Proton-CRAMPS

We introduce research results from recent proton Combined Rotation and Multiple Pulse Spectroscopy (CRAMPS) NMR of a-amino acids, polypeptides and proteins. Proton CRAMPS NMR research has only just begun and has the possibility of wide-ranging future development. 

Proton CRAMPS demands a highly efficient spectrometer to avoid measurement difficulties. The main demands on the machine are as follows. 

The recently developed high-resolution solid-state NMR technique proton CRAMPS NMR has become a very useful research tool, corresponding to X-ray crystallography, and enabling the study of crystal structure polymorphs of amino acids. In this chapter, we first discuss a recent research example application to crystal structure analysis of polymorphic forms of some typical a-amino acids in order to test the power of H CRAMPS NMR, compared with C and N NMR methods. 

Lesage et al. have shown that the resolution of the proton NMR spectroscopy of powdered solids can be improved significantly when multi-pulse sequences are employed [44a]. In the approach based on combined rotation and multipulse spectroscopy (CRAMPS) (Figure 7.9) the problem of dipolar line broadening is usually overcome. 

Employing CRAMPS-type proton spectroscopy of powdered solids by using a 2D acquisition experiment can enhance the resolution by a factor of 2 or 3. A H FSLG CRAMPS-MAS experiment was used to study sample 6. Figure 7.10 shows the H MAS spectrum of sample 6 recorded at a spinning rate of 10 kHz. 

Figure 7.12 shows the FI traces taken from the 2D plots at 7.3, 3.0 and Oppm. The aromatic resonances at 7.3 ppm (bottom trace in Figure 7.12) are very well resolved, while the aliphatic and vinyl protons of cod ligand (middle trace) are broad and hidden in the baseline. The upper trace, taken at Oppm, represents the pure signal of methyl proton bonded to silicon residue. Despite the fact that 2D CRAMPS experiment enabled us roughly to assign the position of cod protons, the exact chemical shifts of other hydrogens remain equivocal. 

Two recent studies have examined the NMR spectra of coal macerals and lithotypes respectively. Retcofsky and VanderHardt (12) reported the aromaticities of the vitrinite, exinite, micrinite, and fusinite from Hershaw hvAb coal using non-spinning cross-polarization techniques. The fa values of 0.85, 0.66, 0.85, and 0.93 -0.96 for these macerals demonstrate clear variations between the materials at a given rank. Gerstein et. al. (13) used carbon-13 CP/MAS proton combined rotation and multiple pulse spectroscopy (CRAMPS) to examine Iowa vitrain (Star coal) and a Virginia vitrain (Pocahontas 4 coal) with aromaticities of 0.71 and 0.86 respectively. 

We think that the main reason for this decline of interest is the limited analytical value of proton shielding tensors. Even if one has the necessary equipment (few people have it) and the necessary know-how, it takes a considerable effort to actually measure a proton shielding tensor. For analytical purposes, the combination of line-narrowing m.p. sequences with magic angle sample spinning (CRAMPS) applied to a powder sample will usually be the method of choice (Scheler et al., 1976 Burum et al., 1993 for a recent review, see Maciel et al., 1990). However, the combination... 

Figure 3, High-resolution NMR spectra of protons in Pocahontas No, 4 vitrain (top) and Star vitrain (bottom). Combined rotation and multiple-pulse spectroscopy, (top) in Pocahontas No. 4 vitrain CRAMPS at t = 36 jjisec f = 2.5 KHz = 0.73 corrected for hydroxyl, (bottom) H in Star vitrain CRAMPS at t = 36 xsec f =2.5 kHz f = 0.23 corrected for hydroxyl.

Figure 9.6. A. H CRAMPS-MAS correlation spectrum of hydrated sodium disilicate glass showing projections in both dimensions. B. Slices through the CRAMPS dimension of spectrum (A) showing the separate spectra from the H2O resonance at 4.0 ppm (upper) and the OH resonance at 14.0 ppm (lower). Note that the different sideband distributions from the 2 protonated groups are clearly distinguishable. From Schaller and Sebald (1995), by permission of the copyright owner.

FIGURE 5 Solid-state proton spectra of (L)-alanine, showing the differences between spectra acquired in static, spinning, and CRAMPS modes. All 500 MHz spectra were acquired at ambient conditions. 

The CRAMPS experiment puts a large demand on the NMR hardware (especially on NMR probes), since high-power radio frequency (rf) pulses are applied between each acquisition point. High homogeneity of the irradiation field, as well as careful setup of experimental variables, is required to avoid distortions of the proton peaks. Typical line widths afforded by the CRAMPS experiment are approximately 1 ppm, limiting the application of the CRAMPS experiment to compounds with a small number of well-resolved protons. 

The validity of the structural model in Figure 17d is proven by the XH DQ MAS spectrum in Figure 17b. First, the observation of two hydrogen-bonded resonances is explained A and B correspond to the O—H "N and O—H "0 protons, respectively (this assignment is on the basis of CRAMPS spectra and also results subsequently obtained for a 15N labeled sample). Remembering that a DQ peak is only... 

Combined rotation and multiple-pulse spectroscopy (CRAMPS). A special pulse sequence, in addition to MAS, is required for high-resolution proton NMR in solids. This technique is known as CRAMPS. 

From these results, it was concluded that the polymorphic forms of L-histidine can be readily distinguished by H CRAMPS NMR spectra, even when it is quite difficult to distinguish using the C and N CP-MAS NMR spectra. From the H CRAMPS NMR measurement, it is concluded that the H chemical shifts of a-amino acid crystals are very sensitive to a small difference in the magnetic surroundings of protons as well as that in the hydrogen-bond network. Accordingly, the solid H CRAMPS NMR is a very useful means for structural analysis of L-histidine crystals. 

Figure 22 shows the H CRAMPS NMR spectra of poly(L-leucines) (A) [Leu]n-1 (mostly /3-sheet and a few a-helix form) and (B) [Leu]n-2 (a-helix) in the solid state. These H spectra are solid high-resolution signals separated into three regions (NH, H , and side-chain protons), which is a similar result to that of PLA. From these spectra, it is clear that (1) the chemical shift of the H ... 

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