TOCSY

TOCSY

TOCSY (Total Correlation Spectroscopy) creates correlations between all protons within a given spin system, not just between geminal or vicinal protons as in COSY. Correlations are seen between distant protons as long as there are couplings between every intervening proton. This is extremely useful for identifying protons on sugar rings or amino acids: All protons on a given sugar ring will have a correlation with all other protons on the same ring but not with protons on different rings.
Magnetization is transferred successively over up to 5 or 6 bonds as long as successive protons are coupled. Transfer is interrupted by small or zero proton-proton couplings. The presence of hetero-atoms, such as oxygen, usually disrupts TOCSY transfer. The number of transfer steps can be adjusted by changing the spin-lock time. A short time such as 20ms will give only one-step transfers and its TOCSY spectrum will be very similar to its COSY spectrum. More usefully, a long spin-lock time such as 80ms or 120ms will give up to 5 or 6-step transfers. The number of transfers depends on exact coupling details. A useful paper detailing TOCSY transfer in various sugars is Gheysen, K. et. al., Chem. Eur. J. 2008, 14, 8869-8878.

Shown below is a 400 MHz spectrum of sucrose. The red circles show the connections between proton 4 and all the other protons of the glucose ring.

TOCSY (TOtal Correlation SpectroscopY) is very similar to COSY in that it is a homonuclear correlation technique. It differs from COSY in that it not only shows nuclei that are directly coupled to each other, but also signals that are due to nuclei that are in the same spin system, as shown in Figure below. This technique is useful for interpreting large, interconnected networks of spin couplings. The pulse sequence is arranged in such a way to allow for isotropic mixing during the sequence that transfers magnetization across a network of atoms coupled to each other. An alternative technique to 2D TOCSY is selective 1D TOCSY, which can excite certain regions of the spectrum by using shaped pulses. By specifying particular chemical shift values and setting a desired excitation width, one can greatly simplify the 1D experiment. Selective 1D TOCSY is particularly useful for analyzing polysaccharides, since each sugar subunit is an isolated spin system, which can produce its own subspectrum, as long as there is at least one resolved multiplet. Furthermore, each 2D spectrum can be acquired with the same resolution as a normal 1D spectrum, which allows for an accurate measurement of multiplet splittings, especially when signals from different coupled networks overlap with one another.

An example of a spectrum of xylobiose (structure shown in Figureb) taken from a TOCSY 2D NMR experiment. The diagonal peaks are again highlighted by the red dotted line. Figure from F. Sauriol, NMR Webcourse, Department of Chemistry, Queen’s University, Ontario, www.chem.queensu.ca/facilities/nmr/nmr/webcourse/.