2D PASS

Song et al.261 showed that rotor-synchronized 2D PASS experiment can also be used to find the orientation distribution function of chemical shift tensors. Because it permits the use of MAS, the sensitivity is higher than when static or slow-turning samples are used. 

C. "Pb NMR spectra of PbZi03 obtained under MAS conditions (upper) and from a two-rotor-cycle 2D PASS experiment (lower) showing the simpliheation obtained by this method giving essentially an isotropic spectrum with no sidebands. From Vogt et al. (2000), by permission of the copyright owner. 

Fig. 19. C spectra of L-tyrosine hydrochloride powder at a rotor frequency of 1.03 kHz. Top ID MAS spectrum (3888 transients). Bottom 2D-PASS spectrum showing sidebands separated in the loi dimension. The toj slices are labelled with the order of the sidebands, A . Sixteen tj increments were taken, each the sum of 243 transients. (Reproduced from Antzutkin et with permission.)...

Fig. 9.24 A 2D PASS spectrum recorded for the antibiotic, penicillin-V The ID CP MAS spectrum is shown in (a). (Reproduced by permission of Academic Press from [116].)...

An alternative means by which the isotropic and anisotropic chemical shift interactions can be separated is the 2D PASS (phase-adjusted spinning sidebands) experiment due to Levitt and co-workers [115]. By changing the timings of the application of five 71 pulses in the tj dimension, it is possible to separate the spinning sidebands by order. As a specific example, Fig. 9.24 shows the 2D PASS spectrum for the antibiotic, penidUin-V. [116] An analysis of this spectrum allowed the determination of the CSA prindpal values for aU the resonances. A distinct advantage of this approach is that only very few (typically 16) increments must be made in the indirect dimension. 

Another technique, enabling a complete separation of isotropic and anisotropic chemical shift was introduced and developed by Antzutkin and Levitt with coworkers [116,117]. This sequence, called 2D PASS (phase adjusted spinning sidebands), consist of a r/2 pulse followed by a train of five n pulses (see Fig. 2.16) The timings between the pulses are modulated to satisfy the PASS equations. The PASS equations are complex and wiU not be discussed here timings for the pulses are shown in paper published by Antzutkin (see Ref [116]). 

D PASS approach was extensively exploited by Potrzebowski group in the course of investigation of the structural and dynamical properties of the signal sequences of the deltorphin and dermorphin—naturally occurring opioid peptides. This experiment was applied for the structure and dynamics elucidation of Tyr-D-Ala-Phe [92] and investigation the influence of hydratation on the changes in the dynamical properties of the Tyr-L-Ala-Phe tripeptides [93]. 

Chemical shift parameters for rigid and flexible Tyr-o-Ala-Phe and Tyr-L-Ala-Phe tripeptides in the soHd state were established on the basis of 2D PASS experiments [94]. The same approach was employed to investigate the influence of the stereochemistry of alanine residue on molecular properties in Tyr-Ala-Phe-Gly tetrapeptide [95]. Very recently, this experiment was performed on Leu- and Met-enkephalins (Tyr-Gly-Gly-Phe-Leu and Tyr-Gly-Gly-Phe-Met, respectively) [96]. 

Figure 2.17 2D PASS spectra of two forms of Tyr-i-Ala-Phe tripeptide before (ieft) and after (right) data shearing. See Ref. [93] for detaiis. Reprinted from Ref. [93], Copyright 2009 American Chemicai Society.

The analysis of CP buildup curves and C chemical shift tensor (CST) parameters obtained by a 2D PASS experiment have revealed a dramatic difference in the molecular motion of both modifications. C Ti relaxation times have provided further evidence confirming distinct molecular dynamics. 

CST parameters were estabHshed by means of 2D PASS technique. In order to precisely assign the Su values, the sufficient number of spinning side-... 

C solid-state NMR measurements were carried out on poly(p-xylylene). CP MAS and DFT calculations of NMR shielding parameters were performed for the optimized geometry and structure of a xylylene trimer acquired from the X-ray data, including intermolecular interactions. 2D PASS correlation was employed for the assignment of the values of the principal elements of the CSA tensor. 

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