Cross-polarization heteronuclear shift

In the following, we will discuss heteronuclear polarization-transfer techniques in four different contexts. They can be used as a polarization-transfer method to increase the sensitivity of a nucleus and to shorten the recycle delay of an experiment as it is widely used in 1H-13C or 1H-15N cross polarization. Heteronuclear polarization-transfer methods can also be used as the correlation mechanism in a multi-dimensional NMR experiment where, for example, the chemical shifts of two different spins are correlated. The third application is in measuring dipolar coupling constants in order to obtain distance information between selected nuclei as is often done in the REDOR experiment. Finally, heteronuclear polarization transfer also plays a role in measuring dihedral angles by generating heteronuclear double-quantum coherences. 

NMR in dichloromethane-d1. The spectrum was run quantitatively with broad band heteronuclear decoupling, b) 1JC CPMAS NMR of uncured PTEB. Cross-polarization time was 4.5ms the rotor speed was 2.3 kHz the cross polarization field was 50 kHz. Chemical shifts are relative to TMS. 

Cross polarization can also be used for magnetization transfer between low-gamma nuclei like and as a mixing sequence in a multidimensional NMR experiment. Due to the substantial shift separation of about 120 ppm between carbonyl and Ca carbon atoms in proteins, the CP transfer can be tailored specifically to NCa- or NCO-transfer (SPECIFIC-CP) [81]. In protein NMR spectroscopy such a heteronuclear correlation experiment may be incorporated into multidimensional experiments and thus be used to residue-specific resonance assignment in NMR spectroscopy of immobilized peptides or proteins [82]. 

As shown in the list of relay COSY experiments heteronuclear correlation experiments are possible. In Check it 5.4.1.14 the H-H-X relay IR, DC COSY experiment for the crotonaldehyde type spin system is calculated. Implementing a relay step to a heteronucleus enables complex IR COSY spectrum to be disentangled by including a heteronuclear polarization transfer to link the IR signals to the heteronuclear chemical shift dimension. Fig. 5.26 illustrates this schematically for two spin systems. Since 8(Ra) = 5(Rd) and 5(Rt>) = 5(Rg) at least two IR, IR relay cross peaks which belong to two different relayed spin systems overlap in the IR, IR spectrum. Rowever because 5(Cc) 4 5(Cf) these peaks may be separated if the correlation peaks can be related to the heteronucleus which has a different chemical shift for each peak. 

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