X-half-filter

Figure 6 X-half filters used for filtering or selecting 13C and 15N-attached protons. Thick and thin closed rectangles are 180° and 90° pulses, respectively, open rectangles are spin lock pulses. (A) A simple X-half filter (2). The delay t is equal to 0/(2[1JXH]) where 1JXH is the one-bond coupling between proton and either 13C (120 to 140 Hz) or 15N (95 Hz). The second 90° pulse is the editing...

This scheme is applied in the so-called X-half-filter technique (Fig. 17.4a, c), with the only difference of an additional 90 (X) pulse with constant phase [16, 17]. Instead of generating heteronuclear multiple quantum coherence 2 Iy Sy, which cannot readily be detected (in case of selecting the 1H-X pairs), one now always ends up with proton antiphase coherence, but with the same phase alternation ... 

The single filter elements (as shown in Figs. 17.4a-c) are often called X-halffilters since each of them acts only in one dimension of a 2D experiment, to be distinguished from Xfilters that select (or suppress) 1H-X pairs in both dimensions of a 2D experiment [17, 20, 21]. Of course, X-half filters can be employed twice in a 2D experiment, to yield isotope selection in both dimensions (see Sect. 17.3.3). 

Fig. 17.4 Common filter elements a X-half filter based on X pulse phase cycling [16, 17], b X-half filter with purge gradient [18], c X-half filter as in a, but with refocusing period for the hetero-nuclear antiphase magnetization [16, 17]. Sequences d [22], e [23] and f [18] show double filters based on single filter elements the delays r and r can be set to slightly different values to cover a broader range of ]J coupling constants (see text for a more detailed description).

The simplest double tuned filter can be constructed by a concatenation of two X-half filters and removal of redundant 180° pulse pairs (Fig. 17.4d) [22]. Alternatively, it can also be realized by keeping the 180° pulse pairs and adding short spin-lock periods (to dephase the 1H-13C magnetization which is orthogonal to the spin-lock axis, Fig. 17.4e) [23], or it is based on the gradient-purging scheme of Fig. 17.4b, resulting in the double filter shown in Fig. 17.4f [18]. 

Otting G, et al. Editing of 2D IH NMR spectra using X half-filters, combined use with residue-selective 15N labeling of proteins. J. Magnet. Reson. (1969) 1986 70 500-505. 

The tryptophane so produced (7-8 g.) varies somewhat in quality in different runs. It is purified by recrystallization from 60 cc. of dilute alcohol (2 vol. of 95 per cent alcohol to 1 vol. of water), filtering off from the hot solution an appreciable quantity of insoluble matter and subjecting this to a second extraction with a further 10-cc. quantity of aqueous alcohol. The solution is decolorized by the addition of x g. of decolorizing carbon (Norite) and allowed to stand in the ice-box the silvery leaflets of tryptophane are filtered off and washed successively with cold 70 per cent, 80 per cent, 95 per cent alcohol and finally with a little ether. Less than half the tryptophane is obtained in each crystallization (Note 12). The yield of pure (Note 13) tryptophane is 4.0-4.1 g., together with under o.x g. of less pure product. 

Figure 9 Timing diagram of the BIRD-HMBC pulse sequence for the detection of nJch correlations, including an additional two-step low-pass J filter. Thin and thick bars represent 90° and 180° pulses, respectively. 13C180° pulses are replaced by 90°y — 180°x — 90°y composite pulses. <5 is set to 0.5/(Vch) and A is set to 0.5/("JCH). Phases are cycled as follows fa = y, y, —y, —y 4>j = x, —x fa — 8(x), 8(—x) fa = 4(x), 4(— x) ( rec = 2 (x, — x), 4(—x, x), 2(x, —x). Phases not shown are along the x-axis. Gradient pulses are represented by filled half-ellipses denoted by Gi-G3. They should be applied in the ratio 50 30 40.1.

Males were anesthetised and mental glands (N 200) were surgically removed. Secreted components were extracted into 0.8 mM acetylcholine chloride in l/2x PBS (cf Rollmann, Houck and Feldhoff 1999). Gland extracts were centrifuged for 10 min at 14,000 g and the supernatant was removed. The supernatant was filtered (0.2 pm) and loaded as aliquots onto a Sephadex Superfine G-75 gel filtration column (1.6 cm x 15.5 cm Pharmacia, Piscataway, NJ) on a Waters HPLC system (Mil-lipore, Milford, MA). The column had previously been equilibrated with one-half strength Dulbecco s phosphate buffered saline (l/2x PBS). The column was eluted... 

The first XRF spectrometers employed the wavelength dispersive methodology, which is schematically illustrated in the upper half of Fig. 7.17. The x-rays emanating from the source are passed through a suitable filter or filters to remove any undesired wavelengths, and collimated into a beam that is used to irradiate the sample. For instance, one typically uses a thin layer of elemental nickel to isolate the Ka lines of a copper x-ray source from contamination by the Kp lines, since the K-edge absorption of nickel will serve to pass the Kq, radiation but not the Kp radiation. 

The 2-bromoacetal (0.1 mol), prepared by the Dowex X8 (H+)-catalysed reaction of the diol with l-bromo-2,2-dimethoxyethane, is added to r-BuOK (16.8 g) and Aliquat (0.81 g, 2 mmol) in THF (150 ml) and the mixture is stirred at 80-90°C until the reaction is complete (2-4 h). n-C5H12 (4 x 200 ml) is added portionwise, with stirring after each addition, and the reaction mixture is then filtered through alumina. The filtrate is concentrated, H20 (5.5 ml) and AcOH (ca. 0.2 ml) are added to the residue, and the solution stirred at room temperature for 3 min. The aqueous mixture is extracted with Et20 (3 x 20 ml) and the extracts are dried (Na2S04) and evaporated to yield the diol half ester [e.g. AcO(CH2)2OH, 70% AcO(CH2),OH, 85% MeCH(OH)(CH2)2OAc, 80% MeCH(OAc)CH2CMe2OH, 85%]. 

For this work, the spectrometer function s(x) was determined by the method outlined in Section II.G.3 of Chapter 2. In digitizing the data, a sample density was chosen to accommodate about 70 samples taken across the full width at half maximum of s(x). A 25-point cubic polynomial smoothing filter was used in the deconvolution procedure to control high-frequency noise. Instead of the convolution in Eq. (13), the point-successive modification described in Section III.C.2 of Chapter 3 was employed. In Eq. (24) of Chapter 3, we replaced k with the expression... 

Reaction of Alkali Metals with Water. Work in a fume cupboard, wear eye protection or a protective mask ) Fill a crystallizer with water and put it into a fume cupboard. Make a small box (2x2 cm) from filter paper and put a small piece of purified lithium the size of a pea into it. With the window of the fume cupboard half lowered, carefully place the bo.x with the lithium into the crystallizer with water. Observe what occurs. Test the action of the formed solution... 

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