A Comparative Study of Platinum Di (t-butyl) (15N2)diimine and trans-[(PtCl2(N-ligand)PBu3)] complexes by 195Pt-, 31P- and 15N-NMR

¹⁹⁵Pt-, ³¹P- and ¹⁵N-NMR. data are presented for [PtCl₂ (t-Bu ¹⁵N?CH? CH?¹⁵N (t-Bu)) (η²-styrene)] ( 1 ), trans-[{PtCl₂ (PBu₃)}₂ (t-Bu ¹⁵N?CH? CH?¹⁵N (t-Bu))] ( 2 ), trans-[PtCl₂ (t-Bu ¹⁵N?CH? CH?¹⁵N (t-Bu)) (PBu₃)] ( 3 ) and various complexes of the type trans-[PtCl₂ (N-ligand) (PBu₃)]. In solution the gross structural features of 1 and 2 are shown to be in agreement with those found in the solid state; namely, 1 contains five-coordinate Pt and 2 is dinuclear. In 3 Pt is four-coordinate with only one N-atom of the diimine ligand being coordinated at ?50° in CD₂Cl₂. The NMR. data for 2 and 3 are compared with those of trans-[PtCl₂ (N-ligand) (PBu₃)] (N-ligand = pyridine, 2, 6-dimethylpyridine, (¹⁵N)-hexylamine, (¹⁵N)-t-butylamine and (¹⁵N)-aniline) and are discussed in terms of the donor and acceptor properties of the N-ligands.     

Hyperpolarization of cis-15N2-Azobenzene by Parahydrogen at Ultralow Magnetic Fields**

The development of nuclear spins hyperpolarization, and the search for molecules that can be efficiently hyperpolarized is an active area in nuclear magnetic resonance. In this work we present a detailed study of SABRE SHEATH (signal amplification by reversible exchange in shield enabled alignment transfer to heteronuclei) experiments on ¹⁵N₂-azobenzene. In SABRE SHEATH experiments the nuclear spins of the target are hyperpolarized through transfer of spin polarization from parahydrogen at ultralow fields during a reversible chemical process. Azobenzene exists in two isomers, trans and cis. We show that all nuclear spins in cis-azobenzene can be efficiently hyperpolarized by SABRE at suitable magnetic fields. Enhancement factors (relative to 9.4 T) reach up to 3000 for ¹⁵N spins and up to 30 for the ¹H spins. We compare two approaches to observe either hyperpolarized magnetization of ¹⁵N/¹H spins, or hyperpolarized singlet order of the ¹⁵N spin pair. The results presented here will be useful for further experiments in which hyperpolarized cis-¹⁵N₂-azobenzene is switched by light to trans-¹⁵N₂-azobenzene for storing the produced hyperpolarization in the long-lived spin state of the ¹⁵N pair of trans-¹⁵N₂-azobenzene.     

Ring-Transformationen bei der Umsetzung von 3-(Dimethylamino)-2,2-dimethyl-2H-azirin mit 1-substituierten Imidazolidin-2,4,5-trionen

Ring-Transformations in the Reaction of 3-(Dimethylamino)-2,2-dimethyl-2H-azirines with 1-Substituted Imidazolidine-2,4,5-triones Reaction of 1-substituted imidazolidine-2,4,5-triones ( = N-substituted parabanic acids; 2 ) and 3-(dimethylamino)-2,2-dimethyl-2H-azirine ( 1 ) in i-PrOH or MeCN at room temperature yields 5,6,7,7a-tetrahydro-3H-imidazo[3,4-a]imidazole-5,7-diones 3 (Scheme 1). By ¹⁵N-NMR studies, using (3-¹⁵N)- 2a , it has been shown that only N( 1 ) in (¹⁵N)- 3a is labelled and, hence, N(4) stems from 1 , e.g. the azirine reacts via cleavage of the N(1)=C(3) bond. In MeCN at room temperature, the azacyclols 3 rearrange slowly to give monocyclic 2H, 5H-imidazol-2-ones 4 (Scheme 3); the ¹⁵N-label in (¹⁵N)- 4a is in position 1. Both reactions proceed via deep-seated skeletal rearrangements, most probably via ring-expansion/ring-contraction processes.     

2D relayed15N,1H correlated NMR spectroscopy of a pentadecapeptide at natural abundance of15N

Summary Proton-detected H-relayed N,H correlation NMR spectroscopy at natural abundance of¹⁵N has been used to demonstrate the enormous value of heteronuclear NMR spectroscopy for the proton assignment of medium-sized oligopeptides.

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Zweidimensionalerelayed ¹⁵N,¹H-korrelierte NMR-Spektroskopie an einem Pentadecapeptid bei natürlicher Häufigkeit von¹⁵N

Zusammenfassung Anhand¹H-detektierterrelayed-N,H-korrelierter NMR-Spektroskopie bei natürlicher Häufigkeit von¹⁵N wird die große Bedeutung der heteronuklearen Kernresonanzspektroskopie für die Protonenzuordnung mittlerer Oligopeptide demonstriert.

     

Hyperpolarizing Concentrated Metronidazole 15NO2 Group over Six Chemical Bonds with More than 15 % Polarization and a 20 Minute Lifetime

The NMR hyperpolarization of uniformly ¹⁵N-labeled [¹⁵N₃]metronidazole is demonstrated by using SABRE-SHEATH. In this antibiotic, the ¹⁵NO₂ group is hyperpolarized through spin relays created by ¹⁵N spins in [¹⁵N₃]metronidazole, and the polarization is transferred from parahydrogen-derived hydrides over six chemical bonds. In less than a minute of parahydrogen bubbling at approximately 0.4 μT, a high level of nuclear spin polarization (P_15N) of around 16 % is achieved on all three ¹⁵N sites. This product of ¹⁵N polarization and concentration of ¹⁵N spins is around six-fold better than any previous value determined for ¹⁵N SABRE-derived hyperpolarization. At 1.4 T, the hyperpolarized state persists for tens of minutes (relaxation time, T₁≈10 min). A novel synthesis of uniformly ¹⁵N-enriched metronidazole is reported with a yield of 15 %. This approach can potentially be used for synthesis of a wide variety of in vivo metabolic probes with potential uses ranging from hypoxia sensing to theranostic imaging.     

Darstellung,Kristallstrukturen und Schwingungsspektren von [Pt(N3)6]2– und [Pt(N3)Cl5]2–, 195Pt‐ und 15N‐NMR‐Spektren von [Pt(N3)nCl6–n]2– und [Pt(15NN2)n(N215N)6–n]2–, n = 0–6

Synthesis, Crystal Structures, and Vibrational Spectra of [Pt(N₃)₆]^2– and [Pt(N₃)Cl₅]^2–, ¹⁹⁵Pt and ¹⁵N NMR Spectra of [Pt(N₃)_nCl_6–n]^2– and [Pt(¹⁵NN₂)_n(N₂¹⁵N)_6–n]^2–, n = 0–6 By ligand exchange of [PtCl₆]^2– with sodium azide mixed complexes of the series [Pt(N₃)_nCl_6–n]^2– and with ¹⁵N‐labelled sodium azide (Na¹⁵NN₂) mixtures of the isotopomeres [Pt(¹⁵NN₂)_n(N₂¹⁵N)_6–n]^2–, n = 0–6 and the pair [Pt(¹⁵NN₂)Cl₅]^2–/[Pt(N₂¹⁵N)Cl₅]^2– are formed. X‐ray structure determinations on single crystals of (Ph₄P)₂[Pt(N₃)₆] ( 1 ) (triclinic, space group P1, a = 10.175(1), b = 10.516(1), c = 12.380(2) Å, α = 87.822(9), β = 73.822(9), γ = 67.987(8)°, Z = 1) and (Ph₄As)₂[Pt(N₃)Cl₅] · HCON(CH₃)₂ ( 2 ) (triclinic, space group P1, a = 10.068(2), b = 11.001(2), c = 23.658(5) Å, α = 101.196(14), β = 93.977(15), γ = 101.484(13)°, Z = 2) have been performed. The bond lengths are Pt–N = 2.088 ( 1 ), 2.105 ( 2 ) and Pt–Cl = 2.318 Å ( 2 ). The approximate linear azido ligands with N^α–N^β–N^γ‐angles = 173.5–174.6° are bonded with Pt–N^α–N^β‐angles = 116.4–121.0°. In the vibrational spectra the PtCl stretching vibrations of (n‐Bu₄N)₂[Pt(N₃)Cl₅] are observed at 318–345, the PtN stretching modes of (n‐Bu₄N)₂[Pt(N₃)₆] at 401–428 and of (n‐Bu₄N)₂[Pt(N₃)Cl₅] at 408–413 cm^–1. The mixtures (n‐Bu₄N)₂[Pt(¹⁵NN₂)_n(N₂¹⁵N)_6–n], n = 0–6 and (n‐Bu₄N)₂[Pt(¹⁵NN₂)Cl₅]/(n‐Bu₄N)₂[Pt(N₂¹⁵N)Cl₅] exhibit ¹⁵N‐isotopic shifts up to 20 cm^–1. Based on the molecular parameters of the X‐ray determinations the vibrational spectra are assigned by normal coordinate analysis. The average valence force constants are f_d(PtCl) = 1.93, f_d(PtN^α) = 2.38 and f_d(N^αN^β, N^βN^γ) = 12.39 mdyn/Å. In the ¹⁹⁵Pt NMR spectrum of [Pt(N₃)_nCl_6–n]^2–, n = 0–6 downfield shifts with the increasing number of azido ligands are observed in the range 4766–5067 ppm. The ¹⁵N NMR spectrum of (n‐Bu₄N)₂[Pt(¹⁵NN₂)_n(N₂¹⁵N)_6–n], n = 0–6 exhibits by ¹⁵N–¹⁹⁵Pt coupling a pseudotriplett at –307.5 ppm. Due to the isotopomeres n = 0–5 for terminal ¹⁵N six well‐resolved signals with distances of 0.03 ppm are observed in the low field region at –201 to –199 ppm.     

15N-Markiertes 3-(Dimethylamino)-2,2-dimethyl-2H-azirin zur mechanistischen Untersuchung von Reaktionen mit NH-aciden Heterocyclen

₁₅N-Labelled 3-(Dimethylamino)-2,2-dimethyl-2H-azirine for Mechanistic Studies of Reactions with NH-Acidic Heterocycles The synthesis of 3-(dimethylamino)-2,2-dimethyl(1-¹⁵N)-2H-azirine ( 1 *) was accomplished via reaction of 1-chloro-N,N,2-trimethyl-1-propenylamine ( 9 ) and sodium (1-¹⁵N) azide (Scheme 3). The earlier reported reactions of 1 with saccharin ( 10 , Scheme 4), phthalimide ( 12 , Scheme 5), and 2H-1,3-benzoxazin-2,4(3H)-dione ( 16 , Scheme 6) were repeated with 1 *, and the position of the ¹⁵N-label in the products was determined by ¹⁵N-NMR spectroscopy. Whereas the postulated reaction mechanisms for 10 and 12 were confirmed by these experiments, the mechanism for the reaction of 16 had to be revised. With respect to the position of ¹⁵N in the products 17 and 18 , a new mechanism is formulated in Scheme 7. Treatment of 5,5-dimethyl-1,3-oxazolidine-2,4-dione ( 19 ) with 1 * led to 3,4-dihydro-2H-imidazol-2-on 20 in which only N(3) was labelled. The mechanism of a ring expansion and transannular ring contraction as shown in Scheme 8 is in agreement with this finding.     

Boron heterocycles derived from 2-guanidinobenzimidazole

The syntheses and structure determinations of a series of boron heterocycles derived from 2-guanidinobenzimidazole 1 are reported. Structures of new compounds, 2-guanidino-1-methyl-benzimidazole 2, diphenyl-(2-guanidinobenzimidazole-N,N′)-borate 3, diphenyl-(2-guanidino-1-methyl-benzimidazole-N,N′)borate 4, hydroxy-phenyl-(2-guanidino-benzimidazole-N,N′)borate 5, hydroxy-phenyl-(2-guanidino-1-methyl-benzimidazole-N,N′)borate 6,methoxy-phenyl-(2-guanidinobenzimidazole-N,N′)borate 7, isopropoxy-phenyl-(2-guanidinobenzimidazole-N,N′)borate 8, acetoxy-phenyl-(2-guanidinobenzimidazole-N,N′)borate 9, methoxy-phenyl-(2-guanidino-1-methyl-benzimidazole-N,N′)borate 10, dihy-droxy-(2-guanidino-1-methyl-benzimidazole-N,N′)borate 16, difluoro-(2-guanidinobenzimidazole-N,N′)borate, 17, dihydroxy-(2-guanidino-1-benzimidazole-N,N′)borate potassium salt 19, diphenyl-(2-guanidinium-10H-benzimidazole-N,N′)borate hydro-chloride 20, methoxy-phenyl-(2-guanidinobenzimidazole-N,N′)borate hydrochloride 21, and N10-borane-(diphenyl-2-guanidinobenzimidazole-N,N′)borate 22, were determined based on ¹H, ¹³C, ¹⁵N, and ¹¹B spectroscopy. The X-ray diffraction structures of 3–7, 19, and 20 were obtained. The formation of N3-borane adducts 11 and 12 derived from compounds 1 and 2, respectively, and the dihydride-(2-guanidinobenzimidazole-N,N′)borate 13 and dihydride-(2-guanidino-1-methyl-benzimidazole-N,N′)borate 14 were observed by ¹¹B NMR. The results show that 2-guanidinobenzimidazole gives stable borate heterocycles with a delocalized π electronic system. A dynamic exchange of N–H protons was observed with preferred protonation at N-12. The new heterocycles are protonated at N-10 by acidic substances to give pyridinium-type heterocycles or can lose a proton to give iminium salts. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:399–409, 1998     

Terminal Diazirines Enable Reverse Polarization Transfer from 15N2 Singlets

Diazirine moieties are chemically stable and have been incorporated into biomolecules without impediment of biological activity. The ¹⁵N₂ labeled diazirines are appealing motifs for hyperpolarization supporting relaxation protected states with long‐lived lifetimes. The (‐CH¹⁵N₂) diazirine groups investigated here are analogues to methyl groups, which provides the opportunity to transfer polarization stored on a relaxation protected (‐CH¹⁵N₂) moiety to ¹H, thus combining the advantages of long lifetimes of ¹⁵N polarization with superior sensitivity of ¹H detection. Despite the proximity of ¹H to ¹⁵N nuclei in the diazirine moiety, ¹⁵N T₁ times of up to (4.6±0.4) min and singlet lifetimes T_s of up to (17.5±3.8) min are observed. Furthermore, we found terminal diazirines to support hyperpolarized ¹H₂ singlet states in CH₂ groups of chiral molecules. The singlet lifetime of ¹H singlets is up to (9.2±1.8) min, thus exceeding ¹H T₁ relaxation time (at 8.45 T) by a factor of ≈100.     

Nitrogen-15 coupling constants: Geminal coupling, 2J(15NH), in cis- and trans-aldonitrones and vicinal coupling, 3J(15NH), in cis-and trans-ketimines,oxaziridines and nitrones

Nitrogen-15-hydrogen ²J(¹⁵NH), ¹⁵N, ¹³C ¹J(¹⁵N¹³C) and ¹³C, H ¹J(¹³CH) coupling constants have been measured and their signs determined for cis-and trans-[9-anthryl(¹³C)methylene](²H₃)methylamine (¹⁵N)-oxide. Values of ²J(¹⁵NH) were of similar magnitude (～2 Hz) but were of opposite sign. The results are compared and contrasted with those reported for related imino and quaternary imino systems. Vicinal ³J(¹⁵NH) coupling constants have been measured in cis- and trans-¹⁵N-[1-(α-naphthyl)ethylidene]benzylamine and ¹⁵N-[1-(p-nitrophenyl)ethylidene]-t-butylamine and were found to be larger when the imino methyl group was cis to the nitrogen lone pair. The corresponding cis and trans ketonitrones formed by photoisomerization of the derived oxaziridines had ³J(¹⁵NH) values of c. 3.3 Hz. A study of the signs and magnitudes of observed and calculated ¹⁵N,H coupling constants for all of the ¹⁵N labelled imines, oxaziridines, imine N-oxides (nitrones) and similar model systems which were synthesized is described.     

Characterization of 2‐aryl‐1,3,4‐oxadiazoles by 15N and 13C NMR spectroscopy

¹⁵N NMR chemical shifts of 2‐aryl‐1,3,4‐oxadiazoles were assigned on the basis of the ¹H–¹⁵N HMBC experiment. Chemical shifts of the nitrogen and carbon atoms in the oxadiazole ring correlate with the Hammett σ‐constants of substituents in the aryl ring (r² ≥ 0.966 for N atoms). ¹⁵N NMR data are a suitable and sensitive means for characterizing long‐range electronic substituent effects. Additionally, ¹³C NMR data for these compounds are presented. Copyright © 2003 John Wiley & Sons, Ltd.     

Regioselectively [15NO2]-labeled N-methoxypicramide and DPPH prepared by using crown ether and solid sodium [15N]nitrite

The present paper reports the regioselective [¹⁵NO₂]-labeling of N-methoxy-2,4,6-trinitroaniline and 2,2-diphenyl-1-picrylhydrazine (reduced DPPH). Starting from N-methoxy-2,6-dinitroaniline, or N-methoxy-2,4-dinitroaniline, nitration in methylene chloride with solid sodium [¹⁵N]nitrite and 15-crown-5-ether afforded N-methoxy-2,6-dinitro-4-[¹⁵N]nitroaniline and N-methoxy-2,4-dinitro-6[¹⁵N]nitroaniline, respectively. The same compounds could be prepared in higher purity by nitrodecarboxylation (ipso-substitution) under the same conditions starting from N-methoxy-4-carboxy-2,6-dinitroaniline (4-methoxyamino-3,5-dinitrobenzoic acid) and N-methoxy-2-carboxy-4,6-dinitroaniline (2-methoxyamino-3,5-dinitrobenzoic acid). Similarly,ipso-substitution of 2,2-diphenyl-1-(4-carboxy-2,6-dinitrophenyl)-hydrazine afforded, under the same reaction conditions, 2,2-diphenyl-1-(2,6-dinitro-4-[¹⁵N]nitrophenyl)-hydrazine. By¹H-NMR and¹³C-NMR it was also observed that under these reaction conditions a¹⁴NO₂ group can be replaced by a¹⁵NO₂ group.     

Acid-catalyzed attack of the dinitrogen ligand in thecis-(Me2PhP)4Mo(N2)2 andtrans-(Ph2PCH2CH2PPh2W(N2)2 complexes by nitronium and nitrosonium cations with the formation of nitrogen oxides

The interaction of labeled dinitrogen complexescis-(Me₂PhP)₄Mo(¹⁵N₂)₂ andtrans-(dppe)₂W(¹⁵N₂)₂ with non-labeled nitronium and nitrosonium fluoroborates,¹⁴NO₂BF₄ and¹⁴NOBF₄, in sulfolane at room temperature in the presence of H₂SO₄ results in rapid formation of labeled nitrous and nitric oxides (¹⁵N¹⁴NO,¹⁵NO), as well as¹⁵N¹⁴N. The yield of the products depends on the reagent ratio and reaches 10–20 mol. % per mole of a complex under optimum conditions. The mechanism of the reactions found is proposed. It involves the step of protonation of the dinitrogen ligand to form the corresponding hydrazido(2–) derivatives, which are then attacked by nitronium or nitrosonium cations. In accordance with the mechanism proposed, it was established that the hydrazido(2–) complexes, (Me₂PhP)₃Mo(¹⁵N₂H₂)Cl₂ and (dppe)₂W(¹⁵N₂H₂)Cl₂, are capable of forming¹⁵N¹⁴NO,¹⁵NO, and¹⁵N¹⁴N under the action of¹⁴NO₂BF₄ and¹⁴NOBF₄ in the absence of an acid.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 13–13, July, 1995.     

One-bond 1J(15N,H) coupling constants at sp2-hybridized nitrogen of Schiff bases,enaminones and similar compounds: A theoretical study

¹J(¹⁵N,H) coupling constants for enaminones and NH-forms of intramolecularly hydrogen-bonded Schiff bases as model compounds for sp²-hybridized nitrogen atoms are evaluated using density functional theory (DFT) to find the optimal functionals and basis sets. Ammonia is used as a test molecule and its one-bond coupling constant is compared with experiment. A methylamine Schiff base of a truncated molecule of gossypol is used for checking the performance of selected B3LYP, O3LYP, PBE, BHandH, and APFD density functionals and standard, modified, and dedicated basis sets for coupling constants. Both in vacuum and in chloroform, modeled by the simple continuum model of solvent, the modified basis sets predict significantly better the ¹J(¹⁵N,H) value in ammonia and in the methylamine Schiff base of a truncated molecule of gossypol than the standard basis sets. This procure is then used on a broad set of intramolecularly hydrogen-bonded molecules, and a good correlation between calculated and experimental one-bond NH coupling constants is obtained. The ¹J(¹⁵N,H) couplings are slightly overestimated. The calculated data show for hydrogen-bonded NH interatomic distances that the calculated values depend on the NH bond lengths. The shorter the bond lengths, the larger the ¹J(¹⁵N,H). A useful correlation between ¹J(¹⁵N,H) and NH bond length is derived that enables realistic predictions of one-bond NH coupling constants. The calculations reproduce experimentally observed trends for the studied molecules.     

Synthesis of Selectively 15N‐Labeled 2′‐O‐{[(Triisopropylsilyl)oxy]methyl}(=tom)‐Protected Ribonucleoside Phosphoramidites and Their Incorporation into a Bistable 32Mer RNA Sequence

We present optimized reaction conditions for the conversion of 2′‐O‐{[(triisopropylsilyl)oxy]methyl}(=tom) protected uridine and adenosine nucleosides into the corresponding protected (3‐¹⁵N)‐labeled uridine and cytidine and (1‐¹⁵N)‐labeled adenosine and guanosine nucleosides 4, 6, 12 , and 18 , respectively (Schemes 1–4). On a DNA synthesizer, the resulting ¹⁵N‐labeled 2′‐O‐tom‐protected phosphoramidite building blocks 19 – 22 were efficiently incorporated into five selected positions of a bistable 32mer RNA sequence 23 (known to adopt two different structures) (Fig. 1). By 2D‐HSQC and HNN‐COSY experiments in H₂O/D₂O 9 : 1, the ¹⁵N‐signals of all base‐paired ¹⁵N‐labeled nucleotides could be identified and attributed to one of the two coexisting structures of 23 .     

The effects of differentiated water supply after anthesis and nitrogen fertilization on δ15N of wheat grain

The δ¹⁵N signature of plants integrates various processes in soil and plant. In this study, the effect of different water regimes applied during the period of grain growth of winter wheat on grain δ¹⁵N was examined in a 4‐year field experiment. The treatments comprised water shortage (S), an ample water supply (W), and rain‐fed crop (R). Zero fertilization (N0) and 200 kg N.ha^?1 in mineral fertilizer (N1) treatments were studied. The grain ¹⁵N was determined during grain growth and at maturity. The water regime, nitrogen application and year had a significant effect on mature grain δ¹⁵N (p < 0.001). Water and nitrogen explained 54.6% of the variability of δ¹⁵N in the experiment, the year accounted for 10.7% and the interactions for another 19.6% of the total variability. The analysis of non‐mature grain δ¹⁵N showed significant effects of N and year but not of water. Nitrogen fertilization reduced the δ¹⁵N of mature grain in years by 0.7–6.3‰ in comparison with N0 plants; the reduction was more pronounced under stress (average reduction by 4.1‰) than under rain‐fed (2.4‰) and ample water supply (2.2‰). Water stress decreased the grain δ¹⁵N in fertilized wheat, by 0.1–2.1‰ and 0.6–3.6‰ in experimental years, on average by 1.30‰ and 1.79‰ in comparison with the R and W water supply, respectively. The effect of water supply was not significant in non‐fertilized wheat. A significant negative linear relationship between grain N concentration and δ¹⁵N in maturity or during the grain growth (R² = 0.83, R² = 0.76, respectively) was found. The observed sources of grain δ¹⁵N variability should be taken into consideration when analyzing and interpreting the data on the δ¹⁵N signature of plant material from field conditions. Copyright © 2010 John Wiley & Sons, Ltd.     

Electrospray-Ionization Mass Spectrometry. Part III. Acid-Catalyzed Isomerization of N,N′-Bis[(E)-3-(4-hydroxyphenyl)prop-2-enoyl]spermidines by the Zip Reaction

The electrospray tandem mass spectra (ESI-MS/MS) of the three N,N′-bis[(E)-3-(4-hydroxyphenyl)prop-2-enoyl]spermidines 1–3 displayed the same fragment-ion signals. These isomers could not be differentiated by ESI-MS/MS, since their fragmentation patterns are similar. (E,E)-N-(3-[¹⁵N]Aminopropyl)-3,3′-bis(4- hydroxyphenyl)-N,N′-(butane-1,4-diyl)bis[prop-2-enamide] ([¹⁵N(1)])-( 1 ) was synthesized in order to get further information about the fragmentation mechanisms. The comparison of the ESI-MS/MS of 1 and [¹⁵N(1)]- 1 revealed a transamidation, the Zip reaction, under mass-spectral conditions of the [ 1 + H]⁺ ions. Because of this reaction, the three isomers 1–3 could not be distinguished.     

Structure and dynamics of silk fibroin studied with 13C, 15N and 2H solid state NMR

[1-¹³C]Gly, L-[1-¹³C]Ala, [¹⁵N]Gly, L-[¹⁵N]Ala, [2,2-²H₂]Gly, L-[3,3-²H₂]Ser and [3,3,3-²H₃]Ala labeled silk fibroin fibers from Bombyx mori and Samia cynthia ricini silkworms were prepared in order to analyze structure of backbone and dynamics of side chain. The torsion angles ϕ and Ψ were determined from the angular dependent ¹³C and ¹⁵N solid state NMR spectra for uniaxially oriented fiber samples. In addition, the characteristic side chain dynamics of Ser residue determined from solid state ²H NMR measurements was compared with those of Ala and Gly residues.     

15N Chemically Induced Dynamic Nuclear Polarization (15N‐CIDNP) Investigations of the Peroxynitrite Decay and Nitration of N‐Acetyl‐L‐tyrosine

During the decay of (¹⁵N)peroxynitrite (O?¹⁵NOO ^? ) in the presence of N‐acetyl‐L ‐tyrosine (Tyrac) in neutral solution and at 268 K, the ¹⁵N‐NMR signals of ¹⁵NO and ¹⁵NO show emission (E) and enhanced absorption (A) as it has already been observed by Butler and co‐workers in the presence of L ‐tyrosine (Tyr). The effects are built up in radical pairs [CO , ¹⁵NO ]^S formed by O? O bond scission of the (¹⁵N)peroxynitrite? CO₂ adduct (O?¹⁵NO? OCO ). In the absence of Tyrac and Tyr, the peroxynitrite decay rate is enhanced, and ¹⁵N‐CIDNP does not occur. This is explained by a chain reaction during the peroxynitrite decay involving N₂O₃ and radicals NO ^. and NO . The interpretation is supported by ¹⁵N‐CIDNP observed with (¹⁵N)peroxynitrite generated in situ during reaction of H₂O₂ with N‐acetyl‐N‐(¹⁵N)nitroso‐dl ‐tryptophan ((¹⁵N)NANT) at 298 K and pH 7.5. In the presence of Na¹⁵NO₂ at pH 7.5 and in acidic solution, ¹⁵N‐CIDNP appears in the nitration products of Tyrac, 1‐(¹⁵N)nitro‐N‐acetyl‐L ‐tyrosine (1‐¹⁵NO₂‐Tyrac) and 3‐(¹⁵N)nitro‐N‐acetyl‐L ‐tyrosine (3‐¹⁵NO₂‐Tyrac). The effects are built up in radical pairs [Tyrac ^. , ¹⁵NO ]^F formed by encounters of independently generated radicals Tyrac ^. and ¹⁵NO . Quantitative ¹⁵N‐CIDNP studies show that nitrogen dioxide dependent reactions are the main if not the only pathways for yielding both nitrate and nitrated products.     

Analysis of fine splittings in the fourier transform 1H magnetic resonance spectra of some 15N labeled borazine derivatives

Proton NMR spectra are reported for ¹⁵N enriched borazine and a series of ¹⁵N enriched derivatives: N-methyl-borazine, N,N′-dimethylborazine and a new photochemical product, 1-methyl-2-aminoborazine. Chemical shifts for the ring (¹⁵N? H) protons have been measured. Using a Fourier transform spectrometer, fine structure in the ¹⁵N? H doublet is resolved. Ortho and meta ring proton and three-bond ¹⁵N to H coupling constants have been determined. Substituent effects on chemical shifts and coupling constants for borazine derivatives are compared with those for analogous benzene derivatives.