Relayed COSY
Relayed spectroscopy introduces one more magnetization step into a COSY-type experiment. This is accomplished by an additional 180 º pulse and an 90 º compared to the COSY). The pulse sequence is:
We can describe the pulse sequence by the following product operator terms. Consider three spins, I1, I2, and I3, in which I1 and I2, and I2 and I3 are coupled:
I1z -I1x -2I1yI2z 2I1zI2y (-I2x) -2I2yI3z
The pathways of the magnetization transfer correspond to the COSY, therefore COSY and RELAYED cross peaks are found in this spectrum.
The advantage of the RELAYED COSY are the added cross peaks which can be helpful to overcome overlap/ missing peaks in COSY spectra, as we can transfer magnetization to spins not coupled (or coupled only with a small coupling constant) via two well-coupled resonances. However, otherwise the RELAYED COSY has the same disadvantages as the COSY spectrum (different phase modulation for COSY, and relayed peaks).
Notes:
In the last lecture, we have seen how we can use 2D-NMR to transfer magnetization between spins, and connect resonances by different properties. In COSY and DQF-COSY spectra, we connect spins through bonds. There is only one transfer step (mixing pulse ) in both experiments. Hence, magnetization is transferred from one nucleus to another one which is coupled to it ( 2IxSz - 2IzSx), which means we obtain cross peaks for vicinal or geminal protons only. These can be used to stepwise connect all protons coupled to another (spin system), but this is in practice often difficult. Next to signal overlap, in (DQF)COSY spectra small coupling constants, (i.e. smaller than the peptide/protein line widths) will lead to low signal intensity or even lack of some cross peaks due to their antiphase structure.