15N-NMR. Spectra of pterins and folic acid derivatives

¹⁵N-Chemical shifts of a series of 5, 6, 7, 8-tetrahydropterins, 7, 8-dihydropterins and pterins have been measured in acidic solution by means of a probe for 20 mm sample tubes. Included are the relevant data of folic acid (11) , 5, 6, 7, 8-tetrahydrofolic acid (5) and N (5, 10)-metheno-5, 6, 7, 8-tetrahydrofolic acid (6) . The different oxidation states are clearly relected in the chemical shifts of N (5) and N (8). Assignment of the nitrogen resonances was achieved by protonation effects (discrimination between N (1) and N (3)) and with the aid of alkyl substitution at C (6) and C(7), to distinguish between N (5) and N (8).     

15N-NMR Spectra of Azoles with Two Heteroatoms

The ¹⁵N-NMR spectra of azoles, with natural isotope abundance, have been measured under different experimental conditions, and chemical shifts are reported for imidazoles, pyrazoles, oxazoles, isoxazoles, thiazoles, and isothiazoles. General trends of substituent effects in this heterocyclic series are discussed based on the data of 67 substituted azoles, dihydro- and tetrahydroazoles. ¹⁵N, ¹H spin-coupling constants have been determined from spectra obtained by [¹H] → ¹⁵N polarizationtransfer experiments, i.e. an application of INEPT and DEPT pulse sequences. Two-bond and three-bond coupling constants are fully assigned and are discussed in terms of the specific pathways in azoles. The potential of structural applications of the new data is illustrated for isomeric nitro-imidazoles and highly-substituted pyrazoles, and in the case of ring-chain tautomerism of 2-substituted tetrahydrooxazoles.     

A 13C- and 15N-NMR. Study of Some 1-Aryl-3, 3-dimethyl Triazene Derivatives

¹³C- and ¹⁵N-NMR. spectra have been measured for a series of triazenes. The results are compatible with the significant contribution of a structure such as II. One-bond nitrogen-nitrogen coupling constants are reported.     

13C- and 1H-NMR. Spectra of ortho-benzoquinones. On the assignment problem in 13C spectra

¹³C- and ¹H-NMR. spectra of ortho-benzoquinone 1 and its methyl derivatives have been analysed. By means of heteronuclear double resonance experiments it is shown that assignments given in the literature for the olefinic carbon resonances of 1 and of a series of substituted ortho-benzoquinones have to be inverted. ¹³C-carbonyl frequencies of various six-membered cyclic ketones are presented.     

锆吸氘和氚的动力学同位素效应

应用反应速率分析方法,在高真空金属系统上测定了锆在恒容体系和450～630 ℃范围内吸收氘和氚的p-t曲线。由p-t曲线可知,在同一温度下,锆吸氚的速率低于锆吸氘的速率;而随着温度的升高,由于解吸逆反应的影响,锆吸氚(氘)的反应速率降低。根据p-t曲线计算了吸气反应在不同温度的速率常数,得到锆吸氘和氚的表观活化能分别为(-25.9±0.7)和(-16.8±0.8)kJ·mol^-1。从活化能数据可以看出     

Deuterium Isotope Effects on Acid-Base Equilibrium of Organic Compounds

Deuterium isotope effects on acid–base equilibrium have been investigated using a combined path integral and free-energy perturbation simulation method. To understand the origin of the linear free-energy relationship of ΔpKa=pKaD2O−pKaH2O versus pKaH2O, we examined two theoretical models for computing the deuterium isotope effects. In Model 1, only the intrinsic isotope exchange effect of the acid itself in water was included by replacing the titratable protons with deuterons. Here, the dominant contribution is due to the difference in zero-point energy between the two isotopologues. In Model 2, the medium isotope effects are considered, in which the free energy change as a result of replacing H₂O by D₂O in solute–solvent hydrogen-bonding complexes is determined. Although the average ΔpKa change from Model 1 was found to be in reasonable agreement with the experimental average result, the pKaH2O dependence of the solvent isotope effects is absent. A linear free-energy relationship is obtained by including the medium effect in Model 2, and the main factor is due to solvent isotope effects in the anion–water complexes. The present study highlights the significant roles of both the intrinsic isotope exchange effect and the medium solvent isotope effect.     

15N-NMR Spectroscopy. XVIII. Sequence Analysis of Linear Polyureas

Various homo- and copolyureas were prepared either by conversion of diamines with diisocyanates or by heating diamines with N, N'-bisphenoxycarbonyl diamines. The ¹³C-NMR spectra and the natural abundance ¹⁵N-NMR spectra were measured in trifluoroacetic acid. In contrast to polyamides, the carbonyl signals of polyureas are not sensitive to neighboring residue effects, so that ¹³C-NMR spectra are in most cases useless for the sequence analysis of copolyureas. The ¹⁵ N-NMR signals of urea groups are, however, sensitive to the influence of both substituents and thus contain information on the sequence. Structure/ shift-relationships are discussed, and ¹⁵ N-NMR spectra of various copolyureas are presented.     

15N-NMR. and 31P-NMR. Studies of palladium and platinum complexes

¹⁵N-NMR. parameters for the complexes trans-[MCl₂ (¹⁵NH₂ (CH₂)₅CH₃)L] are reported; M = Pt, Pd, L = PBu, PMePh₂, P (p-CH₃? C₆H₄)₃, AsBuⁿ₃, AsMePh₂, As (p-CH₃C₆H₄)₃, NH₂ (CH₂)₅CH₃ and (for Pt) C₂H₄. For both metals, the NMR. parameters depend on the trans-influence of the ligand L. The values ¹J (¹⁹⁵Pt, ¹⁵N) vary from 138 to 336 Hz and can be shown to correlate with the values ¹J (¹⁹⁵Pt, ³¹P) in the complexes trans-[PtCl₂ (PBu)L]. There is a linear relation between the ¹⁵N chemical shifts in the complexes of the two metals. The reactions of the complexes sym-trans-[M₂Cl₄L₂], M = Pd, Pt, L = a tertiary phosphine or arsine, with neutral ligands are described. ¹⁹⁵Pt-, ³¹P- and ¹³C-NMR. data are reported.     

Deuterium isotope effects on 13C and 15N nuclear shielding in o-hydroxyazo dyes

Deuterium isotope effects on the ¹³C and ¹⁵N nuclear shieldings of o-hydroxyazo compounds are described. Both the direct and the equilibrium contributions were determined. Large direct deuterium isotope effects on ¹⁵N nuclear shieldings for ¹Δ, ²Δ and ⁴Δ and negative Δ¹J(¹⁵NH) [D] values were observed. Isotope effects on ¹⁵N nuclear shieldings, because of their magnitudes, are shown to be very useful in determining changes in the azo–hydrazone equilibrium. Isotope effects on ¹³C nuclear shieldings are smaller, but the effects observed at the carbon resonances of the N-phenyl ring are likewise very useful in determining the shift in the equilibrium. Deuterium substitution leads in all cases to a shift in the equilibrium so that the content of the predominant form of the protio compound is increased.     

Measurement of multiple psi torsion angles in uniformly 13C,15N-labeled alpha-spectrin SH3 domain using 3D 15N-13C-13C-15N MAS dipolar-chemical shift correlation spectroscopy

We demonstrate the simultaneous measurement of several backbone torsion angles psi in the uniformly (13)C,(15)N-labeled alpha-Spectrin SH3 domain using two different 3D 15N-13C-13C-15N dipolar-chemical shift magic-angle spinning (MAS) NMR experiments. The first NCCN experiment utilizes double quantum (DQ) spectroscopy combined with the INADEQUATE type 13C-13C chemical shift correlation. The decay of the DQ coherences formed between 13C'(i) and 13C(alphai) spin pairs is determined by the "correlated" dipolar field due to 15N(i)-13C(alphai) and 13C'(i)-15N(i+1) dipolar couplings and is particularly sensitive to variations of the torsion angle in the regime |psi| > 140 degrees. However, the ability of this experiment to constrain multiple psi-torsion angles is limited by the resolution of the 13C(alpha)-(13)CO correlation spectrum. This problem is partially addressed in the second approach described here, which is an NCOCA NCCN experiment. In this case the resolution is enhanced by the superior spectral dispersion of the 15N resonances present in the 15N(i+1)-13C(alphai) part of the NCOCA chemical shift correlation spectrum. For the case of the 62-residue alpha-spectrin SH3 domain, we determined 13 psi angle constraints with the INADEQUATE NCCN experiment and 22 psi constraints were measured in the NCOCA NCCN experiment.     

15N-NMR spectroscopy 32. Synthesis and characterization of polyelectrolytes based on polyaminamides

The titrations of the polyammamides prepared from adipic acid and diethylenetriamine or di(3-aminopropyl)methylamine were monitored by means of natural-abundance ¹⁵N-(NMR) nuclear magnetic resonance spectra. Opposite shift effects were found for amine and amide nitrogens upon protonation of the amino groups. These shift effects allow one to distinguish substitutions involving exchange of electric charges from other substitutions without isolation of the polymers. Reactions of the polyaminamides with β-propiolactone, γ-butyroslution, methyl-methane sulfonate, prolyleneoxide, epichlorohydrine, maleic acid anhydride, succinic acid anhydride, carbon dioxide, ethylchloroformate, and methyne sulfonylchloride were carried out in water, and the resulting polyelectrolytes were characterized by means of ¹⁵N-NMR spectra.     

15N-NMR. Studies of Aminopyridines,Aminopyrimidines and of Some Diazine N-Oxides

¹⁵N-NMR. spectra of mono- and diaminopyridines, and mono-, di- and triaminopyrimidines including trimethoprim and other dihydrofolate reductase inhibitors have been studied in neutral and acidic media. Complete chemical shift assignments are given. Ring-nitrogen shifts are discussed in terms of β-, χ- and δ-substituent effects of amino and alkyl groups. Protonation states in TFA- and FSO₃H-solution and protonation increments for the ¹⁵N-shifts of ring and amino N-atoms are determined. A linear correlation is observed between amino substituent effects (Δδ(¹⁵N)) on the ring N-atom in aminopyridines and corresponding Δδ (¹³C) values in aminobenzenes and, similarly, between Δδ(¹⁵N) values in aminopyrimidines and Δδ(¹³C) values in aminopyridines. Assignment of the ¹⁵N-NMR. spectra of pyrimidine N-oxides, pyrazine N-oxides and pyridazine N-oxides is achieved by comparison with ¹⁴N-NMR. data and with the aid of Yb(fod)₃-induced shifts. One-bond ¹⁵N, ¹H-coupling constants are reported for aminopyridines and aminopyrimidines and discussed in terms of conjugative interaction between NH₂-group and ring system.     

Accurate measurement of 13C-15N distances with solid-state NMR

Solid-state NMR technique for measuring distances between heteronuclei in static powder samples is described. It is based on a two-dimensional single-echo scheme enhanced with adiabatic cross polarization. As an example, the results for intramolecular distances in alpha-crystalline form of glycine are presented. The measured NMR distances (13)C(alpha)-(15)N and (13)C(')-(15)N are 1.496+/-0.002 and 2.50+/-0.02 A, respectively.     

2,6-二甲基哌啶+水体系临界性质的氘代同位素影响

用等体积法分别确定2,6-二甲基哌啶+水和2,6-二甲基哌啶+重水体系的临界组成, 并测定各自的临界温度. 采用折射率法在较宽温度范围内测定两个体系的温度-折射率(T-n)共存曲线, 利用标φ曲线法将其转化为温度-摩尔分数(T-x)和温度-体积分数(T-φ)共存曲线, 求得临界指数β、指前因子B、Wegner校正项因子B1及与共存曲线直径ρd相关的参数. 研究结果表明, 氘代后, 体系低临界温度下降了约3.2 K, 临界组成和共存曲线形状不变, 在近临界点临界指数β仍符合三维伊辛模型(3D-Ising)的理论值(0.327).     

Substituent effects on 13C-15N spin couplings and 13C chemical shifts in benzenediazonium ions

Similar trends are observed for ¹J(¹³C-¹⁵N) values for a series of aryldiazonium tetrafluoroborates and comparable anilinium fluorosulfonates. Contributions from resonance structures involving double-bond character in the CN bond are judged to be relatively unimportant in the ground state structure of aryldiazonium ions.     

Peptide conformations. Part 30. Assignment of the 1H-, 13C-, and 15N-NMR spectra of cyclosporin A in CDCl3 and C6D6 by a combination of homo- and heteronuclear two-dimensional techniques

The ¹H-, ¹³C-, and ¹⁵N-NMR spectra of the immunosuppressive cyclic undecapeptide cyclosporin A ( 1 ) have been analyzed at 300 MHz in CDCl₃, C₆D₆, and mixtures of these solvents. A combination of different homo- and heteronuclear two-dimensional NMR techniques enable complete assignment of all H-, C- and 4 N-signals. Recognition of the proton spin systems has been achieved via ¹H,¹H–COSY and double-quantum-¹H-NMR spectroscopy. NOESY spectra yield some sequence assignments, but two techniques using coupling across amide bonds have been applied to get independent assignments of all amino acids in the sequence: (i) An ¹H,¹H-COSY spectrum optimized for small coupling constants enables the detection of long-range couplings from N-methyl groups to both α-protons attached to that amide bond. (ii) An ¹H, ¹³C-COSY spectrum optimized for C,H-long-range couplings (J = 5 to 10 Hz) to the eleven CO groups again yields coupling to both α-protons attached to that amide bond. Additionally these two experiments yield the assignment of N-methyl protons and carbonyl C-atoms. Normal and relayed ¹H,¹³C-COSY in both solvents have been applied to assign all C-atoms via their directly attached and remote protons. An ¹H,¹³C-COLOC spectrum at 500 MHz in CDCl₃, which uses H,C-long-range couplings confirms the assignment of all proton spin systems as well as the C-signals of each individual amino acid. Ambiguities in the assignment of the C(δ)'s of MeLeu have thus been removed. An ¹H,¹⁵N-COSY spectrum enables the assignment of the 4 NH N-atoms.