The [2Fe-2S]ferredoxin I (Fd I) from parsley leaves (M r ϭ 10500; 96 amino acids) in the Fe(III)-Fe(III) oxidized form has been studied by 1 H-NMR spectroscopy. Sequence-specific 1 H-NMR assignments were obtained through two-dimensional classical double-quantum-filtered-COSY, NOESY and TOCSY spectra. NOEs between protons as close as 5.6 Å from the paramagnetic Fe(III) atoms were observed at 800 MHz. A total of 3066 NOEs (of which 2533 are meaningful) and 18 distance constraints taken from X-ray crystallography of the Fe 2 S 2 active site were used to obtain the solution structure. From inversion recovery NOESY experiments, 33 longitudinal relaxation rate (AE para ) constraints were used for the structural refinement. The final structure was obtained by a process of restrained energy minimization. Root-mean-square (rmsd) deviation values obtained for the family of 18 structures (with reference to the average structure) are 0.52 Ϯ0.10 Å and 0.91Ϯ 0.12 Å for backbone and all heavy atoms respectively. The structure consists of seven-strands of β-sheets and four short A-helices. The quality of the present solution structure is among the best of those reported for [2Fe-2S]ferredoxins. The secondary structure and overall folding are compared with those of Anabaena variabilis Fd and the higher plant Equistum arvense (horse tail) protein determined through X-ray crystallography. The groups believed to be responsible for electron transfer have been analysed.Keywords : ferredoxin; solution structure; paramagnetism. Plant [2Fe-2S]ferredoxins are very important redox components involved in photosystem I (PSI) electron transport [1Ϫ3].They perform one-electron transfer (ET) from subunit PsaC of PSI to ferredoxin NADP-reductase (FNR), ferredoxin nitrite oxidoreductase, glutamate synthase and sulfite reductase [4]. There are now more than 75 amino acid sequences for this class of protein from higher plants, algae and cyanobacteria [1] (Table 1). Plant ferredoxins are very acidic (pI ϭ 3.4), with a charge balance of Ϫ18(Ϯ 2) at pH 7.5 from amino acid compositions. From X-ray crystallography on cyanobacteria PSI (6 Å resolution), it appears that the PSI subunits PsaD, PsaE and PsaC are involved in establishing an electrostatic orientation of the located on Fe(I), which is the Fe closest to the protein surface (Ϸ5 Å ) [17].The NMR technique is a powerful method for determining solution structures of small biomolecules. At first it was believed that it would be very difficult (or impossible) to detect a sufficiently large number of constraints for solution structure determination of paramagnetic metalloproteins. Since the first NMR solution structure of a paramagnetic protein in 1994 [18], new strategies and approaches have been introduced. In the few NMR solution structures of oxidized [2Fe-2S]ferredoxins reported from Pseudomonas putida [19], Synechocystis sp. PCC 6803 [20] and Synechococcus elongatus [21], the quality of the structures is severely hampered by the lack of NOEs closer than about 8 Å from the iron atoms, as previou...