Pulsed-FT spectrometers

Creatine kinase was purified from rabbit muscle by the method of Kuby et al, (4). Rabbit muscle pyruvate kinase was purchased from Boehringer. Porcine muscle adenylate kinase was purchased from Sigma, and was further purified by gel filtration on Sephadex G-50. The enzymes were homogeneous as judged by their specific activities and by their migration as single components in sodium dodecyl sulfate gel electrophoresis. Proton NMR spectra at 250 MHz of 0.5-2.0 mM enzyme sites in 0 solution were obtained with a Bruker WM 250 MHz pulse FT spectrometer at 25°. At least 256 transients were accumulated over 8192 data points using 16 bit A/D conversion. Relaxation rates and histidine pK values were determined by standard NMR methods (5, 6),... 

The availability of pulsed FT spectrometers at high frequencies (300-750 MHz for protons and 75-188 MHz for 13C) with signal averaging has greatly enhanced the possibility of polymer analysis in HR NMR. 

Tube Tubes—Usually a 5 or 10 mm outside diameter tube compatible with the configuration of the CW or pulse FT spectrometer. 

Xl.4.5 The NMR plectrum obtained after Fourier transformation on a pulse FT spectrometer should have a computer-limited ectral resolution suffident to accurately define the aromatic and aliphatic absorption bands. 

The original method employed was to scan eitiier the frequency of the exciting oscillator or to scan the applied magnetic field until resonant absorption occiined. Flowever, compared to simultaneous excitation of a wide range of frequencies by a short RF pulse, the scanned approach is a very time-inefficient way of recording the spectrum. Flence, with the advent of computers that could be dedicated to spectrometers and efficient Fourier transfomi (FT) algoritluns, pulsed FT NMR became the nomial mode of operation. 

Figure Bl.12.3. Schematic representation showing the components of a pulse FT NMR spectrometer.

The key dimension m NMR is the frequency axis All of the spectra we have seen so far are ID spectra because they have only one frequency axis In 2D NMR a stan dard pulse sequence adds a second frequency axis Only pulsed FT NMR spectrometers are capable of carrying out 2D experiments... 

Carbon-13 nmr. Carbon-13 [14762-74-4] nmr (1,2,11) has been available routinely since the invention of the pulsed ft/nmr spectrometer in the early 1970s. The difficulties of studying carbon by nmr methods is that the most abundant isotope, has a spin, /, of 0, and thus cannot be observed by nmr. However, has 7 = 1/2 and spin properties similar to H. The natural abundance of is only 1.1% of the total carbon the magnetogyric ratio of is 0.25 that of H. Together, these effects make the nucleus ca 1/5700 times as sensitive as H. The interpretation of experiments involves measurements of chemical shifts, integrations, andy-coupling information however, these last two are harder to determine accurately and are less important to identification of connectivity than in H nmr. 

A considerable improvement in speed and sensitivity can be achieved with a pulsed Fourier transform (FT) spectrometer. Here the sample is subjected to a series of short duration high intensity RF pulses (1-100 us)... 

The most abundant isotope of earbon ( C) cannot be observed by NMR. is a rare nucleus (1.1% natural abundance) and its low concentration coupled with the fact that has a relatively low resonance frequency, leads to its relative insensitivity as an NMR-active nucleus (about 1/6000 as sensitive as iff). However, with the increasing availability of routine pulsed FT NMR spectrometers, it is now common to acquire many spectra and add them together (Section 5.3), so C NMR spectra of good quality can be obtained readily. 

C-RMR spectra of atenolol and compound (presented in Fig. 4.) were run in pulse FT mode on a Jeol FX-loo spectrometer operating at 25. o5 MHz. The samples were measured in 5 mm tubes with TMS as an internal standard. An 8K computer memory and internal deuterim lock w re used. (5)... 

The basic spectrometer we have employed for the study of solid polymers by cross-polarization/magic-angle spinning techniques is a Nicolet TT-14 pulse-FT system operating at 1.4T with resonance frequencies of 15.087 MHz, 60.0 MHz and 56.4 MHz for 1r and... 

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