Single crystal EPR study of electronic structure and exchange interactions for copper(II)(L-arginine)2(SO4).(H2O)6: a model system to study exchange interactions between unpaired spins in proteins.

We report EPR measurements at 9.77 and 34.1 GHz in powder and single crystal samples of the ternary copper amino acid complex Cu(L-arginine)(2)(SO(4)).(H(2)O)(6). The single crystal Electron Paramagnetic Resonance spectra display a single resonance for all magnetic field orientations in the ca and cb crystal planes. In the ab plane they display two resonances for most orientations of the magnetic field, and only one resonance for orientations close to the crystal axes. This behavior is a result of the selective collapse of the resonances corresponding to the four copper sites in the unit cell produced by the exchange interactions between copper ions. From the characteristics of the collapse and the angular dependences of the position and width of the resonances we evaluate the g-tensors of the copper molecules and estimate exchange interactions |J(1)/k(B)|=0.9 K and |J(2)/k(B)|=0.009 K between copper neighbors at 5.908 A and at 15.684 A, respectively. J(1) is assigned to a syn-anti equatorial-apical carboxylate bridge with a total bond length of 7.133 A. J(2) is assigned to a long bridge of 12 atoms with a total bond length of 19.789 A, that includes two hydrogen bonds. The results are discussed in terms of the crystal and electronic structure of Cu(L-arginine)(2)(SO(4)).(H(2)O)(6). We show that J(2) is in excellent agreement with the observed magnetic interaction between the reduced quinone acceptors in the photosynthetic reaction center protein of the bacterium Rb. sphaeroides, which is transmitted along a similar chemical path containing two hydrogen bonds. Our findings indicate that it is valid to estimate values for the exchange interactions between redox centers in proteins transmitted along long chemical paths containing sigma and H-bonds, from data obtained in model systems, and emphasize the importance of measuring exchange interactions in biologically relevant model systems.

Stereoselective synthesis of functionalized trans-2,5-disubstituted tetrahydrofurans

[reaction: see text] The addition of the titanium enolates of N-acetyl, N-propionyl, and N-bromoacetyl (R)-oxazolidin-2-ones to gamma-lactol 2, derived from (S)-glutamic acid, afforded trans- and cis-2,5-disubstituted tetrahydrofurans (trans/cis ratio: R = H, 2:1; R = Me, 8:1; R = Br, 10:1) after desilylation with aqueous HF/CH3CN. Chromatographic separation and LiBH4 reduction allowed the efficient preparation of the corresponding trans-2,5-disubstituted tetrahydrofuran diols and the recovery of the chiral auxiliary.

5-Methoxyjusticidin A, a new arylnaphthalene lignan from Protium unifoliolatum.

A new arylnaphthalene lignan, 9-(1,3-benzodioxol-5-yl)-4,5,6, 7-tetramethoxynaphtho[2,3-C]furan-1(3H)-one (5-methoxyjusticidin A, 1), was isolated from a Et2O extract of the wood of Protium unifoliolatum. The structure of 1 was determined by both spectroscopic and X-ray crystallographic methods.