The thermodynamic stabilities of the GdIII complexes of five hexadentate ligands, which incorporate the 2,3-dihydroxyterephthalamide and 2,3-hydroxypyridonate chelating moieties, have been determined ...by potentiometric and spectrophotometric titration. The ligands were chosen to span a range of basicities while maintaining a similar tripodal structural motif, facilitating a study of the effect of ligand basicity on the thermodynamic stability of the GdIII complexes. The relative stability of the five complexes is found to be highly pH dependent, with the most acidic ligands forming the most stable complexes at low pH and more basic ligands forming more stable complexes at high pH. The most stable GdIII complex at a physiological pH of 7.4 is formed with a ligand of intermediate basicity and is of stability comparable to that of GdIII complexes that feature eight-coordinate amino−carboxylate ligands and are currently used as magnetic resonance imaging contrast agents in diagnostic medicine. A single-crystal X-ray structure of the intermediate compound 3-hydroxy-6-methyl-2-oxo-1,2-dihydropyridine-4-carboxylic acid ethyl ester is described: This compound crystallizes in the triclinic space group P1̄ with a = 7.4801(3) Å, b = 8.0671(3) Å, c = 8.3457(4) Å, α = 72.242(2)°, β = 80.693(2)°, γ = 69.943(3)°, V = 449.60(3) Å3, Z = 2, and R = 0.042.
The tetradentate imino-carboxylate ligand L2- chelates the equatorial sites of NiII to give the complex Ni(L)(MeOH)2 in which a NiII center is bound in an octahedral coordination environment with ...MeOH ligands occupying the axial sites. Lanthanide (Ln) and Group II metal ions (M) template the aggregation of six Ni(L) fragments into the octahedral cage aggregates {MNi(L)6} x + (1: M = SrII; x = 2, 2: M = BaII; x = 2, 3: M = LaIII; x = 3, 4: M = CeIII; x = 3, 5: M = PrIII; x = 3, and 6: M = NdIII; x = 3). In the presence of Group I cations, however, aggregates composed of the alkali metal-oxide cations template various cage compounds. Thus, Na+ forms the trigonal bipyramidal Na5O3+ core within a tricapped trigonal prismatic Ni(L)9 aggregate to give {(Na5O)⊂Ni(L)9(MeOH)3}(BF4)2·OH·CH3OH, 7. Li+ and Na+ together form a mixed Li+/Na+ core comprising distorted trigonal bipyramidal Na3Li2O3+ within an approximately anti-square prismatic Ni(L)8 cage in {(Na3Li2O)⊂Ni(L)8(CH3OH)1.3(BF4)0.7}(BF4)2.3·(CH3OH)2.75·(C4H10O)0.5, 8, while in the presence of Li+, a tetrahedral Li4O2+ core within a hexanuclear open cage Ni(L)6 in {(Li4O)⊂Ni(L)6(CH3OH)3}2ClO4·1.85CH3OH, 9, is produced. In the presence of H2O, the Cs+ cation induces the aggregation of the Ni(L)(H2O)2 monomer to give the cluster Cs2Ni(L)(H2O)26·2I·4CH3OH·5.25H2O, 10. Analysis by electronic spectroscopy and mass spectrometry indicates that in solution the trend in stability follows the order 1−6 > 7 > 8 ∼ 9. Magnetic susceptibility data indicate that there is net antiferromagnetic exchange between magnetic centers within the cages.
The effect of ligand structure on the magnetic resonance (MR) imaging and biodistribution of six gadolinium (Gd) chelates based on a hydroxypyridonate−terephthalimide (HOPO−TAM) ligand design was ...investigated. Modifications to the molecular structure of the Gd−HOPO−TAM chelates (hydrophilicity and aromatic group substitution) significantly influence the efficacy of imaging and biodistribution. MR imaging was performed on female mice after intravenous (iv) injection of 100 μmol of Gd/kg of body weight of the different complexes. The biodistribution results indicate that the liver uptake of the complexes is enhanced by a short poly(ethyleneoxy) (PEO) chain, while blood pool localization is facilitated by a very long PEO chain. There is a direct correlation between the blood pool localization of the complexes and the signal intensity of blood vessels in the MRI. The imaging results are consistent with in vitro NMR measurements that indicate long PEO chains increase image enhancement capabilities in the presence of serum albumin.
The variation of the size of the capping scaffold which connects the hydroxypyridonate (HOPO) binding units in a series of tripodal chelators for gadolinium (Gd) complexes has been investigated. A ...new analogue of TREN-1-Me-3,2-HOPO (1) (TREN = tri(ethylamine)amine) was synthesized: TREN-Gly-1-Me-3,2-HOPO (2) features a glycine spacer between the TREN cap and HOPO binding unit. TRPN-1-Me-3,2-HOPO (3) has a propylene-bridged cap, as compared to the ethylene bridges within the TREN cap of the parent complex. Thermodynamic equilibrium constants for the acid−base properties of 2 and the Gd3+ complexation strength of 2 and 3 were measured and are compared with that of the parent ligand. The most basic ligand is 2 while 3 is the most acidic. Both 2 and 3 form Gd3+ complexes of similar stability (pGd = 16.7 and 15.6, respectively) and are less stable than the parent complex Gd-1 (pGd = 19.2). Two of the three complexes are more stable than the bis(methylamide)diethylenetriamine pentaacetate complex Gd(DTPA-BMA) (pGd = 15.7) while the other is of comparable stability. Enlargement of the ligand scaffold decreases the stability of the Gd3+ complexes and indicates that the TREN scaffold is superior to the TRPN and TREN-Gly scaffolds. The proton relaxivity of Gd-2 is 6.6 mM-1 s-1 (20 MHz, 25 °C, pH 7.3), somewhat lower than the parent Gd-1 but higher than that of the MRI contrast agents in clinical practice. The pH-independent relaxivity of Gd-2 is uncharacteristic of this family of complexes and is discussed.
Lanthanide, main group, and transition metal ion templates provide different polynuclear cages from M(L) (M=Ni, Mn; (L)2−=CH2CH2N=C(CH3)COO−2). Templating with lanthanum results in a 12‐coordinate ...LaIII ion encapsulated by six Ni(L) units, whereas with sodium four Na+ ions are trapped inside a tricapped trigonal prismatic {Ni(L)}9 cage. With manganese, an octahedrally coordinated MnII ion is surrounded by six Mn(L) fragments in a twisted trigonal‐prismatic configuration (see picture).
Nominal operating cell temperature, NOCT, defined by module heat generation and loss mechanisms, is an important factor relating to energy conversion efficiency of PV modules. Development of ...simulation tools for predicting NOCT is an important element of designing modules that may function to operate at lower NOCT. When modeling the heat generation from incoming radiation, it is highly desirable to accurately simulate the solar energy conversion in the cell, glass, encapsulant and anti-reflective layers, as well as the spaces between the cells. We present herein an approach that starts from the fundamental of electromagnetic wave (EMW) propagation based on Maxwell's equations. This approach enables the EMW energy in individual layers and at different regions of the PV module to be evaluated from the dielectric constants and thickness of each material. Once the light intensity in the cell is known, it becomes possible to predict cell electrical parameters using established electrical models. This methodology was applied to a packaged back-junction-back-contact (BJBC) cell and compared with parameters derived from an experimental I-V curve. Good agreement was observed for both electrical parameters and external quantum efficiency. The distribution of solar energy conversion within the module was further combined with heat transfer and 3D finite element analysis models to enable a prediction of thermal profiles of the module. The simulated temperature distribution was found to agree well with experimental measurements using a thermal imaging camera and thermocouples.
The synthesis and relaxometric properties of hetero-tripodal hydroxypyridonate-terephthalamide gadolinium (Gd(3+)) chelates with differing structural features for probing human serum albumin (HSA) ...interactions are reported. The Gd(3+) complexes are divided into two series. The first series (3-5) features a benzyl derivative connected to the hydroxypyridonate (HOPO) moiety. The second series of complexes (6-10) has the common feature of a poly(ethylene glycol) (PEG) attached to the terephthalamide (TAM) moiety and is nonbenzylated. The water exchange of the complexes is in the fast exchange regime with rates (k(ex)) in the range 0.45-1.11 x 10(8) s(-1). The complexes have a moderate interaction with HSA with association constants (K(A)'s) in the range 0.7-8.6 x 10(3) M(-1). Protein binding results in an enhancement in proton relaxivity from 7.7-10.4 mM(-1) s(-1) (r(1p)) to 15-29 mM(-1) s(-1) (r(1p)(b)). It is concluded that the interaction of the complexes with HSA (i) is enhanced by the presence of benzyl groups, (ii) is entropically driven, and (iii) results in a lower hydration number (q).
The thermodynamic selectivity for Gd(3+) relative to Ca(2+), Zn(2+), and Fe(3+) of two ligands of potential interest as magnetic resonance imaging (MRI) contrast agents has been determined by NMR ...spectroscopy and potentiometric and spectrophotometric titration. The two hexadentate ligands TREN-6-Me-3,2-HOPO (H(3)L2) and TREN-bisHOPO-TAM-EA (H(4)L3) incorporate 2,3-dihydroxypyridonate and 2,3-dihydroxyterephthalamide moieties. They were chosen to span a range of basicity while maintaining a structural motif similar to that of the parent ligand, TREN-1-Me-3,2-HOPO (H(3)L1), in order to investigate the effect of the ligand basicity on its selectivity. The 1:1 stability constants (beta(110)) at 25 degrees C and 0.1 M KCl are as follows. L2: Gd(3+), 20.3; Ca(2+), 7.4; Zn(2+), 11.9; Fe(3+), 27.9. L3: Gd(3+), 24.3; Ca(2+), 5.2; Zn(2+), 14.6; Fe(3+), 35.1. At physiological pH, the selectivity of the ligand for Gd(3+) over Ca(2+) increases with the basicity of the ligand and decreases for Gd(3+) over Fe(3+). These trends are consistent with the relative acidities of the various metal ions;- more basic ligands favor harder metals with a higher charge-to-radius ratio. The stabilities of the Zn(2+) complexes do not correlate with basicity and are thought to be more influenced by geometric factors. The selectivities of these ligands are superior to those of the octadentate poly(aminocarboxylate) ligands that are currently used as MRI contrast agents in diagnostic medicine.
