15N labelling

Synthesis of this compound from a 15N labelled source revealed that the i4N and 15N were equally distributed between the apical bent nitrosyl (NO-) and equatorial linear nitrosyl (NO+) ... 

Hydrazino compounds can react with one or two equivalents of arenediazonium ions. In reactions of arylhydrazines without substituents at the P-nitrogen in mineral acid media the initial product, the 1,4-diaryltetraz-1-ene (6.24), disproportionates rapidly into aryl azide and amine (Scheme 6-17). As shown by 15N labeling experiments (Clusius and Craubner, 1955), equal amounts of all the products shown in Scheme 6-17 are obtained. In acetate buffer the reaction is regiospecifically different. The diazonium ion attacks the a-, not the P-nitrogen, and a 1,3-diaryltetraz-l-ene is formed (6.25). 

There are two mechanisms that are consistent with the experimental data. They are summarized in Scheme 6-23, which shows first of all the sequence diazonium ion - diazoazide - phenyl azide for the major process (M). The second process involves the phenylpentazole as intermediate. There are two possible pathways for this process. Both are consistent with the kinetics and the results of 15N labeling experiments. One was proposed by Huisgen (Huisgen and Ugi, 1957 Huisgen 1984, p. 152), the other by Ugi (1963). In Scheme 6-23 the letters H and U indicate the two... 

The influence of substituents on the solvolysis of benzenediazonium ions in water and in TFE is very similar. This fact suggested that the mechanism is the same in both solvents. Exchange of 3-15N-labeled 4-methoxy- and 2,4,6-trimethyl-benzene-... 

SCHEME 9.13 A dissociative mechanism releases the 15N-labeled dA from its QM adduct. 

In collaboration with Professor Collum and coworkers, 6Li NMR (including 13C-labeled acetylene 37 and 15N-labeled chiral modifier 46 experiments) and Li aggregation studies were implemented to assist in the understanding of some of the factors responsible for the stereoselective nature of this chemistry [35],... 

An advantage of NMR spectroscopy is the analysis of protein dynamics. Measurement and analysis of the relaxation parameters R1 R2, and the 15N NOE of 15N-labeled proteins leads to an order parameter (S2) that can describe the relative mobility of the backbone of the protein. Both collagenase-1 and stromelysin-1 have been studied either as inhibited complexes or the free protein [19, 52], Stromleysin-1 was studied with inhibitors binding to prime or nonprime subsites. Presence or absence of inhibitors in the nonprime sites had minor effects on the highly ordered structure of residues in these subsites, which are in contact with the... 

Boucher W, Laue ED, Campbell SL, Domaille PJ. Improved 4D NMR experiments for the assignment of backbone nuclei in 13C/15N labelled proteins. J Biomol NMR 1992 2 631-637. 

Fig. 8. Amide region of XH NMR spectra for XAO peptide. Each NH peak in the spectrum of XAO is cleanly resolved and allows accurate determination of the coupling constants. The values of 3/hn for A2, A3, A4, and A5 can be determined from unlabeled sample or 15N-labeled sample (with 15N decoupling in the pulse sequence), as seen from the lower panel those for A6 and A7 can be obtained from the sample XA026 (15N-labeled at 2 and 6 positions) without 15N decoupling in the pulse sequence, as seen from upper panel. From Shi et al. (2002). Proc. Natl Acad. Sci. USA 99, 9190-9195, 2002 National Academy of Sciences, USA.

Backbone dynamics are most commonly investigated by measurement of 15N T and T% relaxation times and the fyH -15N NOE in uniformly 15N-labeled protein. To circumvent problems associated with the limited dispersion of the NMR spectra of unfolded proteins, the relaxation and NOE data are generally measured using 2D HSQC-based methods (Farrow et al., 1994 Palmer et al., 1991). 

Ammonia, Ammonium nitrate, Diphenyl carbonate Buzard, J. et al., J. Amer. Chem. Soc., 1952, 74, 2925-2926 During the synthesis of 15N-labelled urea by interaction of labelled ammonium nitrate, liquid ammonia and diphenyl carbonate in presence of copper powder, a series of explosions of the refrigerated sealed tubes was encountered. This was almost certainly caused by condensation of traces of oxygen in the tubes cooled to — 196°C dining condensation of ammonia before sealing the tubes. Cooling to —80°C would have been adequate and have avoided the hazard. 

The 140-residue protein AS is able to form amyloid fibrils and as such is the main component of protein inclusions involved in Parkinson s disease. Full-length 13C/15N-labelled AS fibrils and AS reverse-labelled for two of the most abundant amino acids, K and V, were examined by homonuclear and heteronuclear 2D and 3D NMR.147 Two different types of fibrils display chemical shift differences of up to 13 ppm in the l5N dimension and up to 5 ppm for the backbone and side-chain 13C chemical shifts. Selection of regions with different mobility indicates the existence of monomers in the sample and allows the identification of mobile segments of the protein within the fibril in the presence of monomeric protein. At least 35 C-terminal residues are mobile and lack a defined secondary structure, whereas the N terminus is rigid starting from residue 22. In addition, temperature-dependent sensitivity enhancement is also noted for the AS fibrils due to both the CP efficiency and motional interference with proton decoupling.148... 

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