Polarization effect on the two-photon absorption of a chiral compound.

In this report, we investigate the polarization effect (linear, elliptical and circular) on the two-photon absorption (2PA) properties of a chiral compound based in azoaromatic moieties using the femtosecond Z-scan technique with low repetition rate and low pulse energy. We observed a strong 2PA modulation between 800 nm and 960 nm as a function the polarization changes from linear through elliptical to circular. Such results were interpreted employing the sum-over-essential states approach, which allowed us to model the 2PA circular-linear dichroism effect and to identifier the overlapping of the excited electronic states responsible by the 2PA allowed band.

Solid-state and solution properties of the lanthanide complexes of a new heptadentate tripodal ligand: a route to gadolinium complexes with an improved relaxation efficiency.

The tripodal ligand (alpha,alpha',alpha' 'nitrilotri(6-methyl-2-pyridinecarboxylic acid)) (H(3)tpaa) forms a Gd(III) complex which has a relaxivity (r(1p) = 13.3 mM(-1) s(-1) at 25 degrees C and at 60 MHz) remarkably higher than those of the currently clinically used contrast agents based on octacoordinate polyaminocarboxylate complexes (3.5-4.7 mM(-1) s(-1)) and a reasonably good thermodynamic stability. The crystal structure of the ligand and of its La, Nd, Eu, Gd, Tb, Ho, Tm, Yb, and Lu complexes have been determined by X-ray crystallography. The neutral H(3)tpaa molecule adopts, in the solid state, a preorganized tripodal conformation in which the three H(3)tpaa arms are located on the same side of the molecule, ready to bind a metal ion in a heptadentate coordination mode. The structures of the Ln(III) complexes vary along the series for their nuclearity and number of water molecules coordinated to the metal, and a tetrameric structure is observed for the La(3+) ion (9- and 10-coordinate metal centers), dimeric structures are formed from the Nd(3+) ion through the Yb(3+) ion (9-coordinate), and a monomeric structure results for Lu(3+) (8-coordinate). The relaxivity studies presented here suggest that the high relaxivity of the Gd(tpaa) complex is mainly the consequence of a shorter bound water proton-Gd(III) distance associated with a probable water coordination equilibrium between tris(aqua) and bis(aqua) complexes, giving raise to a mean number of coordinated water molecules q > 2. Both effects are strongly related to the ligand flexibility, which allows for a large volume available for water binding. The observed rapid water exchange rate is probably due to the presence of a low-energy barrier between 10-, 9-, and 8- coordinate geometries. Although the low solubility of the Gd complex of tpaa prevents its practical application as an MRI contrast agent, the straightforward introduction of substituents on the pyridine rings allows us to envisage ligands with a higher water solubility, containing functional groups leading to macromolecular systems with very high relaxivity.

Protection against radiation-induced degradation of DNA bases by polyamines.

Polyamines have been reported to protect DNA against the formation of radiation-induced strand breaks and crosslinks to proteins. The present study was aimed at investigating the protective effect of spermine, spermidine and putrescine against the degradation of DNA bases upon exposure to gamma rays in aerated aqueous solution. The yield of 8-oxo-7,8-dihydroguanine and 5-hydroxycytosine was found to decrease for concentrations of spermine and spermidine greater than 0.1 mM. A protection factor of 10 was observed for a concentration of 1 mM of the latter two polyamines. Putrescine afforded a lower protection. In addition, the formation yield of a series of radiation-induced degradation products of the purine and pyrimidine bases was determined within DNA in the presence or absence of spermine. The protection factor was within the same range for all the lesions measured. The latter observation ruled out the possibility of degradation of DNA by radiation-induced polyamine peroxyl radicals. This was confirmed by studies involving radiolysis of DMSO and decomposition of 2,2'-azobis(2-methyl-propionamidine) as sources of alkylperoxyl radicals. Therefore, it is likely that the polyamine-mediated protection against the radiation-induced degradation of DNA bases is due to the compaction of the DNA structure and the reduction in the accessibility of DNA to .OH rather than by scavenging .OH in the bulk solution or in the vicinity of the DNA.

Solid-state and solution structure of lanthanide complexes of a new nonadentate tripodal ligand containing phenanthroline binding units.

The new nonadentate tripodal ligand trenphen (tris[(1,10-phenanthroline-2-carboxamido)-ethyl]amine) has been synthesized by condensation of tren [tris(2-aminoethyl)amine] with an excess of 1,10-phenanthroline-2-carboxylic acyl chloride. The ligand trenphen and its lanthanide complexes (Sm, Nd, Eu, Tb, and Lu) have been structurally characterized by single-crystal X-ray diffractometry. Crystals of trenphen.H2O.CH3CN, 1, are monoclinic, space group P2(1)/n, a = 14.9923(8) A, b = 17.4451(10) A, c = 17.1880(10) A, beta = 114.8290(10) degrees, V = 4079.9(4) A3, Z = 4. The solid-state crystal structures of the isostructural [Ln(trenphen)](OTf)3.yH2O.xEt2O.zCH3CN (OTf = CF3SO3) (Ln = Nd, y = 0.5, x = 1, z = 3 (2); Ln = Sm, y = 0.5, x = 1, z = 3 (3); Ln = Eu, y = 0.5, z = 3 (4); Ln = Tb, y = 0.5, x = 1, z = 1.5 (5); Ln = Lu, y = 0.5, x = 1, z = 1.5 (6)) (trigonal, P-3, Z = 2) show that the covalent tripod trenphen undergoes a rearrangement in the presence of lanthanide ions yielding three tridentate binding units which encapsulate the nine-coordinated lanthanide ion with a slightly distorted, tricapped, trigonal prismatic coordination geometry. The correlation observed between the decrease of Ln-N distances and the metal ionic radius indicates that trenphen, although containing rigid bidentate phenanthroline units, is sufficiently flexible to self-organize without steric constraints around lanthanide ions of different size. Solution-state NMR studies show that complexes 2-6 exist in acetonitrile solution as discrete rigid C3-symmetric species retaining the triple-helical structure observed in the solid state. NMR and ES-MS titration show the formation of bimetallic and trimetallic species in the presence of an excess of metal, whereas mononuclear bistrenphen complexes are obtained in the presence of an excess of ligand.