Complete resonance assignments are mandatory for de-will refer to as (HB)CB(CG)CDHD. Finally, the assigned side-chain resonances are linked to the backbone by use of tailed NMR studies of protein structures in solution. In many proteins, aromatic residues are involved in the construction an (H)CCH-COSY spectrum, thus determining the position along the amino-acid sequence. The correlations provided of the hydrophobic core and thus a large number of relevant long-range NOE contacts can be identified. A wealth of by each of the experiments are summarized in Fig. 1. Including the 13 C g chemical shifts leads to an overdetermination triple-resonance methods have been developed which allow establishment of sequential connectivities via the scalar cou-of the connectivity between the aliphatic and aromatic nuclei which can prove useful in the case of overlapping 13 C b or pling network (1). In contrast, the assignment of aromatic 1 H and 13 C resonances usually relies on the detection of NOE 13 C d resonances. Also, knowledge of the characteristic 13 C g chemical shifts allows distinguishing amino-acid types be-effects between aromatic and aliphatic protons of the same residue, although the use of HMBC experiments at natural fore complete assignment of the aromatic spin systems.
The pulse sequences in Fig. 2 likewise are of the ''out-13 C abundance has also been reported for this purpose (2). Standard HCCH-COSY (3) and HCCH-TOCSY (4, 5) ex-and-back'' type. The HCBCG experiment is similar to ct-HCACO ( 10) , which has already been proposed for periments, however, are not designed for a transfer of magnetization between the b-and g-carbon spins of aromatic the correlation of 1 H b and 13 C g( aromatic ) resonances in a two-dimensional version ( 11 ) . However, employing an side chains. The NOE-based approach may lead to ambiguities concerning the positions of the 1 H nuclei both in the HMQC-rather than an INEPT-type transfer between the two 13 C species has the advantage of completely remov-ring system and in the amino-acid sequence. It has been demonstrated that intraresidual connectivities in aromatic ing the effect of the passive 1 J CaCb coupling ( 12 -14 ) .
The delays T and e should be tuned to maximize the amino acids can be obtained reliably in carbon-detected 13 C, 13 C double-quantum correlations (6, 7). More recently, ex-expression periments have been introduced which correlate the chemical shifts of b-carbons and of aromatic protons either in a multi-sin 2 ͫ pJ CbCgͩ T 2 0 2e ͪͬ cos(pJ CaCb T )exp ͫ 0 T T 2 ( 13 C b ) ͬ .