N-methyl Resonance Research Articles

Synthesis and Characterization of N-Methylporphyrins, Heteroporphyrins, Carbaporphyrins, and Related Systems.

MacDonald-type "3 + 1" condensations of an N-methyltripyrrane with a series of dialdehydes afforded a matched set of N-methylporphyrins, N-methylheteroporphyrins, N-methyloxybenziporphyrin, N-methyloxypyriporphyrin, N-methyltropiporphyrin, and a N-methylcarbaporphyrin aldehyde. meso-Unsubstituted heteroporphyrins have been little explored previously, and this strategy was also used to prepare N-unsubstituted 21-oxa-, 21-thia-, and 21-selenaporphyrins. In every case, the N-methylporphyrinoids exhibited weaker, bathochromically shifted UV-Vis absorptions compared to their core unsubstituted congeners. However, proton NMR spectroscopy demonstrated that these derivatives retained strong diamagnetic ring currents and the presence of the internal alkyl substituents had little effect on the global aromatic characteristics. Nevertheless, the UV-Vis spectra of N-methyl-oxybenzi- and N-methyl-oxypyriporphyrins were dramatically altered and gave greatly weakened absorptions. N-Methyl-oxybenzi- and N-methyltropiporphyrins reacted with palladium(II) acetate to give stable palladium(II) complexes, demonstrating that N-alkylation alters the metalation properties for these carbaporphyrinoids. The organometallic derivatives also retained strongly aromatic properties, and the proton NMR spectra showed the N-methyl resonances near -3 ppm. N-Methylcarbaporphyrin-2-carbaldehyde also gave a palladium(II) complex, but this gradually rearranged at higher temperatures to afford a C-methyl complex. The results demonstrate that core alkylation of porphyrinoids greatly alters the reactivity and spectroscopic properties for these systems.

Iron Complexes of C- and N-Methylated 2-Aza-21-carbaporphyrin: NMR Studies

Insertion of iron(II) into methylated derivatives of N-confused porphyrins 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-carbaporphyrin (MeCTPPH)H, 2-aza-5,10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (CTPPMe)H2, and 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (MeCTPPMe)H yielded N- or C-methylated high-spin iron(II) complexes (MeCTPPH)Fe(II)Br, (HCTPPMe)Fe(II)Br, and (MeCTPPMe)Fe(II)Br. One electron oxidation of (Me-CTPPH)Fe(II)Br using Br2, accompanied by deprotonation of a C(21)-H(21) fragment and formation of an Fe-C(21) bond, produces an intermediate-spin, five-coordinate iron(III) complex (MeCTPP)Fe(III)Br. Simultaneously, a high-spin complex [(MeCTPPH)Fe(III)Br]+ was formed which preserved the side-on interaction between the metal ion and the inverted pyrrole ring. &[(MeCTPPH)Fe(III)Br]+ was also obtained by titration of (MeCTPP)FeIIIBr with TFA due to the C(21) protonation. A titration of (HCTPPMe)Fe(II)Br and (MeCTPPMe)Fe(II)Br with Br2 yielded solely corresponding high-spin iron(III) species [(HCTPPMe)Fe(III)Br+ and [(MeCTPPMe)Fe(III)Br+. Dioxygen reacts cleanly with (MeCTPPH)Fe(II)Br carbaporphyrin to form solely (MeCTPP)Fe(III)Br. The 1H NMR spectra of paramagnetic iron(II) and iron(III) complexes were examined. The characteristic patterns of pyrrole, C-methyl, and N-methyl resonances were found diagnostic of the ground electronic state of iron and the coordinating nature of the N-confused pyrrole. The characteristic C-Me resonances occur in a unique window (520-420 ppm) for iron(III) C-methylated N-confused porphyrins which remains in contrast with relatively small values found for iron(II) C-methylated derivatives (50-80 ppm).

1,2-Bis(1-aryl-3-alkyltriazen-3-yl)ethanes and related compounds

A series of novel bistriazenes, the 1,2-bis(1-aryl-3-methyltriazen-3-yl)ethanes, Ar-N T N-NMe-CH2CH2-NMe-N T N-Ar, have been synthesized by diazonium coupling with N,N'-dimethylethylenediamine. These bistriazenes are stable crystalline compounds and have been unequivocally characterized by IR and NMR spectroscopy (1H and 13C), and elemental analysis. The structures of two compounds in the series have been confirmed by X-ray crystallography. The 1H NMR spectra show significant line broadening of the N-methyl resonances arising from the restricted rotation around the N2-N3 bond of the triazene units. The presence of strongly electron-withdrawing groups on the aryl ring restricts the rotation to the point where the N-methyl signals of the rotamers are distinct even at room temperature; four resonances of the N-methyl signal are clearly evident and these can be assigned to the anti-anti, syn-syn, and syn-anti conformations of the bistriazene. Diazonium coupling with N,N'-diethylethylenediamine affords the N,N'-diethyl homologues of the bistriazenes, which have been similarly characterized. As model compounds to assist in spectroscopic analysis, a series of related triazenes, the 1-(1-aryl-3-methyltriazen-3-yl)-N,N-dimethyl-2-ethanamines, were prepared by diazonium coupling with N,N,N'-trimethylethylenediamine. These dialkyltriazenes exist mainly as oils, but characterization was achieved by IR, 1H NMR, and 13C NMR spectroscopy, also showing the presence of two rotamers in solution when strongly electron-withdrawing substituents are bonded to the aryl moiety.Key words: triazene, bistriazene, diazonium, ethylenediamine, molecular dynamics, NMR.

Investigation by NMR spectroscopy of the interaction between synthetic soluble (?)-dopa melanin and drugs

In order to understand the molecular interactions of drugs with melanin, synthetic soluble (-)-dopa-melanin was prepared in deuterium buffer. The spectra of various drug moieties with the pigment at 30 degrees C were studied employing the line width measurements obtained with a pulse NMR (AF270) instrument. As compared to drug effects in fresh melanins (48 h), the aged melanins (greater than or equal to 168 h) gave consistent spectral measurements, even in dilute solutions of pigment. NMR signals of aromatic and N-methyl protons of drugs were relatively easy to quantify and, in the presence of melanin, line broadening of various drug moieties occurred. The line widths of the N-methyl groups of acetylcholine (3.02 ppm), the N-methyl group of atropine (2.52 ppm), N-isopropyl of isoprenaline bitartrate (1.14 ppm) and N-ter-butyl of timolol maleate (1.22 ppm) in the presence of the pigment were increased. Line widths associated with acetate, bitartrate, maleate or tropic acid, however, were not altered by the melanin. This indicates the specificity of the interaction between drug moieties and the site(s) of melanin. Based on the line width measurements of N-methyl protons of ephedrine, two dissociation constants were obtained (Kd1 2.08 mM and Kd2 greater than 20 mM). The constants for atropine melanin complex were Kd1 0.79 mM and Kd2 greater than 6 mM. Furthermore, based on N-methyl resonances, it appears that atropine and ephedrine compete for at least one common interacting site of the melanin polymer.

N.m.r. spectroscopic and X-ray crystallographic studies on some o-NO2substituted aromatic sulphur amides

Solid state 13C c.p.-m.a.s. and solution 13C, 15N, 14N, and 17O n.m.r. spectra have been measured for N,N-dimethyl-o-nitrobenzenesulphenamide, o-nitrobenzenesulphenamide, and N,N-dimethyl-onitrobenzenesulphonamide. The 13C c.p.-m.a.s. n.m.r. resonance signals of the methyl carbon atoms bonded to nitrogen show a characteristic doublet pattern. In addition, like the signal of N,N-dimethyltoluene-p-sulphonamide, the N-methyl resonance signal of N,N-dimethyl-o-nitrobenzenesulphenamide shows extra splitting. In the solution spectra the 15N nuclei of the amide group of the sulphonamides are considerably less shielded than those of the corresponding sulphenamides. The nitro-substituent of the aromatic ring has hardly any effect on the electron density of the nitrogen atom of the sulphonamide group.The crystal structures of N,N-dimethyl-o-nitrobenzenesulphenamide (1) and N,N-dimethyl-o-nitrobenzenesulphonamide (2) have been determined by the single crystal X-ray diffraction technique and refined to final R values 0.043 and 0.049, respectively. The configurations of these sulphur(II) and sulphur(VI) amides, with respect to the geometry around the sulphur atom and the orientation of the substituents in the phenyl ring, are different. The sulphenamide adopts crystallographic σ symmetry, with all atoms except the methyl groups lying in the crystallographic mirror plane. N,N-Dimethyl-o-nitrobenzenesulphonamide has no high molecular symmetry and there is clear steric repulsion between the NO2 group and the oxygen atoms of the SO2N group. The nitro group is rotated 53.5(2)° relative to the plane through the phenyl group. The amide nitrogen displays a pseudo-trigonal arrangement in both compounds.

Phospholipid packing and conformation in small vesicles revealed by two-dimensional 1H nuclear magnetic resonance cross-relaxation spectroscopy

Two-dimensional 1H-NMR spectroscopy has been used to examine cross-relaxation in sonicated phospholipid vesicle systems. The observed pattern of proton cross-relaxation reveals several important features of these vesicle systems. For example, cross-relaxation rates on each monolayer of the vesicle system can be resolved and reflect the expected geometric packing constraints of the vesicle system. Small but significant magnetization-exchange is also seen to develop between the headgroup N-methyl resonance and the terminal methyl resonance. Spectra taken with deuterated lipids indicate that this exchange is not mediated by spin-diffusion down the length of the alkyl chains. Since spin-diffusion is the only process that is expected to facilitate magnetization-exchange over distances of 15-20 A, a close proximity of headgroup and terminal methyl protons in a fraction of the membrane lipid is indicated by these results. This could occur by events such as lipid interdigitation or alkyl chain bends that terminate lipid alkyl chain ends near the membrane surface.

Chlorophyll a in bilayer membranes. II. Interaction with distearoylphosphatidylcholine by NMR

Bilayer membranes containing distearoylphosphatidylcholine and chlorophyll a have been investigated via 1H- and 31P-NMR. Measurements as a function of temperature and composition support the basic interpretation of the phase diagram for this mixed bilayer system advanced in the preceding report. The 31P-NMR spectrum of mixed vesicles above the solidus temperature of the phase diagram shows a single resonance, but below the solidus two peaks are observed. Observation of two distinct 31P resonances from the phospholipid at 46°C conclusively demonstrates that phase separation occurs at temperatures below the solidus. Observation of a 5.8 ppm upfield shift for one of the 31P resonances at low temperature provides strong evidence for an interaction between the phospholipid headgroup and chlorophyll a in one of the low-temperature phases. This conclusion is supported by the observation of increased 1H linewidths for the lipid and a longer 1H T 1 for the choline N-methyl resonance of the lipid below the solidus temperature, indicating that headgroups of lipid molecules in at least one of the phases are motionally restricted. The observation of an interaction between chlorophyll a and lipid molecules in a bilayer membrane lends considerable support to the idea that chlorophyll molecules in the chloroplast antenna are physically separated from each other by strongly coordinating lipid molecules.

The conformation of heterocyclic spiro compounds. VIII. 3,7-Dimethyl-3,7-diazabicyclo[3.3.1]nonane-9-spiro-5'-hydantoin

This structure determination is part of a study being carried out in this laboratory on heterocyclic spirohydantoin derivatives with pharmacological properties. The present compound was obtained by Trigo, Galvez & Avendafio (1978). The results of proton N M R studies seemed consistent with an adamantanelike structure, in agreement with Douglass & Ratliff (1968), Fig. 1. Support for this structure was derived from the position of the N-methyl resonance. However, the same authors admit that this fact does not constitute a definitive proof of this supposition. The present investigation was performed in order to verify the above statement and to elucidate the structure in the solid state.

Protonation and solvation in aqueous systems of N,N-dimethylalkylthioamides

The protonation of N,N-dimethylalkylthioamides in water—sulfuric acid media has been followed through nmr spectroscopy. Bunnett and Olsen treatment of the data gives (for protonation of the methyl derivative at 25 °C) pK = 2.7 and ø = 1.1. The ø value suggests protonation at the sulfur atom (ø sulfides = −0.26/−0.35) rather than at the nitrogen atom (ø amides = +0.42/+0.55) and wide delocalisation of the charge: ▪ On the other hand, two different N-methyl resonances are observed in water, which collapse into a single resonance upon protonation. This may suggest either a lowering of the rotational barrier upon protonation at nitrogen or protonation of sulfur with collapse of the signals due to a smaller N-methyl resonance difference in the protonated rather than in the free thioamide. Rational barriers have been calculated as a function of the medium acidity.

Cytotoxic compounds. Part 23. Reactions of the methanesulphonates of 2-(N-aryl-N-methylamino)propan-1-ols and of 1-(N-aryl-N-methylamino) propan-2-ols with nucleophiles

Syntheses of the primary alcohols p-RC6H4·NMe·CHMe·CH2·OH (from ethyl 2-bromopropanoate) and of the secondary isomers p-RC6H4·NMe·CH2·CHMe·OH (from propylene oxide) are described (R = H, Cl, OMe, or NO2). In solution, the primary methanesulphonates rearrange to the secondary isomers, a process facilitated by an electrondonating aryl substituent. Reactions of the primary and the secondary methanesulphonates with methanol, sodium methoxide, acetic acid, potassium acetate, tetramethylammonium acetate, sodium phenyl sulphide, sodium benzyl sulphide, lithium bromide, and sodium azide show that with weak nucleophiles the intermediate aziridinium ion is opened almost exclusively to give the secondary substitution product; with more powerful nucleophiles some sterically-controlled opening occurs to give also some primary product. When the formation of the aziridinium ion is retarded by the presence of an electron-attracting aryl substituent, strong nucleophiles can effect direct SN2 substitution on the methanesulphonates to give unrearranged products. The N-methyl resonances in the 1H n.m.r. spectra of the primary compounds are consistently at higher field than those of the secondary isomers.

Studies of Hindered Rotation in Some Group IV Esters of Dimethylcarbamic and Dimethylthiocarbamic Acids

Activation parameters for hindered rotation about the C—N bond have been obtained for some trimethylgermyl, trimethylplumbyl, and tert-butyl esters of N,N-dimethylcarbamic, N,N-dimethylmonothiocarbamic, and N,N-dimethyldithiocarbamic acids. These are compared with previous results for some group IV esters and general considerations as to factors influencing barriers to rotation are discussed. It was found that the degeneracy of the N-methyl resonances observed in the room or low temperature 1H spectra appeared also to extend to the room temperature 13C spectra. A 'steric' effect was noted on the carbonyl and thiocarbonyl resonances.

Proton‐Magnetic‐Resonance Studies of the Lysine Residues of Ribonuclease A

The amino groups of ribonuclease A (RNase-A) have been methylated with formaldehyde and borohydride to provide observable resonances for proton magnetic resonance (PMR) studies. Although enzymatic activity is lost, PMR difference spectroscopy and PMR studies of thermal denaturation show native conformation is largely preserved in methylated RNase-A. Resonances corresponding to the NH2-terminal alpha-amino and 10 xi-amino N-methyl groups are titrated at 220 MHz to obtain pK values. After correction for the effects of methylation, using values previously derived from model compound studies, a pK of 6.6 is found for the alpha-amino group, a pK of 8.6 for the xi-amino group of lysine-41 and pK values ranging from 10.6 to 11.2 for the other lysine xi-amino groups. Interactions between lysine-7 and lysine-41 or between the alpha-amino and xi-amino groups of lysine-1 have been proposed to account for deviations from simple titration behaviour. The correct continuities for the titration curves of the histidine H-2 proton resonances have been confirmed by selective deuteration of the H-2 protons. Titration curves for the H-2 proton resonances of histidine-12 and histidine-119 of methylated RNase-A show deviations from the titration curves for the native enzyme, indicating some alteration of the active-site conformation. In the presence of phosphate, titration curves for the H-2 proton resonances of histidine-12 and histidine-119 of methylated RNase-A indicate binding of phosphate at the active site, but these curves continue to show deviations from the titration behaviour of native RNase-A. The titration curve for the N-methyl resonance of lysine-41 is perturbed considerably by the presence of phosphate, which indicates a possible catalytic role for lysine-41.

Selective Simultaneous Proton Magnetic Resonance Labelling of Hydrophilic and Hydrophobic Regions of Phospholipid Vesicles. Effect of Polar and Apolar Reagents

Methyl-12-dimethylsilastearate was synthesized and incorporated by sonication into lecithin vesicles. In the resulting mixed vesicle the nuclear magnetic resonance line width of the Si-methyl resonance line is proportional to the lecithin–silastearate ratio, indicating interaction between the hydrocarbon chains of the phospholipid and the labelled fatty acid ester reporter molecule. The Si-methyl line reports selectively perturbations of the structure of hydrophobic regions as shown by addition of hydrocarbons. The N-methyl resonance line of lecithin is not affected by these changes, but indicates selectively effects on the hydrophilic exterior of the micelle produced by salts. The mixed vesicle may therefore be used as a membrane model allowing simultaneous observation of effects on the exterior and interior of the lipid bilayer.The silastearate label was successfully incorporated into Acholeplasma laidlawii membrane, which however failed to give a sufficiently resolved nuclear magnetic resonance spectrum.

NMR and EPR studies of the coordination of vanadyl perchlorate in dimethylformamide and dimethylacetamide solutions and the rate of exchange of axially coordinated ligands

The temperature dependence of the NMR linewidths of the solvent resonances in solutions of VO 2+ in dimethylformamide (DMF) and in dimethylacetamide (DMA) indicates that chemical exchange from axial and equatorial coordination sites can control the formyl proton relaxation in DMF, but axial exchange does not produce broadening of the methyl resonances in either DMF or DMA which could be distinguished from outer sphere broadening. The rates at 25 °C and activation parameters for equatorial exchange in DMA are 4.7× 103 sec−1, Δ Heq†= 10.1± 0.5 kcal/mole and Δ Seq†= −7.9± 1.6 eu, and in DMF are 2.0× 102 sec−1, Δ Heq†= 13.1± 0.6 kcal/mole and Δ Seq†= −4.1± 1.8 eu. The rate of axial exchange in DMF is 4.6× 104 sec−1 at 25°C, and the activation parameters are Δ Hax†= 15.3± 2.5 kcal/mole and Δ Sax†= 13.9± 9.0 eu. The rotational tumbling times of the vanadyl complex ions in DMF and in DMA were determined from EPR linewidth studies of VO 2+ in these solvents. The tumbling times were used to show that inner sphere broadening of the formyl proton resonance in DMF and the C-methyl resonance in DMA is dominated by dipole-dipole interactions, while the broadening of the N-methyl resonances is caused primarily by hyperfine interactions.

ON THE USE OF NUCLEAR MAGNETIC RESONANCE FOR THE DETECTION, ESTIMATION, AND CHARACTERIZATION OF N-METHYL GROUPS

A useful n.m.r. technique for the detection, estimation, and characterization of N-methyl groups is described. The method depends on the decreased shielding of Nbasic-methyl protons when the solvent is changed from deuterochloroform to perdeuteroacetic (PDA) (or acetic) acid to trifluoroacetic acid (TFA). The decreased shielding results in a downfield shift of the N-methyl resonance of ca. 0.3–1.3 p.p.m. depending on the type of amine. Most N-methyl groups on neutral nitrogen atoms show little movement. For the majority of aliphatic amines in trifluoroacetic acid the NH exchange rate is slow enough at 38° to permit observation of coupling of H(N) with the N-methyl protons, and thus determination of whether the amine is secondary or tertiary. In some cases information can be obtained about methylene and methine groups attached to basic nitrogen. In acid solution compounds with basic nitrogen attached to, or part of, an aromatic ring show considerable shift of the aromatic proton resonance signals to lower magnetic field. Combination of the n.m.r. technique with the Herzig-Meyer analysis can also reveal the number of methyl groups attached to neutral nitrogen.