Vicinal Proton Research Articles

Three-dimensional 1H-TOCSY-relayed ct-[13C,1H]-HMQC for aromatic spin system identification in uniformly 13C-labeled proteins

Three-dimensional (1)H-TOCSY-relayed ct-[(13)C,(1)H]-HMQC is a novel experiment for aromatic spin system identification in uniformly (13)C-labeled proteins, which is implemented so that it correlates the chemical shift of a given aromatic proton with those of the directly attached carbon and all vicinal protons. The ct-HMQC scheme is used both for overlay of the indirect (1)H and (13)C chemical shift evolution periods and for the generation of (1)H-(1)H antiphase magnetization to accelerate the (1)H-TOCSY magnetization transfer at short mixing times. As an illustration, data recorded for the 18 kDa protein cyclophilin A are presented. Since transverse relaxation of (13)C-(1)H zero-quantum and double-quantum coherences is to first order insensitive to (13)C-(1)H heteronuclear dipolar relaxation, the new experiment should work also for proteins with molecular weights above 20 kDa.

Molecular Dynamics of Covalently-Linked Multi-porphyrin Arrays

The molecular dynamics of a series of diphenylethyne- or diphenylbutadiyne-linked porphyrin arrays in solution at 296 K have been investigated using high-field proton NMR spectroscopy. Measurements of the direct dipolar interactions between vicinal proton pairs located on the pyrrole and phenylene moieties provide information about the flexibility of the para-substituted diphenylethyne linker. The magnitude of the 2-fold rotational barrier is about 3.3 kJ/mol, with the most stable form having coplanar phenylene moieties. This is in rough agreement with the behavior reported for diphenylacetylene. The diphenylethyne linker undergoes large bending motions, causing the phenylene proton−proton axis to be bent out of the plane of the more remote porphyrin ring by an average angle of 26°. This large bending motion presumably is distributed at sites over the entire linker, including bending at the ethyne and possible out-of-plane bending of the phenylene group relative to the attached porphyrin. The diphenylbuta...

Two Highly Populated Conformations at Room Temperature of Monterine and Granjine, Antitumor Bisbenzylisoquinoline Alkaloids: Origin and Tridimensional Structures.

Monterine 1 as well as granjine 3, 1R,1'S configured biphenylic bisbenzylisoquinoline alkaloids, generate two highly populated conformers. The interconversion of two forms was detected by saturation tranfer in (1)H NMR NOEs experiments. Tridimensional structure of the conformers was determined on the basis of (1)H NMR analysis of anisotropic shielding protons, by NOEs measurements and vicinal proton coupling constants of CH1-CH(2)alpha and CH1'-CH(2)alpha'. The structures established from NMR data were further refined to observe the mobility of 3D conformations by molecular dynamics simulation in vacuo. The highly populated conformers, monterine 1a and 1b, as well as granjine 3a and 3b, are interconvertible by rotation about the C1'-Calpha', Calpha'-C9', and C11'-C11 bonds and inversion of the benzyl D ring by reference to CH(2)alpha'. The slow exchange system was investigated by dynamic NMR spectroscopy: DeltaG()(c) 77.9 KJ/mol and k(c) 200 s(-)(1) for monterine 1; DeltaG()(c) 77.7 KJ/mol and k(c) 211 s(-)(1) for granjine 3. Natural and synthetic biphenylic bisbenzylisoquinolines displayed, in vitro, cytotoxic activities against human prostate and breast cancer cell lines.

Calculation of nuclear spin–spin couplings. VIII. Vicinal proton–proton coupling constants in ethane

Ab initio self-consistent-field (SCF) and electron correlation calculations have been carried out for the dihedral angle dependence of the vicinal proton–proton coupling constants, 3JHH, in ethane molecule. The four contributions to 3JHH, (JFC, JSD, JOP, and JOD) have been computed with the three different basis sets, [5s2p1d/2s1p], [5s3p1d/3s1p], and [7s4p2d/5s2p]. The Fermi contact (FC) contribution was largest and the spin–dipole (SD) contribution was smallest. The FC and orbital paramagnetic (OP) contributions showed large basis set dependence, but the SD and orbital diamagnetic (OD) contributions presented little basis set dependence. The calculated total SCF contribution to 3JHH was higher than the experimental coupling. Using the Mo/ller–Plesset perturbation theory we have introduced electron correlation effects on the FC and OP terms. The correlation effects on the OP term was shown to be negligible. The second-order correlation in the FC term was very large and amounted to half of its SCF value in magnitude with opposite sign. However, the third-order correlation in the FC contribution was small. Unfortunately, the calculated 3JHH value including correlation corrections through third order was too small compared to the experimental one. The poor agreement between calculation and experiment is claimed to be due to higher than third-order correlations in the FC term.

Solution structures of 13 2-methoxychlorophyll a epimers

The absolute configurations at C-13 2 of 13 2-methoxychlorophyll a (13 2-MeO-Chl a) epimers, synthetic derivatives of Chl a, were determined by the 2D ROESY NMR technique. The ROESY spectra and the vicinal coupling of the C-17 and C-18 protons also provided information on the conformation of ring D which was found to be more distorted in both MeO-Chl a epimers compared with 13 2( R)-Chl a due to increased steric crowding between the C-13 2 and C-17 substituents. This conclusion was supported by the conformational analysis of the C-17 propionic ester side-chain. The populations of the side-chain rotamers were calculated from vicinal proton coupling constants, obtained from spectral simulations of the 17 1- and 17 2-CH 2 proton multiplets. For the R and S epimers of 13 2-MeO-Chl a, the most populated conformers were found to be: ap 1a (populations 0.7 ( R), 0.5 ( S)) and g + 1b (0.3 ( R), 0.5 ( S)) of the C 17 1 C 17 bond rotamers, and ap 2a (0.88 ( R), 0.82 ( S)) of the C 17 2 C 17 1 bond rotamers (Fig. 5). In comparison, for 13 2( R)-pheophytin a, the previously found most populated conformers are: ap 1a (0.7) and g + 1b (0.3) of the C 17 1 C 17 bond rotamers, and ap 2a (0.63) of the C 17 2 C 17 1 bond rotamers. The conformers of the ester fragment and the front part of the phytyl chain in the Chl derivatives are discussed.

Empirical group electronegativities for vicinal NMR proton–proton couplings along a CC bond: Solvent effects and reparameterization of the Haasnoot equation

AbstractEmpirical group electronegativities (substituent parameters λi), valid for 3J(HH) in saturated HCCH fragments, were derived from the coupling to methyl in substituted ethanes and isopropyl derivatives according to the equation In contrast to earlier work, it was found advantageous to differentiate between the λi values of hydrogen acting as substituent in CH3 as compared with H in CH2. Special attention was paid to solvent effects, in particular the influence of D2O, on the vicinal couplings and thus on λi. The previously derived λi values remain valid in all common organic solvents but a special effect of D2O on λ is manifest in cases where the α‐substituent carries one or two non‐conjugated lone pairs of electrons that readily act as hydrogen bond acceptors: Δλ= −0.11 ± 0.03 for NH2, NHR, NR2, OH, OR, R = alkyl. Protonation of NH2 to give NH3+ lowers λi by 0.28 units. The λi values for the nucleic acid bases (Ade, Gua, Ura, Thy, Cyt), as determined from the N‐isopropyl derivatives, are 0.56 ± 0.01 irrespective of the solvent. Secondary amides display similar values. The parameters of the Haasnoot equation, originally derived with the aid of a Pauling‐type electronegativity scale, were reoptimized on the basis of the present λi scale; the previous overall r.m.s. error of 0.48 Hz now drops to 0.36 Hz and separate parameterization of HCCH fragments with different substitution patterns appears to be no longer necessary.

Solution structures of active and inactive forms of the DP IV (CD26) inhibitor Pro-boroPro determined by NMR spectroscopy.

Synthesis of the boronic acid analog of the dipeptide Pro-Pro yields a mixture of diastereomers Pro-L-boroPro and Pro-D-boroPro, one of which is a potent inhibitor [Ki = 16 pM; Gutheil, W. G., & Bachovchin, W. W. (1993) Biochemistry 32, 8723-8731] of dipeptidyl amino peptidase type IV (DP IV), also known as CD26. The structures of both diasteremers are determined here in aqueous solution by means of 1D and 2D NMR of 1H, 13C, and 11B, and force-field calculations, and the inhibitor is proven to have the L-L configuration. At low pH values (approximately 2), both diastereomers are trans with respect to the peptide bond. Populations of proline ring conformers are determined by pseudorotation analysis, using vicinal proton spin-coupling constants obtained by computer analysis of 1D1H NMR spectral fine structure. At neutral pH values, the Pro-boroPro inhibitor of DP IV undergoes slow, reversible inactivation (Gutheil & Bachovchin, 1993). By structural determination of the decomposition products of both diasteromers, the process is shown here to involve formation of a six-membered ring between the residues by means of trans-cis conversion and formation of a B-N bond, producing chiral nitrogen atoms in both cases having the S configuration. Analogy to cyclic dipeptides suggests the new compounds be named cyclo(Pro-L-boroPro) and cyclo(Pro-D-boroPro).

Structure and conformational features of tetrahydrothienothiepinoisoxazole as a novel heterocyclic system

The crystal and molecular structure of a novel heterocyclic system, 8-methyl-3,3a,4,5-tetrahydrothieno[3′,2:6,7]thiepino[4,5-c]isoxazole, has been determined by X-ray analysis. The seven-membered ring has the boat conformation (B), while the isoxazoline cycle has the flat chair conformation (3E). There are strong steric strains between the vicinal protons at the C(3a), C(3), and C(4) atoms.

Synthesis, molecular modelling, and NMR structure determination of four cyclic peptide antagonists of endothelin

A combined distance geometry and molecular mechanics/dynamics (MM/MD) protocol was unable to predict the active conformation of the cyclic pentapeptide inhibitor of endothelin-1 receptor, BQ-123, and two analogues. However, the MM/MD method alone is sufficient to predict the solution conformation of a third analogue. In that one case, the combination of proline at residue 3 and an N α-methyl substitution at residue 5 provides enough internal constraints to eliminate conformational flexibility seen in the other three analogues. For this constrained analogue, the 50 lowest energy conformations (out of a set of 500 DGEOM-generated, MM/MD minimized conformations) differ by no more than 3.9 kcal/mol. Thirty three of these 50 conformations have backbone atom RMSDs of less than 0.33 Å, relative to the lowest energy conformation. The accuracy of this MM/MD model is verified by determining the solution structure of each of the four analogues with 2D NMR techniques. Each of the cyclic pentapeptides has a well defined solution conformation where a proline residue is clearly in a γ-turn, leaving the remaining residues in a loose β-turn. All four experimental NMR conformations agree closely with the MM/MD model. MM/MD molecular mechanics/molecular dynamics, DG distance geometry, RMS root mean square, RMSD root mean square deviation. NMR nuclear magnetic resonance, 2D NMR 2-dimensional NMR, ROESY 2-dimensional rotating frame nuclear Overhauser effect spectroscopy, COSY 2-dimensional J-correlated spectroscopy, PECOSY 2-dimensional exclusive correlation spectroscopy, TOCSY 2-dimensional total correlation spectroscopy, 3J Nα, NH-C αH vicinal proton coupling constant, 3J αβ C αH-C βH vicinal proton coupling constant. FMOC 9-(fluorenylmethoxy)carbonyl, PyBOP benzotriazole-1-yl-oxy-tris-(pyrrolidino)phosphonium hexafluorophosphate, PyBrOP, bromo-tris-(pyrrolidino)phosphonium hexafluorophosphate, HOBt hydroxybenzotriazole, DMSO dimethyl sulfoxide, DCM dichloromethane, DMF dimethyl formamide, DIEA diisopropylethanolamine, TFA trifluoroacetic acid, EtOH ethanol, HOAc acetic acid, TMS tetramethylsilane, DMAP dimethylaminopyridine, DIC diisopropylcarbodiimide. RP-HPLC reverse phase high pressure liquid chromotography, AAA amino acid analysis, FAB-MS fast atom bombardment mass spectral analysis. We use the designations ETR A and ETR B, rather than the convention ET A and ET B to ensure clarity that we are discussing the two receptor subtypes of the Endothelin family.

Konformation und Bromierung von Bicyclo[3.3.1]nonan‐2,6‐dion sowie Dehydrobromierung der Bromketone. – Verbesserte Synthese von Bicyclo[3.3.1]nona‐3,7‐dien‐2,6‐dion

Conformation and Bromination of Bicyclo[3.3.1]nonane‐2,6‐dione and Dehydrobromination of the Bromo Ketones. – Improved Synthesis of Bicyclo[3.3.1]nona‐3,7‐diene‐2,6‐dioneBromination by copper(II) bromide of bicyclo[3.3.1]nonane‐2,6‐dione (1) affords the epimeric α‐bromo diketones exo‐and endo‐5 and, eventually, the α, α′‐dibromo diketones exo‐ and endo‐7 (3:7) in high yield. Dehydrobromination of the mixtures of bromo diketones by calcium carbonate in dimethylacetamide yields the unsaturated diketones 6 (30%) and 2 (72%), respectively. Bromination of 1 by molecular bromine is less selective and furnishes mixtures of brominated diketones consisting of exo‐ and endo‐7, 8 and the tetrabromo diketone 10. An excess of bromine yields pure 10. – The ethylene acetal 12 is brominated by pyridinium perbromide to produce the α,α′‐dibromo acetal 13 which is very reluctant towards 1,2‐dehydrobromination by potassium tert‐butoxide. Thus, with an excess of potassium tert‐butoxide in tetrahydrofuran, 1,2‐dehydrobromination is achieved only on one side to afford 14. In contrast, potassium tert‐butoxide in dimethyl sulphoxide induces a transannular dehydrobromination leading to the diacetal 15 of 3‐bromonoradamantane‐2,6‐dione. – The conformations of 1,10, and 13 in the solid state are determined by X‐ray diffraction analyses. From torsional angles, vicinal proton coupling constants are calculated with the aid of the Karplus equation and compared to those determined from solutions by high‐field NMR spectroscopy. The reasonable agreement provides the basis for the assessment of the configuration and conformation for exo‐ and endo‐7. magnified image

Solution structure of a conserved DNA sequence from the HIV-1 genome: restrained molecular dynamics simulation with distance and torsion angle restraints derived from two-dimensional NMR spectra.

The three-dimensional solution structure of a trisdecamer DNA duplex sequence, d(AGCTTGCCTTGAG).d(CTCAAGGCAAGCT), from a conserved region of HIV-1 genome's long terminal repeat, has been investigated using NMR spectroscopy and restrained molecular dynamics calculations. Interproton distances derived from two-dimensional nuclear Overhauser enhancement (2D NOE) experiments, using the iterative complete relaxation matrix algorithm MARDIGRAS, and torsion angles for sugar rings, estimated from stimulated fitting of double-quantum-filtered correlation (2QF-COSY) spectra, were obtained [Mujeeb, A., Kerwin, S. M., Egan, W. M., Kenyon, G. L., & James, T. L. (1992) Biochemistry 31, 9325-38]. These structural restraints have now been employed as the basis for structure refinement using restrained molecular dynamics (rMD) to search conformational space for structures consistent with the experimental restraints. Specifically, upper and lower bounds on the restraints were incorporated into the AMBER (version 4.0) total potential energy function of the system, the bounds being used to define the width of a flat-well penalty term in the AMBER force field. Confidence in the time-averaged structure obtained is engendered by convergence to essentially the same structure (root-mean-square deviation approximately 0.9 A) when two quite different DNA models, A-DNA and B-DNA (RMSD approximately 6.5 A), were employed as starting structures and when various initial trajectories were used for the rMD runs. The derived structure is further supported by the total energy calculated, the restraint violation energy, the restraint deviations, and the fit with experimental data. For the latter, the sixth-root residual index indicated a good fit of the determined structure with experimental 2D NOE spectral intensities (R1x < 0.07), and the RMS difference between vicinal proton coupling constants calculated for the derived structure and experimental coupling constants were also in reasonable agreement (JRMS = 0.9 Hz). While the structure of the trisdecamer is basically in the B-DNA family, some structural parameters manifest interesting local variations. The helix parameters indicate that, compared with classical B-DNA, the structure is longitudinally more compressed. Local structural variations at the two TG steps in particular together create bending into the major groove of the duplex. Comparison of the two-CTTG-tetrads in the duplex reveals that they have similar structures, with the TT moieties being almost identical; however, the -CTTG-pur sequence has a larger roll and slide for the -TG- step than for the -CTTG-pyr sequence, in accord with published X-ray crystallographic conclusions.

MM2 conformational analysis of diastereoisomers with phenyl groups on neighbouring carbon atoms and intramolecular hydrogen bonding

The conformations of a set of compounds containing phenyl groups on neighbouring carbon atoms and substituents with the potentiality for formation of an intramolecular hydrogen bond (diols, diamines and amino-alcohols) have been studied with the MM2 force field, enhanced with the addition of electrostatic corrections to the aromatic force field; as proposed by Allinger and Lii (dipole-dipole interactions), and the special MM2 hydrogen bonding parameters for the van der Waals function. The molecular mechanics results are in satisfactory agreement with experimental conformational data (mainly NMR and IR). An average deviation of 1.0 Hz was obtained between 20 observed vicinal proton coupling constants and those estimated with the Haasnoot-Altona equation. Previous indications regarding the electrostatic corrections for modelling phenyl-phenyl interactions and the parameters for the C sp 2 C sp 3 C sp 3 C sp 2 torsional potential are further supported. The molecular mechanics results for the low-energy conformations of six compounds have been supplemented with AM1, PM3 and MM-EHMO estimates of their conformational energies. The results are encouraging for attempting a complete SE-MO conformational analysis of such systems.

Metropolis Monte Carlo calculations of DNA structure using internal coordinates and NMR distance restraints: an alternative method for generating a high-resolution solution structure.

A new method, a restrained Monte Carlo (rMC) calculation, is demonstrated for generating high-resolution structures of DNA oligonucleotides in solution from interproton distance restraints and bounds derived from complete relaxation matrix analysis of two-dimensional nuclear Overhauser effect (NOE) spectral peak intensities. As in the case of restrained molecular dynamics (rMD) refinement of structures, the experimental distance restraints and bounds are incorporated as a pseudo-energy term (or penalty function) into the mathematical expression for the molecular energy. However, the use of generalized helical parameters, rather than Cartesian coordinates, to define DNA conformation increases efficiency by decreasing by an order of magnitude the number of parameters needed to describe a conformation and by simplifying the potential energy profile. The Metropolis Monte Carlo method is employed to simulate an annealing process. The rMC method was applied to experimental 2D NOE data from the octamer duplex d(GTATAATG).d(CATTATAC). Using starting structures from different locations in conformational space (e.g. A-DNA and B-DNA), the rMC calculations readily converged, with a root-mean-square deviation (RMSD) of < 0.3 A between structures generated using different protocols and starting structures. Theoretical 2D NOE peak intensities were calculated for the rMC-generated structures using the complete relaxation matrix program CORMA, enabling a comparison with experimental intensities via residual indices. Simulation of the vicinal proton coupling constants was carried out for the structures generated, enabling a comparison with the experimental deoxyribose ring coupling constants, which were not utilized in the structure determination in the case of the rMC simulations. Agreement with experimental 2D NOE and scalar coupling data was good in all cases. The rMC structures are quite similar to that refined by a traditional restrained MD approach (RMSD < 0.5 A) despite the different force fields used and despite the fact that MD refinement was conducted with additional restraints imposed on the endocyclic torsion angles of deoxyriboses. The computational time required for the rMC and rMD calculations is about the same. A comparison of structural parameters is made and some limitations of both methods are discussed with regard to the average nature of the experimental restraints used in the refinement.

Synthese und Struktur Phenyl‐substituierter Bicyclo[3.3.1]nonene

Synthesis and Structure of Phenyl‐Substituted Bicyclo[3.3.1]nonenesAddition of one mole of phenyllithium to bicyclo[3.3.1]nonane‐2,6‐dione (5) affords the hydroxy ketone 6 besides small amounts of the known diol 7. Dehydration of this mixture by sulfuric acid in acetic acid yields the unsaturated ketone 8 and the diene 1 which are separated by chromatography. Wolff‐Kishner reduction of 8 furnished the hydrocarbon 2. In the same way, the known ketones exo‐ and endo‐10 are converted into 4. The preferential formation of the less stable endo‐10 is interpreted in terms of the relative stability of the conformations required in the intramolecular aldol reaction of diketone 9. The conformations of 4, 8, exo‐ and endo‐10 are determined by X‐ray diffraction analyses. Vicinal proton coupling constants are calculated from torsional angles and compared to those determined in solutions by high‐field NMR spectroscopy.

2,6‐Dicyan‐4,8‐diphenylbarbaralan

2,6‐Dicyano‐4,8‐diphenylbarbaralane[1]Conjugate addition of the phenylcuprate reagent, obtained from phenyllithium, copper(I) cyanide, and boron trifluoride–diethylether, to the bicyclo[3.3.1]nonadienedione 3 affords the diphenylbicyclo[3.3.1]nonanedione 4 in high yield. Catalyzed by the potassium cyanide/18‐crown‐6 complex, addition of trimethylsilyl cyanide produces a mixture of the diastereomeric bis[O‐(trimethylsilyl)cyanohydrins] exo,exo‐, exo,endo‐ and endo,endo‐5. The hydrogen fluoride – pyridine complex in phosphorus oxychloride as solvent and, subsequently, an excess of pyridine convert the diastereomers 5 into the unsaturated γ,γ′1‐diphenyldinirile 6. This is brominated by N1‐bromosuccinimide to yield the γ,γ′1‐dibromodinitriles exo‐ and endo‐ 7 (6:1). The predominant diastereomer exo‐7 is debrominated by the zinc‐copper couple to afford the orange‐red title compound 2 in 78% yield. More conveniently, the unsaturated dinitrile 6 is converted to 2 in a single step by treatment with hexachloroethane and concentrated aqueous sodium hydroxide in the presence of tetrabutylammonium hydroxide as phase‐transfer catalyst. Surprisingly, low yields of 2 are also obtained when the bis[O‐(trimethylsilyl)cyanohydrins] 5 or the unsaturated dinitrile 6 are treated with phosphorus oxychloride in boiling pyridine. – The structures of the new compounds are based on spectroscopic evidence and X‐ray diffraction analyses of 2, 4, and endo,endo‐5. The conformations of 4 and endo,endo‐5 in solution are inferred on the basis of vicinal proton coupling constants and a comparison with coupling constants calculated with the aid of the Karplus equation and torsional angles obtained by X‐ray diffraction analyses. – While the barbaralane 2 exists as a pair of very rapidly rearranging degenerate valence tautomers in solution, the degeneracy is lifted in the crystal lattice. As a result, the crystal consists of two rapidly rearranging but non‐equivalent valence tautomers in a ratio of 9:1 as estimated from the apparent atomic distance C2–C8 of 2 and the C2–C8 bond length of non‐rearranging barbaralanes. – The colour of 2 in the crystal and in solution results from a maximum at 436 nm which increases on heating of the solution to 450 K. Cooling to 77 K results in reversible fading and the disappearance of the maximum. Thus, 2 is a barbaralane like 1 which exhibits colour though it is lacking a classical long‐wavelength chromophore.

Two-dimensional 1H-NMR study of the spatial structure of neurotoxin II from Naja naja oxiana.

The spatial structure of neurotoxin II from the venom of the central Asian cobra Naja naja oxiana was determined by two-dimensional 1H-NMR techniques and computational analysis. Nearly complete proton resonance assignments for 61 amino acid residues have been made using two-dimensional (2D) homonuclear total correlated spectroscopy, 2D homonuclear double-quantum-filtered correlated spectroscopy and 2D homonuclear NOE spectroscopy (NOESY) experiments. The cross-peak volumes in NOESY spectra spin-spin coupling constants of vicinal protons NH-C alpha H and C alpha H-C beta H and the observation of slow deuterium exchange of amide protons were used to define local structure and a set of constraints for distance geometry program DIANA. The average root-mean-square deviations are 53 pm for backbone heavy atoms and 118 pm for all heavy atoms of 19 final neurotoxin II conformations. The spatial structure is characterized by a short double-stranded (residues 1-5 and 13-17) and a triple-stranded (residues 22-30, 33-41 and 50-54) antiparallel beta-sheets.

2,4,6,8‐Tetraphenylbarbaralan – ein orangeroter, thermochromer Kohlenwasserstoff ohne Chromophor[1,2]

2,4,6,8‐Tetraphenylbarbaralane – an Orange‐Red, Thermochromic Hydrocarbon Devoid of a Chromophore[1,2]The diphenylbicyclo[3.3.1]nonanedione 6 adds phenylcerium‐(III) dichloride to afford a high yield of the triphenyl‐2‐oxa‐twistanol 8. In contrast, phenyllithium yields a mixture of tri‐phenylhydroxyketone exo‐7, tetraphenylbicyclo[3.3.1]nona‐nediol 9, and 8. The latter is dehydrated by sulfuric acid in acetic acid to produce the triphenylbicyclo[3.3.1]nonenone 10 in almost quantitative yield. Addition of phenylcerium(III) dichloride to 10 affords a 3:2 mixture of the tetraphenylbicy‐clo[3.3.1]nonenols exo‐ and endo‐11 which may be separated by chromatography. Dehydration of the mixture yields quantitatively the tetraphenyldiene 12. On treatment with an excess of butylpotassium in pentane, 12 is converted to the deep violet, crystalline dipotassium salt 17 which may be purified by reprecipitation from its tetrahydrofuran solution with pentane. When the solution of 17 in tetrahydrofuran is slowly added to an excess of 1,2‐dibromoethane at –60°C, the dianion is immediately oxidized to produce the tetraphenylbarbara‐lane 4 which is isolated in 30% yield as orange‐red crystals after chromatography. – The structures of the new compounds are based on spectroscopic evidence and X‐ray diffraction analyses of 4, 8, exo‐11, and 12. The conformations in solution are inferred on the basis of vicinal proton coupling constants and a comparison with coupling constants calculated with the aid of the Karplus equation from torsional angles obtained by X‐ray diffraction analyses. The conformation of exo‐11 in solution closely resembles that present in the crystal. – While the barbaralane 4 exists as a pair of very rapidly rearranging degenerate valence tautomers in solution, the degeneracy is lifted in the crystal lattice. As a result, the crystal consists of two rapidly rearranging but non‐equivalent valence tautomers in a ratio of 9:1 as estimated from the apparent atomic distance C2–C8 of 4 and the C2–C8 bond length of the model barbaralane 18. – The orange‐red colour of 4 in the crystal and in solution results from a pronounced shoulder in the UV/Vis spectrum at 430 nm, the intensity of which strongly depends on the temperature. Thus, 4 is the first barbaralane which exhibits colour though it is lacking a classical long‐wavelength chromophore.

Proton NMR studies of syn‐ and anti‐tricyclo [5.1.0.03,5]octanes and their dichloromethyl derivatives

AbstractThe carbon and proton NMR spectra of syn‐ and anti‐tricyclo[5.1.0.03,5]octanes and their dichlorocarbene CH bond insertion products were determined by conventional 1D and 2D methods. Partially relaxed proton spectroscopy was carried out in some instances to aid in the separation of badly overlapped multiplets. A detailed analysis of the spectra was carried out with the aid of a new equation relating both the torsion angles and the CCH angles to the vicinal proton coupling constants. These values were combined with molecular modeling to draw conclusions regarding the conformational preferences for these molecules.

The solution structure of a cyclic endothelin antagonist, BQ-123, based on 1H1H and 13H1H three bond oupling constants

A cyclic pentapeptide endothelin antagonist, cyclo(dTrp-dAsp-Pro-dVal-Leu), recently reported (K. Ishikawa et al., 13th Am. Pept. Symp., Cambridge MA, 1991) has been studied by NMR spectroscopy and molecular modeling. A stable structure has been determined without the use of nuclear Overhauser effects and is based primarily on homonuclear and heteronuclear three bond coupling constants. The 13C-edited TOCSY experiment is demonstrated at natural abundance and ∼30 mM peptide concentrations. Three bond 13C1H coupling constants obtained by this method are shown to reduce the ambiguity in φ angle determination which exists when only interproton coupling constants are used. Three out of four φ angles were determined uniquely by this method and the fourth was reduced to two possible values. The proline φ angle was determined to be −78° based on the 3JHzHα and 3JHzHβ coupling constants. Comparison of amide proton temperature dependence, chemical shifts and vicinal proton coupling constants in a 20% acetonitrile/80% water solvent mixture and in (CD3)2SO indicates that the structure is similar in both solvents.

Improved three-dimensional1H−13C−1H correlation spectroscopy of a13C-labeled protein using constant-time evolution

An improved version of the three-dimensional HCCH-COSY NMR experiment is described that correlates the resonances of geminal and vicinal proton pairs with the chemical shift of the 13C nucleus attached to one of the protons. The experiment uses constant-time evolution of transverse 13C magnetization which optimizes transfer of magnetization and thus improves the sensitivity of the experiment over the original scheme. The experiment is demonstrated for calmodulin complexed with a 26-residue peptide comprising the binding site of skeletal muscle myosin light chain kinase.