HCCH-TOCSY and HCCH-COSY

HcCH TOCSY experiment is based upon 2D TOCSY experiment (x and y axes are ¹H) that is extended into a third dimension (¹³C). Spectrums obtained by HCCH TOCSY method are used for side-chain assignment. Magnetization is transferred from a side – chain or backbone proton to directly attached carbon, this is followed by isotropic ¹³C mixing and finally transfer back to their attached protons for detection (Fig 1).

We have HCCH, e.g. 4 spins. However, we would like to construct a 3D spectrum. This means that chemical shift for one out of 3 spins is not encoded. The 4^th spin, H, is used to observe signal, therefore its chemical shift is always detected. There are 3 variants of this experiment:

hCCH-TOCSY

HcCH-TOCSY

HCcH-TOCSY

All of these have their advantages and all are used.

As a result we obtain strips at each carbon frequency in which all side-chain hydrogen resonances are visible. Those experiments are usually displayed in such w way that the ¹³C dimension is shown along x-axis rather than the y-axis. The HCCH-COSY spectrum is a slightly less crowded version in which only the hydrogen resonances of the own and neighboring carbons are visible (Fig 2).

Fig 1. Schematic representation of magnetization transfer in the HCCH TOCSY experiment.

Fig 2. The comparison between HCCH TOCSY and HCCH COSY spectra.

Fig 3. HCCH TOCSY experiment is based upon 2D TOCSY experiment (x and y axes are ¹H) that is extended into a third dimension (¹³C).

1. Pulse sequence for HCCH TOCSY:

The original HCCH-TOCSY pulse sequence consisted of the following steps (Fig 4):

1. After the initial 90º ¹H pulse, ¹H chemical shift evolution during the variable t₁ period takes place.

2. Fixed evolution delay to achieve antiphase ¹H magnetization with respect to ¹³C via ¹J_CH.

3. Magnetization transfer to ¹³C by applying simultaneous 90º ¹H and ¹³C pulses.

4. ¹³C chemical shift evolution during the variable t₂ period.

5. An isotropic mixing ¹³C period transfers magnetization along the ¹³C side chain via ¹J_CC.

6. ¹³C magnetization is transferred back to the protons by reversing the transfer steps described above.

7. Proton acquisition under ¹³C decoupling.

Fig 4. Pulse sequence for experiment: A HCcH-TOCSY– from original paper Kay, et al. 1993, B – HCcH COSY pulse sequence from Bruker Biospin book.

References :

1. L.E. Kay, G.Y. Xu, A.U. Singer, D.R. Muhandiram & J. D. Forman-Kay J. Magn.Reson. B 101, 333 - 337 (1993).

2. W.Peti, C. Griesinger & W. Bermel, J. Biomol. NMR 18, 199 - 205 (2000).