ct-[1H,13C]-HSQC

This experiment provides correlation between the aliphatic carbon and its attached protons (which are separated by 1 bond). The basic scheme of experiment involves the transfer of magnetization on the proton to the second nucleus (may be a ¹⁵N or ¹³C), via an INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) step. The constant time (CT) version of ¹H—¹³C HSQC is normally used as it circumvents the issue of splitting of signal due to homonuclear ¹³C—¹³C J couplings which reduces spectral resolution. The "constant time" refers to the entire evolution period between the two INEPT steps which is kept constant in this experiment. If this evolution period is set to be the inverse of the J-coupling constant, then the sign of the magnetization of those carbons with an odd number of aliphatic carbon attached will be opposite to those with an even number. For example, if the C_β of leucine appears as a positive peak (2 aliphatic carbons attached), then the C_γ (3 aliphatic carbons attached) and C_α (1 aliphatic carbons attached) would appear negative.

In the 2D CT-HSQC pulse sequence, the evolution of ¹³C chemical shift takes place during a constant time period (T), when this T is kept = 1/J (cc), then the sign of the carbons changes.

The use of CT periods has largely been used to enhance sensitivity and resolution in both 2D and 3D NMR experiments. Hence its role in the 3D HNCA would be the same.

References

1) J. Santoro, G. C. King, J. Magn. Reson. 97, 202–207 (1992).

2) G. W. Vuister, A. Bax, J. Magn. Reson. 98, 428–435 (1992).