and related ellagitannins from Phyllanthus amarus

Chemical examination of the polar extractives of the aerial parts of Phyllanthus amarus led to the isolation of amariinic acid, a novel ellagitannin, together with 1-O-galloyl-2,4-dehydrohexahydroxydiphenoyl-glucopyranose elaeocarpusin, repandusinic acid A and geraniinic acid B. On the basis of chemical and NMR data, including long-range correlation experiments, the structure of amariinic acid was established as the ring-opened oxidized cyclohexentrione moiety of DHHDP attached to O-4 of the glucose core.

Structural Elucidation of the Biological Repeating Unit of O‐specific Polysaccharide from Citrobacter Serotype O41

Sugar analysis of the O-specific polysaccharide produced by Citrobacter serotype O41 revealed the presence of a hexasaccharide repeating unit, which includes the unusual 3-amino-N-(D-3'-hydroxybutyryl)-3,6-dideoxy-D-galactosyl residue (Fuc3NAcyl). The structure of the repeating unit was determined by extensive use of homo- and heteronuclear two-dimensional NMR spectroscopy, including the application of long-range 1H-13C correlation experiments and NOE studies to establish the sequence of sugar units. Indentification of the Glc beta 1-->2Fuc3NAcyl beta 1-->6GlcNAc alpha 1-->sequence at the non-reducing terminus establishes the biological repeating unit of this O-specific polysaccharide as: -->2Glc beta 1-->Fuc3NAcyl-beta 1-->6GlcNAc alpha 1-->(Glc alpha 1-->2) 4Gal beta 1-->3GalNAc beta 1-->. This structure is similar to that found for the O-specific polysaccharide isolated from Hafnia alvei strain 1211 [Katzenellenbogen, E., Romanowska, E., Dabrowski, U. & Dabrowski, J. (1991) Eur. J. Biochem. 200, 401-407], which differs in having an acetyl substituent at O4 of the Fuc3NAcyl residue and a branch point of GalNAc alpha substituted by Glc beta at O3; these differences are responsible for the only weak serological cross-reactivity of the two strains.

Solution structure and dynamics of PEC-60, a protein of the Kazal type inhibitor family, determined by nuclear magnetic resonance spectroscopy.

The three-dimensional solution structure of porcine PEC-60, a 60 amino acid residue protein of the Kazal type family of proteinase inhibitors, was determined by nuclear magnetic resonance (NMR) spectroscopy. The structure determination is based on nearly complete 1H, 13C and 15N resonance assignments including stereospecific 1H resonance assignments for 40 pairs of methylene protons and isopropyl methyl groups. The stereospecific resonance assignments of the beta-protons were supported by heteronuclear long-range correlation experiments recorded at natural 13C and 15N isotopic abundances. A group of 20 conformers were calculated using the experimentally derived NMR constraints with the program DIANA, and energy-minimized in a 4 A water shell using the program OPAL. The average of the root-mean-square deviations relative to the mean structure of the 20 conformers selected to represent the solution structure of PEC-60 is 0.55 A for the backbone atoms of residues 6 to 10 and 24 to 60. Disordered conformations are observed for the amino-terminal pentapeptide and the polypeptide segment containing residues 11 to 23. The NMR structure confirms the structural similarity of PEC-60 to the Kazal type family of proteinase inhibitors which had been previously suggested on the basis of amino acid homology. The well-defined part of PEC-60 contains a short three-stranded anti-parallel beta-sheet involving the residues 27 to 29, 33 to 35 and 53 to 56 with a beta-bulge at residue 55, a type I turn comprising residues 29 to 32, and an alpha-helix involving the residues 37 to 48. T1(13C) relaxation measurements of the alpha-carbons and linewidth measurements of the amide proton signals indicate substantially increased mobility on the pico- to nanosecond time-scale for the amino-terminal pentapeptide as well as within the loop comprising residues 11 to 23. The structure of PEC-60 is compared to the X-ray crystal structures of homologous Kazal type proteinase inhibitors and the dynamic properties of PEC-60 are discussed with respect to the observed lack of any substantial trypsin inhibiting activity.

Estimation of31P–1H and1H–1H vicinal coupling constants along the DNA backbone by 2D HELCO measurements

An accurate method for simultaneous estimation of 31P–1H and 1H–1H vicinal coupling constants along the backbone of DNA fragments is described using the two-dimensional heteronuclear long-range correlation (2D HELCO) experiment.