Time dependence of ¹³C-¹³C magnetization transfer in isotropic mixing experiments involving amino acid spin systems

The time dependence of ¹³C-¹³C coherence transfer for the aliphatic portions of amino acid side chains has been examined by calculating ideal coherence-transfer functions. These results were compared to experimentally determined cross- and diagonal-peak intensities obtained from a series of 2D ¹³C TOCSY experiments acquired with 12 mixing times ranging from 0.0 to 34.2 ms on a mixture of uniformly ¹³C-labeled amino acids. In general, the calculated coherence-transfer functions agreed. Unlike ¹H-¹H TOCSY experiments, the time dependence of ¹³C-¹³C coherence transfer for these spin systems is simple and well characterized due to the large and conformationally independent ¹³C-¹³C spin-spin couplings. Spectra obtained at mixing times of 8, 14, 20, and 28 ms cover most features of the coherence-transfer functions such as vanishingly small cross peaks and local maxima and minima. These features, along with the fact that ¹³C-¹³C coherence transfer takes place in times which are short compared to relaxation times typical for small proteins, make this experiment ideal for the assignment of protein ¹³C spectra, which in turn aids in the ¹H NMR assignment.