Selective inversion

Aspects of the experimental implementation of the technique have been studied in detail [41] and it has been shown that double sideband modulation and pulse shaping can be combined to improve the performance of selective pulses in solid state 2H NMR. Applications of the selective inversion-recovery experiment using a DANTE sequence to study ultraslow motions have been demonstrated [42,43]. 

Highly accurate interatomic distances (ultimately 0.05 A) may be obtained from REDOR experiments [49], which are therefore an attractive tool for studies of hydrogen bonding. This technique has been used recently to characterise ex-helix structures in polypeptides by measuring 13C=0---H-15N hydrogen bond lengths [50]. The intrachain 13C- 15N interatomic distances, measured for a number of different samples, were found to be 4.5 0.1 A. This finding was used as evidence for the a-helix structure, which is consistent with the conformation dependent displacements of 13C chemical shifts of the Ca, Cp and carbonyl carbons of the peptide unit [51]. 

A comparatively long N- -H hydrogen bond length in the benzoxazine dimer (see below), measured using an advanced solid state NMR technique (DIP-HSQC),has been reported [52]. This technique employs REDOR-type recoupling under fast MAS to recouple the heteronuclear - N dipolar interaction, such that rotor-encoded spinning sideband patterns are obtained. 

Finally, using both 13C and 15N labelled gramicidin A samples in hydrated phospholipid bilayers, both intermolecular and intramolecular distances have been measured using a solid state NMR technique based on simultaneous frequency and amplitude modulation [53]. By measuring 15N-13C residual dipolar couplings across a hydrogen bond, distances of the order of 4.2 0.2 A were established. 

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Figure B2.4.2. Eyring plot of log(rate/7) versus (1/7), where Jis absolute temperature, for the cis-trans isomerism of the aldehyde group in fiirfiiral. Rates were obtained from tln-ee different experiments measurements (squares), bandshapes (triangles) and selective inversions (circles). The line is a linear regression to the data. The slope of the line is A H IR, and the intercept at 1/J = 0 is A S IR, where R is the gas constant. A and A are the enthalpy and entropy of activation, according to equation (B2.4.1)...

Figure B2.4.8. Relaxation of two of tlie exchanging methyl groups in the TEMPO derivative in figure B2.4.7. The dotted lines show the relaxation of the two methyl signals after a non-selective inversion pulse (a typical experunent). The heavy solid line shows the recovery after the selective inversion of one of the methyl signals. The inverted signal (circles) recovers more quickly, under the combined influence of relaxation and exchange with the non-inverted peak. The signal that was not inverted (squares) shows a characteristic transient. The lines represent a non-linear least-squares fit to the data.

In a selective-inversion experiment, it is the relaxation of the z magnetizations that is being studied. For a system without scalar coupling, this is straightforward a simple pulse will convert the z magnetizations directly into observable signals. For a coupled spur system, this relation between the z magnetizations and the observable transitions is much more complex [22]. 

In a coupled spin system, the number of observed lines in a spectrum does not match the number of independent z magnetizations and, fiirthennore, the spectra depend on the flip angle of the pulse used to observe them. Because of the complicated spectroscopy of homonuclear coupled spins, it is only recently that selective inversions in simple coupled spin systems [23] have been studied. This means that slow chemical exchange can be studied using proton spectra without the requirement of single characteristic peaks, such as methyl groups. 

Bouaboula, M., Perrachon, S., Milligan, L., Canatt, X., Rinaldi-Carmona, M., Portier, M., Barth, F., Calandra, B., Pecceu, F., Lupker, J., Maffrand, J.-P., Le Fur, G., and Casellas, P. (1997). A selective inverse agonist for central cannabinoid receptor inhibits mitogen-activated protein kinase activation stimulated by insulin or insulin-like growth factor. J. Biol. Ckem. 272 22330-22339. 

Figure 7.8 SELINCOR (Selective Inverse Correlation) pulse sequence with BIRD presaturation. (Reprinted from Mag. Reson. Chem. 29, H, Kessler et ai, 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

Fig. 9.2 Schematic representation of the three basic experiments useful for the determination of (A) transient NOE experiment, (B) 2D NOESY and (C) 2D ROESY. The gray-filled half-circle represents a frequency-selective inversion pulse which inverts the spin to which the cross-relaxation...

Iwamura and Ueda [386] described compound (611) as a CB2 selective inverse agonist in a patent application. The potential therapeutic roles of CB2 antagonists are not clearly defined at the moment, although roles in regulation of the immune system and inflammation have been widely proposed. This patent application describes that activity of compound (611) in a mouse model of asthma, in which the compound suppressed immediate and late-phase asthmatic response and airway hyper-responsiveness. 

Fig. 2.6.8 Time-of-flight dispersion curves versus the encoding position of gas flowing through a cylindrically symmetrical glass phantom with large pores on the order of 1-cm diameter, obtained with slice selective inversion of magnetization. The flow direction changes twice as the gas is flowing from inlet to outlet. Slices parallel (upper) and perpen-...

These initial slopes are readily evaluated since we can measure the signal corresponding to Igq (or Seq if S is a nuclear spin) and, since the relevant instrumental factor is identical to the one which prevails in the measurement of leq (or Seq). Moreover, initial conditions can be devised for determining separately one of the three relaxation parameters 7 (, or a. The simplest experiment consists of selectively inverting one of the two magnetizations. Consider first the selective inversion of I magnetization for which the following initial conditions hold 72(0) = —leg and S2(0) = Seq. This yields for the initial slopes... 

The complementary experiment (selective inversion of S magnetization) leads to... 

In summary (Scheme 15), 2-lithiopiperidines and 2-lithiopyrrolidines appear to be very versatile nucleophiles for the elaboration of these heterocyclic systems, affording a variety of 2-substituted heterocycles in excellent yields. The stereoselectivity of the reaction is near 100% in the piperidine series with most carbonyl electrophiles (retention of configuration) and alkyl halides (inversion of configuration). In the pyrrolidine series, the selectivity is also near 100% with carbonyl electrophiles (retention), but less selective (inversion predominates) with alkyl halides (less problematic with Af-aUylpyrrolidines). 

Fig. 3. Numerical simulations of four different selective inversion pulses. (Top) Pulse shapes. From left to right 180° rectangular, 180° Gaussian truncated at 2.5%, Quaternion cascade Q, and l-BURP-2. (Middle) Trajectories of Cartesian operators in the rotating frame during...

Most commonly used selective inversion and selective refocussing pulses. 

The peak rf amplitude required to achieve optimum inversion with a selective inversion pulse is given in comparison to the rf amplitude required to achieve an on-resonance 90° flip-angle with a selective rectangular pulse, the simplest conceivable shape. 

Selectivity of the selective inversion pulses of table 3, all numbers given for properly... 

Fig. 6. Simulation of the effect of longitudinal and transverse relaxation during a 30 ms selective inversion pulse. The trajectories are shown on a Bloch sphere and in the...

For selective inversion or refocussing, a universal pulse is a good choice. In cases where singlets are to be inverted and where relaxation or exchange during pulses is critical, one may need to use a 180° Gaussian pulse which is the shortest selective inversion pulse available [24]. 

Fig. 5a. Part of the spectra obtained with the ID multiple selective INADEQUATE experiment III applied to cembrene 3 dissolved in CDCI3. The spectra were obtained after appropriate processing, showing the responses in the olefinic and the aliphatic regions after selective inversion of carbons...

Fig. 2. (a) 400 MHz partial H spectrum of a trisaccharide in CDClj. (b)-(e) ID TOCSY difference spectra with selective inversion of Hj// by a DANTE pulse train, and mixing times of 30, 60, 120 and 180 ms, respectively. IOtz values (of 16 scans each) were used for the 2-filter. (Reproduced with permission from Wessel et al. [26a]. Copyright 1991 Verlag... 

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