After entry into lymph nodes (LNs), B cells migrate to follicles, whereas T cells remain in the paracortex, with each lymphocyte type showing apparently random migration within these distinct areas. Other than chemokines, the factors contributing to this spatial segregation and to the observed patterns of lymphocyte movement are poorly characterized. By combining confocal, electron, and intravital microscopy, we showed that the fibroblastic reticular cell network regulated naive T cell access to the paracortex and also supported and defined the limits of T cell movement within this domain, whereas a distinct follicular dendritic cell network similarly served as the substratum for movement of follicular B cells. These results highlight the central role of stromal microanatomy in orchestrating cell migration within the LN.

New asymmetrical ligands (H 2 L) have been synthesized to provide both a bridging and a terminal phenolate to a pair of iron ions in order to mimic the binding of a single terminal tyrosinate at the diiron center of the purple acid phosphatases. H 2 L1 is 2-[(bis(2-pyridylmethyl)amino)methyl]-6-[((2-pyridylmethyl)(2-phenol)amino)methyl]-4-methylphenol and H 2 L′1 and H 2 L2 are obtained by replacing the 2-phenol group by the 5-nitro-2-phenol and the 6-methyl-2-phenol residues, respectively. A series of mixed valence diiron complexes [Fe II Fe III L(X) 2 ](Y) have been obtained where (X) 2 is the dianion of m-phenylenedipropionate or (H 2 PO 4 ) 2 and Y )Diferric complexes have been obtained also either by direct synthesis or by iodine oxidation of the mixed valence precursor (L ) L1, 3a (X) 2 ) mpdp, Y ) BPh 4 , 3d: (X) 2 ) (H 2 PO 4 ) 2 , Y ) PF 6 ; L ) L2, 4d: (X) 2 ) (H 2 PO 4 ) 2 , Y ) PF 6 . Complex 1a [Fe II Fe III L(mpdp)](BPh 4 ) has been characterized by X-ray diffraction techniques. 1a crystallizes in the monoclinic space group P21/a with the following unit cell parameters: a ) 22.038 (9) Å, b ) 16.195 (8) Å, c ) 16.536 (7) Å, β ) 97.26 (1)°, Z ) 4. The significant differences in the Fe-O bond lengths indicate that the metal centers are ordered. The complexes have been studied by electronic spectral, resonance Raman, magnetic susceptibility, Mo ¨ssbauer, NMR, and electrochemical techniques. Mo ¨ssbauer and NMR spectroscopies concur to probe that the valences of the mixed valence compounds are trapped in solution as well as in the solid state at room temperature. The electronic spectrum of the mixed-valence compounds are dominated by a charge transfer transition in the 400-600 nm domain which moves to the 550-660 nm range upon oxidation to the diferric state. In addition they exhibit a weak and broad intervalence transition close to 1100 nm. Electrochemical studies show that the systems exist in the three redox states Fe

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Complexes of the nitroxide ligand 2-(2-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-l/f-imidazolyl-1 -oxy 3-oxide (NIT2-Py) with NiCl2 (1, CmHjíCIíNíNíO*) and MnCl2 (2, C26H36Cl6MnN604) have been structurally and magnetically characterized. Both complexes are centrosymmetric three-spin systems in which the nitroxyl group is coordinated to the metal ion. Metal-nitroxide interactions of -110 cm-1 for 1 and -80 cm-1 for 2 (H = -2JS¡Sj) are observed. These coupling constants are weaker than usually observed in previously reported examples (<-200 cm-1 for Ni and -150, -200 cm-1 for Mn). This result was rationalized by considering a reduced overlap of the magnetic orbitals due to a peculiar geometric arrangement in the chelate. Comparison to literature reported species is obtained from the homologous hexafluoroacetylacetonates, 3 (C22H18F12N3Ni06) and 4 (C22Hi8F12MnN3C>6), which exhibit similar structural and magnetic properties. Coordination of the nitroxyl group in these derivatives illustrates the use of the chelate effect for designing exchange-coupled nitroxide derivatives of poor electrophilic metal centers. Relevant structural parameters are as follows: 1, monoclinic, P2\¡c, a = 8.890

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A new hexadentate ligand 1 was used to prepare the first FeIIFeIII complex that mimics the unsymmetrical coordination in the active site of one form of semimet hemerythrin. The different valences of the iron ions in this model complex are localized in solution, in analogy to the iron centers in the protein.