Magnetization transfer experiment

The earliest of the magnetization transfer experiments is the spin population inversion (SPI) experiment [27]. By selectively irradiating and inverting one of the 13C satellites of a proton resonance, the recorded proton spectrum is correspondingly perturbed and enhanced. Experiments of this type have been successfully utilized to solve complex structural assignments. They also form the basis for 2D-heteronuclear chemical shift correlation experiments that are discussed in more detail later in this chapter. 

Peralkylated tetracarba-mdo-hexaboranes(6) 56 [72] are extremely stable, and almost inert in air and stable towards H20, in contrast with the parent carbaborane [73], nB/nB magnetization transfer experiments performed at 140 °C for the per-ethyl derivative show that exchange reactions between boron atoms in positions 1 and 6 are very slow. 

In the isoelectronic series (butadiene)M( j8 — CgHg) (M = Ti, Zr, Hf), the Hf complex exhibits an NMR spectrum at > 30 °C consistent with an envelope flip (AG = 17.6 kcalmol-1). The same process can be detected for the Zr complex at > 40 °C only via magnetization transfer experiments (AG > 20 kcalmol-1). The Ti complex exhibits a static structure by NMR spectroscopy16. 

A. I. Schmid, M. Chmelik, J. Szendroedi, M. Krssak, A. Brehm, E. Moser and M. Roden, Quantitative ATP synthesis in human liver measured by localized spectroscopy using the magnetization transfer experiment. NMR Biomed., 2008, 21,437- 43. 

Modern NMR techniques such as quantitative analysis of multisite exchange using either ID magnetization transfer experiments (JOS) or the 2D exchange spectroscopy (EXSY) method (104,105) promise to be of great help in unraveling the complex stereochemical exchange networks involved in cluster fluxionality. The usefulness of EXSY in the context of this article is illustrated by the phase-sensitive 13C 1H EXSY spectrum (255 K, tm = 0.5 sec)... 

NMR has the capability, using magnetization transfer techniques, to measure very rapid metabolic fluxes in vivo, of the order of 1(F3 M s-1. A magnetization transfer experiment, in which exchange between ATP and Pj was measured in yeast... 

Figure 11 shows the contour plots of the 2D 51V EXSY spectrum of a two-site system (40 mM total vanadate at pH 10.9) in which V exchanges with V2. Quantification of the EXSY spectrum and calculation of the error propagated to the rate constants from the integration precision gives a 25% error on the rate constant. The results (both the rate constants and the errors) correspond nicely to the results obtained from a ID magnetization transfer experiment on the same sample (27). The EXSY spectrum of a sample containing 12.5 mM total vanadate at 1.0... 

Catalyst initiation involves the formation of a metathesis-active ruthenium species from the starting precatalyst and its entry into the catalytic cyde. For both Ru-2 and Ru-4, the initiation event consists of phosphine (PCys) dissodation to produce the 14-electron intermediate [(L)(Cl)2Rr CHR ], where L= PCys for Ru-2 and L = H2lMes for Ru-4) (Figure 6.4). Although this proposed spedes has not been observed in solution, it has been identified in the gas phase [7], and the ligand dissociation step has been studied by NMR magnetization transfer experiments. 

Analysis of the NMR parameters and the dynamic processes depends on the exchange rates. The basic one-dimensional band-shape analysis is best suited to intermediate rates (10—10 per s). Slow exchange rates (—0.1-10 per s) are most accurately measured by using magnetization transfer experiments and Ti-relaxation times. Fast dynamic processes (> 10 per s) can be elucidated by investigating the spin-spin relaxation times. 

Line-shape analysis, magnetization transfer experiments, 2D NOESY etc. can be used for kinetic studies of CTI, but they require relatively high peptide concentrations (Fig. 8.7) [53]. Mainly used for the investigation of model systems, they can also be utilized in complex cases for the determination of individual rate constants and for the monitoring of protein folding. 

Fig. 8.7 Relative intensities ofthe time-dependent magnetization of CH3 of c/s-Ala-Tyr in H20 D20 9 1 pH 5.9 in a 7, experiment ( ) and the magnetization-transfer experiments with the cis signal parallel (A) and antiparallel to the stationary magnetic field (V) at 316 K. Solid lines represent the correspondingly fitted biexponential decays with 7, = 0.984 s, k= 3.7 s-1 and a cis content of 0.61% [53].

---