The identification of amino acid residues at the active site of enzymes will be investigated by search for nulear Overhauser effects, i.e. diminution in the signal amplitude of an appropriate proton resonance of substrate upon irradiation of specific resonance lines of the protein. In particular, the C-2 proton of adenine appears a priori to be a favorable candidate and adenine nucleotide substrates for kinases (creatine, pyruvate, arginine and adenylate) will be examined in the enzyme complexes. Another aspect of nucleotide binding, particulary in cases where binding is highly specific for one base such as adenine at the mononucleotide site of adenylate kinase, is to search for new hydrogen bonds by NMR between base and protein in the enzyme-substrate complex and to assign them to the appropriate atom in the protein, a non-trivial problem. Quantitative determination of interatomic distances from the effect on nuclear relaxation rates due to paramagnetic probe will be studied in three systems: 1) distance from Mn(II) bound to L-arginine in the transition state analog complex of arginine kinase compared to distances between Mn(II) and analogs of L-arginine in the same type of enzyme complex 2) distance from Mn(II) to the single histidine in the active site of carp muscle adenylate kinase and 3) distance from spin label covalently bound to thio-U of t-RNA fmet (e coli) to 19F of trifluoroacetylated met t-RNA.