Water magnetic

Liquid oxygen is pale blue, slightly heavier than water, magnetic, non-flammable and does not produce toxic or irritating vapours. On contact with reducing agents, liquid oxygen can cause explosions. 

ST2-PT thus results in a 2D [15N, H]-correlation spectrum that contains only the most slowly relaxing component of the 2D 15N- H multiplet. The data are processed as described by Kay et al. [44] in an echo/antiecho manner. Water saturation is minimized by keeping the water magnetization along the z-axis during the entire experiment, which is achieved by the application of the water-selective 90° rf pulses indicated by curved shapes on the line H. It was reported that on some NMR instruments the phase cycle mentioned above does select the desired multiplet component. On these instruments, the replacements of S, with S, = y, x for the first FID and 9, =... 

Since water protons are not bound to or nuclei, the water signal is also suppressed by the spin-lock purge pulse. In practice, the suppression of the water signal is sufficient to record HSQC spectra of protein samples dissolved in mixtures of 95% H20/5% D2O without any further water suppression scheme [12]. For optimum water suppression the carrier frequency must be at the frequency of the water resonance. On resonance, the phase of the water magnetization is not affected by imperfections of the first 180°(ff) pulse, so that no solvent magnetization ends up along the axis of the spin-lock purge pulse. 

To a suspension of this product in 200 ml. of water, magnetically stirred and heated at 65°C., a 10% sodium hydroxide solution is added dropwise. The compound gradually dissolves in the alkaline solution, but it is very important to add the sodium hydroxide solution slowly, especially toward the end of the operation when the product is almost all dissolved. The pH of the suspension should never go above 10. At higher pH the compound may readily decompose to a very dark, soft precipitate of higher molybdenum hydroxides with evolution of hydrogen. From 60 to 70 ml. of sodium hydroxide solution is usually needed to dissolve the compound completely. The... 

Now let s return to the 3-9-19 sequence. For the water peak, it s simple. As there is no evolution during the r delays, it is just a sequence of six pulses whose rotation is exactly balanced between ccw rotations (3-9-19) and cw rotations (19 — 9 — 3). The net rotation is zero and the water magnetization ends up on the +z axis, where it started. Water is not affected by the pulse train. The same is true if the offset is v0 — vr = 1/r... 

Another commonly used technique is the water flip-back pulse, a shaped pulse designed to selectively rotate only the water magnetization by 90°, putting it back on the +z axis after a hard (nonselective) pulse has rotated all of the sample magnetization into the x-y plane. Water can be viewed as a wild and powerful bucking bronco—it must be tamed and never allowed to get out of its pen. The best place for water is on the +z axis where it will not do any harm. This is the rationale behind the flip-back pulse every time water is moved from the +z axis, use a selective pulse to put it back there. 

Pour the CH2CI2 layers again into a 1 L conical flask and treat them for 4 h with a large excess of NaBF4 ( 15 g) dissolved in a minimum of water (magnetic stirring, room temperature).6... 

HOHAHA experiment with minimal water saturation was developed by Schleucher et al. (1995a) and Dhalluin et al. (1996) proposed a water-flip-back TOCSY where water-selective pulses are used to flip a maximal fraction of the water magnetization back along the +z axis at the start of the acquisition time. TOCSY experiments with excellent water suppression based on excitation sculpting (Stott et al., 1995 Hwang et al., 1995) were reported by Callihan et al. (1996). 

The existence of deposits of hematite in the Oxia Palus region, as revealed by TES (Christensen et al., 2000b), probably indicates processes requiring water. Magnetic measurements of dust by Mars landers (e.g., Madsen etal., 1999) indicate iron oxides, so the surface of Mars is highly oxidized. 

Pellecchia et al. have proposed the SEA-TROSY technique to identify solvent-exposed amide groups in l5N-labelled proteins. Initially, all amide magnetization is eliminated with a i5N filter, and then the magnetization of solvent-exposed amide protons is restored by exposure to water magnetization followed by detection of these amide protons by water flipback TROSY.101 Figure 5 shows a comparison of a normal TROSY spectrum and an SEA-TROSY spectrum (50-ms mixing time) of a 30-kDa complex of 15N,l3C-0 subunit and the unlabelled N-terminal domain (1-185) subunit of DNA polymerase III. 

In spin-locking-based methods the inhomogeneity of the RF field is used to dephase the water magnetization that is not aligned along the... 

Fig. 7. (A) The WEFT sequence in this sequence the tt pulse is applied to rotate all of the magnetization (i.e. both solute and solvent) to the -z-axis. A delay (I>np) of sufficient length is used to allow the water magnetization to relax to the origin ( >np = InfZ) ) whilst during the same period, by virtue of faster longitudinal relaxation, the solute resonances have reached thermal equilibrium. An excitation pulse (represented here as a tj/2 pulse) is then applied and an almost water-free spectrum is acquired. However, in the presence of radiation damping the water quicldy returns nonexponentially to the equilibrium position at a similar rate to the solute nuclei (see Fig. 2). However, if during D p a series of n very weak and evenly spaced gradient pulses are applied so as to inhibit the effects of radiation damping, the water relaxes according to its natural spin-lattice relaxation rate. This is the basis of the Water-PRESS sequence (B). An example of a spectrum obtained with Water-PRESS is shown in Fig. IB and Fig. 6.

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