Multinuclear magnetic resonance studies of the 2Fe.cntdot.2S* ferredoxin from Anabaena species strain PCC 7120. 1. Sequence-specific hydrogen-1 resonance assignments and secondary structure in solution of the oxidized form

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Abstract
Complete sequence-specific assignments were determined for the diamagnetic 1H resonances from Anabaena 7120 ferredoxin (Mr = 11 000). A novel assignment procedure was followed whose first step was the identification of the 13C spin systems of the amino acids by a 13C{13C} double quantum correlation experiment. Then, the 1H spin systems of the amino acids were identified from the 13C spin systems by means of direct and relayed 1H{13C} single-bond correlations. The sequential resonance assignments were based mainly on conventional interresidue 1H.alpha.i-1HNi+1 NOE connectivities. Resonances from 18 residues were not resolved in two-dimensional 1H NMR spectra. When these residues were mapped onto the X-ray crystal structure of the homologous ferredoxin from Spirulina platensis, it was found that they correspond to amino acids close to the paramagnetic 2Fe.cntdot.2S* cluster. Cross peaks in two-dimensional homonuclear 1H NMR spectra were not observed for any protons closer than about 7.8 .ANG. to both iron atoms. Secondary structural features identified in solution include two antiparallel .beta.-sheets, one parallel .beta.-sheet, and one .alpha.-helix.

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