PHASE LABELING OF C-H AND C-C SPIN-SYSTEM TOPOLOGIES - APPLICATION INCONSTANT-TIME PFC-CBCA(CO)NH EXPERIMENTS FOR DISCRIMINATING AMINO-ACID SPIN-SYSTEM TYPES

Triple-resonance experiments facilitate the determination of sequence- specific resonance assignments of medium-sized C-13,N-15-enriched prot eins. Some triple-resonance experiments can also be used to obtain inf ormation about amino acid spin-system topologies by proper delay tunin g. The constant-time PFG-CBCA(CO)NH experiment allows discrimination b etween five different groups of amino acids by tuning (phase labeling) independently the delays for proton-carbon refocusing and carbon-carb on constant-time frequency labeling. The proton-carbon refocusing dela y allows discrimination of spin-system topologies based on the number of protons attached to C-alpha and C-beta atoms (i.e. C-H phase labeli ng). In addition, tuning of the carbon-carbon constant-time frequency- labeling delay discriminates topologies based on the number of carbons directly coupled to C-alpha and C-beta atoms (i.e. C-C phase labeling ). Classifying the spin systems into these five groups facilitates ide ntification of amino acid types, making both manual and automated anal ysis of assignments easier. The use of this pair of optimally tuned PF G-CBCA(CO)NH experiments for distinguishing five spin-system topologie s is demonstrated for the 124-residue bovine pancreatic ribonuclease A protein.