MREV-8 pulse sequence

Solid-state H CRAMPS NMR spectra were measured with a Chemagnetics CMX 300 spectrometer operating at 300 MHz. The internal standard was Si-rubber ((5 = 0.12) relative to (CH3)4Si (6 = 0). BR-24 and MREV-8 pulse sequences were applied. The radio frequency (RF) powers and duration windows (r) of these pulse sequences were adjusted so as to obtain the best resolution for adipic acid. 

MREV-8 pulse sequence The 90° pulse width is 1.1 /zs for the MREV-8. The cycle time of the MREV-8 is 28.8 /zs, corresponding to a t of 2.4 /zs, and the rotational frequency is 3.5 kHz. The H chemical shift is calculated with a scaling factor of 0.53 for all samples, which was determined experimentally. The carrier frequency must be selected carefully to avoid the carrier noise... 

CRAMPS spectra above 4.0 kHz give poorer resolution than those at 3.5 kHz with the MREV-8 pulse sequence. 

Figure 42 shows the H CRAMPS NMR spectra of N-labelled poly(L-alanines) in the a-helical form (A) BR-24 pulse sequence at 2.0 kHz MAS speed, (B) MREV-8 pulse sequence at 3.5 kHz MAS speed in the solid state. The intensity of the NH proton signal of poly(L-alanine) was increased when measuring with the MREV-8 pulse sequence at a faster MAS speed (3.5 kHz). 

From the H CRAMPS NMR spectra, therefore, it was possible to determine the NH proton chemical shift value for [Ala ]n-2 (a-helix 6 = 8.0) which is identical with that determined using BR-24 (2.0 kHz). Further, it was possible to determine the " NH proton chemical shift for [Ala ]n-1 (/3-sheet, 6 = 8.6) using the MREV-8 pulse sequence at 3.5 kHz. However, unfortunately, the NH proton chemical shift values for [Ala]n-2 and [Ala]n-t could not be determined because the line shapes of the " NH signals exhibit an asymmetric doublet pattern in this system also. Thus, it is found that determination of the true NH chemical shift of poly(L-alanines) can be achieved to measure fully N-labelled samples at higher MAS speed (3.5 kHz) and that these chemical shifts depend on conformation (a-helix 6 = 8.0 /3-sheet S = 8.6). This is the first determination of the true NH proton chemical shifts of poly(L-alanines) by H CRAMPS NMR. 

The NH proton chemical shifts of some polypeptides were successfully determined using fully N-labelled samples at a higher MAS frequency with a MREV-8 pulse sequence. The NH proton chemical shifts determined with the... 

Fig. 44. 300 MHz H CRAMPS NMR spectra of a-helical and /3-sheet poly(L-leucines) using the MREV-8 pulse sequence at 3.5 kHz MAS speed (A) [Leu]n-2 (a-helix) (B) [Leu ] -2 ( N, a-helix) (C) [Leu ]n-1 ( N, /3-sheet + a-helix). Peak assignment H, 9.1 ppm (/3-sheet) and 8.1- 2 ppm (a-helix) H", 5.4 ppm (/3-sheet) and 4.0 ppm (a-helix), H and H >, 1.5-1.7 ppm, H, 0.8-0.9 ppm. The additional peaks around 4.0-5.0 ppm in spectra (A) and (B) were assigned to carrier noise signal (from ref. 45). Reproduced with permission from the American Chemical Society.

The solid-state H CRAMPS NMR measurements were performed on a Chemagnetics CMX 300 spectrometer equipped with a CRAMPS probe with 5 mm rotor. Quadrature-phase detection was carried out according to the phase-cycling technique proposed by Burum et al. Here, we used the MREV-8 pulse sequence for homonuclear decoupling. The experimental conditions are the same as those described in the previous section. 

Linear and carbon-filled, sulfur vulcanized natural rubber were investigated using the eight-pulse sequence MREV-8, which has the following structure 182,183) ... 

The scaling factor of the BR-24 or MREV-8 sequences is a little smaller than the theoretical value (MREV-8 0.471 BR-24 0.385) for ideal square pulses. The scaling factor depends on the pulse length, the cycle time, the pulse sequence, and properties of the machine. However, it is impossible to make a... 

An MAS frequency of 1.5-2.0 kHz is generally used with the BR-24 to prevent interference between the MAS and the multiple-pulse sequence. MAS frequency of 5-6 kHz is possible with MREV-8. 

At the time of this writing, a rapidly growing area of research within NMR is the high resolution study of solids brought about by the development of certain multiple pulse sequences (such as the WAHUHA and MREV-8) on the one hand and an adaptation of the Hartmann and Hahn cross polarization experiment to obtain spectra of dilute spins on... 

MREV-8 (or another homonuclear pulse sequence) must be set up very carefully. The H frequency offset must be optimized using adamantane and the pulse sequence for ID J-resolved spectroscopy in solids. 

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