CACA-TOCSY with alternate ¹³C-¹²C labeling: A ¹³C^α direct detection experiment for mainchain resonance assignment, dihedral angle information, and amino acid type identification

We present a ¹³C direct detection CACA-TOCSY experiment for samples with alternate ¹³C-¹²C labeling. It provides inter-residue correlations between ¹³C^α resonances of residue i and adjacent C^αs at positions i - 1 and i + 1. Furthermore, longer mixing times yield correlations to C^α nuclei separated by more than one residue. The experiment also provides C ^α-to-sidechain correlations, some amino acid type identifications and estimates for ψ dihedral angles. The power of the experiment derives from the alternate ¹³C-¹²C labeling with [1,3-¹³C] glycerol or [2-¹³C] glycerol, which allows utilizing the small scalar ³J_CC couplings that are masked by strong ¹J_CC couplings in uniformly ¹³C labeled samples.