15N chemical shifts

Finally, we should note that a particularly important area of application where density functional techniques, in spite of the deficiencies noted above, are virtually without competition is provided by biochemically relevant molecules, such as enzymes or nucleic acids. The techniques discussed in this section are virtually the only quantum chemical methods which can be applied in this context due to their outstanding price/performance ratio. For example, the 13C and 15N chemical shifts in bacteriochlorophyll A have been studied by Facelli, 1998, and in another investigation the 57Fe, 13C and 170 shifts in iron porphyrin derivatives gave important clues as to the structural details of these species, as shown by McMahon et al 1998. 

Table 11.2 15N chemical shifts of some common nitrogen-bearing compounds... 

Formation of the adduct with dirhodium complex [6] has significantly changed the chemical shift of the compounds in which proton transfer occurred, for example, for the adduct of iV-(5-nitrosalicylidene)-2-ami nobutane, the chemical shift was —198.2, which was shielded by of 110 ppm in comparison to that in the starting Schiff base.12 The large signal shift was due to the shift of the proton transfer equilibrium towards NH tautomer. For the adduct of N-(salicylidene)-2-aminobutane, existing in the OH-form, the 15N chemical shift has changed from —87.5 to — 84.6 ppm. 

The 15N chemical shifts measured for the Schiff base being a derivative of gossypol [7] and L-phenylalanine methyl ester equal —243.7 ppm in CDC13 solution and —237.5 in solid state (relative to external... 

Deuterium isotope effects on 1SN chemical shifts Deuterium isotope effects on 15N chemical shifts AN(D), similarly as for AC(D), can be employed in determination of mole fraction of the proton transferred form Schiff bases.44 A similar correlation between the mole fraction of NFI-form and AN(D) values was found (Figure 2). For the Schiff bases in which proton transfer takes place, the AN(D) values varied from —2 to + 5 ppm and depend on solvent and temperature. The AN(D) values of... 

Deuterium isotope effects on 15N chemical shift in CDC13 solution as well as in solid state were measured for a series of symmetrical and unsymmetrical di-Schiff bases being derivatives of fra s-l,2-diaminocy-clohexane and various aromatic ort/io-hydroxy-aldehydes [22],57 The AN (D) value determined in solid state for symmetrical di-Schiff base which was a derivative of salicylaldehyde was —1.8 ppm, which was typical of... 

The position of the proton transfer equilibrium for the Schiff bases being derivatives of rac-2-aminobutane [24] or rac-a-methylbenzylamine [25] and their adducts with dirhodium complex has been estimated in CDCI3 solution on the basis of measurements of deuterium isotope effects on 15N chemical shift.12 It was shown that adduct formation significantly influenced the position of the equilibrium which was manifested by AN(D) values. 

Dynamic nuclear polarisation (DNP) enhanced 15N CP MAS NMR has been exploited by Mark-Jurkauskas et al.79 in the studies of intermediates of the bacteriorhodopsin photocycle. The data for L intermediate were similar to those found for 13-ds,15-anti retylidene chloride, while those for K intermediate were similar to those of acid blue bacteriorhodopsin in which the Schiff base counterion was neutralised (Table 3). The 15N chemical shifts observed have shown that for bacteriorhodopsin, the Schiff base in K intermediate state loses contact with its counterion and establishes a new one in L intermediate state. The proton energy stored at the beginning in the electrostatic modes has been transformed to torsional modes. The transfer of energy is facilitated by the reduction of bond order alternation in the polyene chain when the counterion interaction is initially broken and is driven by the attraction of the Schiff base to a new counterion. 3D CP MAS experiments of NCOCX, NCACX, CONCA and CAN(CO)CA types have been used in studies of proteorhodopsin.71... 

Table 7 15N Chemical shifts in ppm for 4,5-dihydro-1H-1,2,3-triazoles 51-61 taken in CDCI3 with nitromethane as external...

Carbonyl stretch frequencies, carbonyl 13C and amide 15N chemical shifts for a wide range of A-acyloxy-A-alkoxyamides are listed in Table 2 together with those of the precursor hydroxamic esters. Spectroscopically, mutagens can be categorised into six types ... 

Finally, it is worth mentioning that 15N-NMR spectra of yohimbine (74), reserpine (109), and isoreserpine (514) also have been investigated (319). The 15N chemical-shift differences between C/D-cis- and trans-fused compounds could be explained by a hyperconjugative interaction between the antiperiplanar C—H bonds and the nitrogen lone pair characteristic for trans-fused compounds only. 

Table 3 The 1H, 13C, and 15N chemical shifts of 1 -alkyl, 2-alkyl-, and 3-aryltetrazolo[1,5-a]pyridinium salts...

Kleinpeter et al. <1997JST(435)65> investigated the effect of substituents in position 2 on the 13C and 15N chemical shifts of 7-hydroxy-substituted derivatives. Some representative data are listed in Table 2. These authors found a good coincidence between the calculated chemical shifts and the experimental values. 

Furthermore, protonation results in a significant distortion of the coordination polyhedron, i.e., the metal ion is displaced from the plane formed by the four cyano ligand carbon atoms toward the oxo along the M = 0 axis by as much as 0.34 A, which represents about 20% of the total metal-oxo bond length. In spite of this distortion stronger metal-cyano bonds are observed crystallographically, suggesting a better n back-donation by the metal center to the cyano carbons since d-ff overlap is increased. This observation is in line with both the 13C and 15N chemical shift and kinetic data (Section V) for the protonated complexes (8). 

Fig. 18. The pH dependence of the cyanide exchange rate at 25°C on [Re02(CN)4]3 8 13Cf and 8 15NF are the 13C and 15N chemical shifts of the free HCN/CN and 8 13CB and 8 15NB the chemical shifts of the bound CN. The total complex concentration [Re] = 0.2 m the total cyanide concentration [CN] = 0.3 m and /jl = 1.5-2.8 m KN03 (8). (Adapted with permission from Abou-Hamdan, A. Roodt, A. Merbach, A. E. Inorg. Chem. 1998, 37, 1278-1288. Copyright 1998 American Chemical Society.)...

Thus, the magnetization is transferred from the amide proton to the attached nitrogen and then simultaneously to the intra- and interresidual 13C spins and sequential 13C spin. The 13C chemical shift is labelled during /, and 13C frequency during t2. The desired coherence is transferred back to the amide proton in the identical but reverse coherence transfer pathway. The 15N chemical shift is frequency labelled during t3, and implemented into the 13C 15N back-INEPT step. The sensitivity of the HNCOmCA-TROSY experiment is excellent and nearly similar to HNCA-TROSY except for the inherent sensitivity loss by a factor of /2, arising from additional quadrature detection needed for 13C frequency discrimination in the fourth dimension. The excellent sensitivity is due to a very efficient coherence transfer pathway,... 

When the protein rotational correlation time exceeds 15 ns, the upfield component of the JNH doublet broadens. In such cases the amide JNH coupling can be obtained from the displacement of the 15N chemical shift of the TROSY component (see Chapt. 10) relative to the one from a XH-decoupled HSQC experiment. 

It is worth mentioning that parameter p is insensitive, to first order approximation, to modulation of the residue-specific 15N chemical shift anisotropy tensor and/or dipolar interaction, as the (d2 + c2) term in the R) / R ratio is canceled out. The noncollinearity of the CSA and dipolar tensors will require corrections to Eqs. (10) and (12) for high degrees of rotational anisotropy (D /D > 1.5), as described in detail in Ref. [22]. 

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