In vivo and in vitro (27)Al NMR studies of aluminum(III) chelates of triazacyclononane polycarboxylate ligands.

The metallic radioisotope of a known radiopharmaceutical chelate, (67)Ga(NOTA) (NOTA=1,4,7-triazacyclonane-1,4,7-triacetic acid), used for tumor detection, was substituted by the chemically similar but non radioactive aluminum ion. Our aim was to detect and evaluate the in vivo behavior of the chelate. For this purpose, Al(NOTA) and the related chelate Al(NODASA) (NODASA=1,4,7-triazacyclononane-1-succinic acid-4,7-diacetic acid) were studied using in vitro and in vivo (27)Al NMR spectroscopy in rats. Both chelates showed high stability towards acid catalyzed dissociation and their (27)Al NMR resonances are characteristic of highly symmetrical species, with chemical shifts within the range for octahedral or pseudo-octahedral geometries. The thermodynamic stability constant of the novel chelate Al(NODASA) was estimated using (27)Al NMR. The value obtained suggested that the chelate does not undergo in vivo demetalation by transferrin. The in vivo spectroscopic studies and the analysis of blood and urine samples for Al(III) concentrations indicated that the chelates remain intact under physiological conditions and that they are mainly eliminated from the body through the kidneys.     

Structural and in vivo studies of metal chelates of Ga(III) relevant to biomedical imaging

The solution chemistry and structure of the complex of the triazamacrocyclic ligand NOTP (1,4,7-triazacyclononane-1,4,7-tris(methylenephosphonate)) with Ga3+ in D2O have been investigated by 1H, 71Ga and 31P NMR spectroscopy. These NMR results show the presence of a 1:1 Ga(NOTP)3- complex, with a highly symmetrical, pseudo-octahedral geometry, possibly with a C3 axis. The 1H spectrum shows that the triazamacrocyclic chelate ring is very rigid, with all the ring protons non-equivalent. The complex is stable in aqueous solution in a wide pH range. Its high thermodynamic stability agrees well with previous results from biodistribution and gamma imaging studies in Wistar rats with 67Ga3+ chelates of triaza macrocyclic ligands, which showed that the neutral chelates 67Ga(NOTA) (where NOTA is 1,4,7-triazacyclononane-1,4,7-triacetate) and 67Ga(NOTPME) (where NOTPME is 1,4,7-triazacyclononane-1,4,7-tris(methylenephosphonate monoethylester)) have similar in vivo behaviour, with high stability and rapid renal excretion, but the high negatively charged 67Ga(NOTP)3- has a considerably slower kidney uptake and elimination.     

Positron emission tomography imaging of CD105 expression with a 64Cu-labeled monoclonal antibody: NOTA is superior to DOTA

Optimizing the in vivo stability of positron emission tomography (PET) tracers is of critical importance to cancer diagnosis. In the case of (64)Cu-labeled monoclonal antibodies (mAb), in vivo behavior and biodistribution is critically dependent on the performance of the bifunctional chelator used to conjugate the mAb to the radiolabel. This study compared the in vivo characteristics of (64)Cu-labeled TRC105 (a chimeric mAb that binds to both human and murine CD105), through two commonly used chelators: 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). Flow cytometry analysis confirmed that chelator conjugation of TRC105 did not affect its CD105 binding affinity or specificity. PET imaging and biodistribution studies in 4T1 murine breast tumor-bearing mice revealed that (64)Cu-NOTA-TRC105 exhibited better stability than (64)Cu-DOTA-TRC105 in vivo, which resulted in significantly lower liver uptake without compromising the tumor targeting efficiency. In conclusion, this study confirmed that NOTA is a superior chelator to DOTA for PET imaging with (64)Cu-labeled TRC105.     

High-yielding aqueous 18F-labeling of peptides via Al18F chelation

The coordination chemistry of a new pentadentate bifunctional chelator (BFC), NODA-MPAA 1, containing the 1,4,7-triazacyclononane-1,4-diacetate (NODA) motif with a methylphenylacetic acid (MPAA) backbone, and its ability to form stable Al(18)F chelates were investigated. The organofluoroaluminates were easily accessible from the reaction of 1 and AlF(3). X-ray diffraction studies revealed aluminum at the center of a slightly distorted octahedron, with fluorine occupying one of the axial positions. The tert-butyl protected prochelator 7, which can be synthesized in one step, is useful for coupling to biomolecules on solid phase or in solution. High yield (55-89%) aqueous (18)F-labeling was achieved in 10-15 min with a tumor-targeting peptide 4 covalently linked to 1. Defluorination was not observed for at least 4 h in human serum at 37 °C. These results demonstrate the facile application of Al(18)F chelation using BFC 1 as a versatile labeling method for radiofluorinating other heat-stable peptides for positron emission imaging.     

Solid-Supported NOTA and DOTA Chelators Useful for the Synthesis of 3'-Radiometalated Oligonucleotides

Esterified precursors of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA; 18) and 1,4,7-triazacyclononane-1,4,7-trisacetic acid (NOTA; 17,19) ligands bearing a dimethoxytritylated hydroxyl side arm were prepared and immobilized via an ester linkage to long chain alkyl amine derivatized controlled pore glass (LCAA-CPG). Oligonucleotide chains were then assembled on the hydroxyl function and conjugates were released and deprotected by a two-step cleavage with aqueous alkali and ammonia. The 3'-DOTA and 3'-NOTA conjugated oligonucleotides were converted to (68)Ga chelates by a brief treatment with [(68)Ga]Cl(3) at elevated temperature. Applicability of the conjugates for in vivo imaging with positron emission tomography (PET) was verified.     

Synthesis and radiolabeling of chelator-RNA aptamer bioconjugates with copper-64 for targeted molecular imaging

Ribonucleic acid (RNA) aptamers with high affinity and specificity for cancer-specific cell-surface antigens are promising reagents for targeted molecular imaging of cancer using positron emission tomography (PET). For this application, aptamers must be conjugated to chelators capable of coordinating PET-radionuclides (e.g., copper-64, (64)Cu) to enable radiolabeling for in vivo imaging of tumors. This study investigates the choice of chelator and radiolabeling parameters such as pH and temperature for the development of (64)Cu-labeled RNA-based targeted agents for PET imaging. The characterization and optimization of labeling conditions are described for four chelator-aptamer complexes. Three commercially available bifunctional macrocyclic chelators (1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid mono N-hydroxysuccinimide [DOTA-NHS]; S-2-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid [p-SCN-Bn-NOTA]; and p-SCN-Bn-3,6,9,15-tetraazabicyclo [9.3.1]pentadeca-1(15),11,13-triene-3,6,9-triacetic acid [p-SCN-Bn-PCTA]), as well as the polyamino-macrocyclic diAmSar (3,6,10,13,16,19-hexaazabicyclo[6.6.6] icosane-1,8-diamine) were conjugated to A10-3.2, a RNA aptamer which has been shown to bind specifically to a prostate cancer-specific cell-surface antigen (PSMA). Although a commercial bifunctional version of diAmSar was not available, RNA conjugation with this chelator was achieved in a two-step reaction by the addition of a disuccinimidyl suberate linker. Radiolabeling parameters (e.g., pH, temperature, and time) for each chelator-RNA conjugate were assessed in order to optimize specific activity and RNA stability. Furthermore, the radiolabeled chelator-coupled RNA aptamers were evaluated for binding specificity to their target antigen. In summary, key parameters were established for optimal radiolabeling of RNA aptamers for eventual PET imaging with (64)Cu.     

Macrocyclic complexes of scandium radionuclides as precursors for diagnostic and therapeutic radiopharmaceuticals

The aim of this study was to evaluate new ligands which can be applied for labeling biomolecules with scandium radionuclides. Two radionuclides of scandium, ⁴⁷Sc and ⁴⁴Sc, are perspective radioisotopes for radiotherapy and diagnostic imaging. ⁴⁷Sc decays with a half-life of 3.35 days and a maximum β⁻ energy of 600 keV and could be an alternative to carrier added ¹⁷⁷Lu radionuclide for targeted radionuclide therapy. Another scandium radionuclide ⁴⁴Sc (t_1/2 = 3.92 h) is an ideal β⁺ emitter for PET diagnosis. It can be obtained as a daughter of the long-lived ⁴⁴Ti (t_1/2 = 60.4 y) from ⁴⁴Ti/⁴⁴Sc generator. For complexation of scandium radionuclides macrocyclic ligands having a cavity size similar to Sc³⁺ ionic radius were selected: 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), 1,4,7-triazacyclononane-1,4,7 triacetic acid (NOTA), 1,4,7-triazacyclodecane-1,4,7 triacetic acid and 1,4,7-triazacycloundecane triacetic acid, and analogs of NOTA with 10, 11 and 12 atoms of the carbon in the ring. Our results have shown that from the studied macrocyclic ligands studied DOTA is most efficient for binding scandium radionuclides ⁴⁴Sc and ⁴⁷Sc to biomolecules. The determined stability constant of Sc-DOTA complex logK = 27.0 is comparable with stability constants for Y³⁺ and heaviest lanthanides but is higher than those for In³⁺ and Ga³⁺. Also ⁴⁶Sc-DOTATATE conjugate exhibits high stability in-vitro studies. The ¹³C NMR studies have shown that Sc-DOTA like Lu-DOTA forms in solution complexes with eight-coordination geometry. The lipophilicity of Sc-DOTATATE is nearly identical to that of Lu-DOTATATE, which suggests similar receptor affinity of both radioconjugates.     

Synthesis of multi-galactose-conjugated 2′-O-methyl oligoribonucleotides and their in vivo imaging with positron emission tomography

⁶⁸Ga labelled 2′-O-methyl oligoribonucleotides (anti-miR-15b) bearing one, three or seven d-galactopyranoside residues have been prepared and their distribution in healthy rats has been studied by positron emission tomography (PET). To obtain the heptavalent conjugate, an appropriately protected 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) precursor bearing a 4-[4-(4,4′-dimethoxytrityloxy)butoxy]phenyl side arm was first immobilized via a base labile linker to the support and the oligonucleotide was assembled on the detritylated hydroxyl function of this handle. A phosphoramidite building block bearing two phthaloyl protected aminooxy groups and one protected hydroxyl function was introduced into the 5′-terminus. One acetylated galactopyranoside was coupled as a phosphoramidite to the hydroxyl function, the phthaloyl protections were removed on-support and two trivalent galactopyranoside clusters were attached as aldehydes by on-support oximation. A two-step cleavage with aqueous alkali and ammonia released the conjugate in a fully deprotected form, allowing radiolabelling with ⁶⁸Ga in solution. The mono- and tri-galactose conjugates were obtained in a closely related manner. In vivo imaging in rats with PET showed remarkable galactose-dependent liver targeting of the conjugates.     

4-N-Alkanoyl and 4-N-alkyl gemcitabine analogues with NOTA chelators for 68-gallium labelling

The conjugation of 4-N-(3-aminopropanyl)-2′-deoxy-2′,2′-difluorocytidine with 2-(p-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (SCN-Bn-NOTA) ligand in 0.1?M Na₂CO₃ buffer (pH 11) at ambient temperature provided 4-N-alkylgemcitabine-NOTA chelator. Incubation of latter with excess of gallium(III) chloride (GaCl₃) (0.6?N AcONa/H₂O, pH?=?9.3) over 15?min gave gallium 4-N-alkylgemcitabine-NOTA complex which was characterized by HRMS. Analogous [⁶⁸Ga]-complexation of 4-N-alkylgemcitabine-NOTA conjugate proceeded with high labeling efficiency (94%–96%) with the radioligand almost exclusively found in the aqueous layer (～95%). The high polarity of the gallium 4-N-alkylgemctiabine-NOTA complex resulted in rapid renal clearance of the ⁶⁸Ga-labelled radioligand in BALB/c mice.     

Al18F labeled sulfonamide-conjugated positron emission tomography tracer in vivo tumor-targeted imaging

An ideal positron emission tomography (PET) tracer should be highly extractable by the tumor tissue or organ that contains low toxicity and can provide high-resolution images in vivo. In this work, the aim was to evaluate the application of Al¹⁸F-labeled 1,4,7-triazacyclononane-1,4,7-triacetic acid containing sulfonamide group (¹⁸F-Al-NOTA-SN) as a potential tumor-targeting signal-enhanced radioactive tracer in PET. SN as a tumor-targeting group was incorporated to NOTA to make a ligand. Subsequently, this ligand reacted with Na¹⁸F and AlCl₃ to produce a compound ¹⁸F-Al-NOTA-SN. This compound was further characterized and its property in regard to cell cytotoxicity assay, microPET imaging, biodistribution, cell uptake assay, and tumor selectivity in vitro and in vivo, was also investigated. ¹⁸F-Al-NOTA-SN possessed low cell cytotoxicity and uptake to COS-7 and 293T healthy cells and high cell cytotoxicity and uptake to MDA-MB-231, HepG2, and HeLa tumor cells in vitro. Moreover, ¹⁸F-Al-NOTA-SN showed good tumor-targeting property and high PET signal enhancement of HeLa tumors, liver, and kidneys in mice, as well as the uptake ratios of tumor to blood and tumor to muscle, were 4.98 and 3.87, respectively. ¹⁸F-Al-NOTA-SN can be accepted to be kidney and liver eliminated earlier and show a potential tumor-targeting signal-enhanced radioactive tracer in PET.     

古蔺雪胆中的新三萜皂苷

从采自四川汉源县的古蔺雪胆(Hemsleya penxianensis var.gulinensiks)中分到9个三萜皂苷化合物,通过化学反应和光谱方法鉴定了它们的结构。其中7个为已知化合物,分别为齐墩果酸-28-O-β-D-比喃葡萄糖苷(1),3-O-β-D-吡喃葡萄糖醛基齐墩果酸苷(3),3-O-β-D-吡喃葡萄糖醛基—齐墩果酸—28-O-α-L-吡喃阿拉伯糖苷(4),3-O-β-D-吡喃葡萄糖醛基—齐墩果酸—28-O-β-D-吡喃葡萄糖苷(5),3-O-α-L-阿拉伯糖基—(1→3)—β—D-吡喃葡萄糖醛基—齐墩果酸—28—O—β—D—吡喃葡萄糖苷(6),3—O—(6′—丁酯)—β-D-吡喃葡萄糖醛基—齐墩果酸—28-O-α-L-阿拉伯糖苷(7),3-O-(6′-丁酯)—β—D吡喃葡萄糖醛基—齐墩果酸—28-O-β-D-吡喃葡萄糖苷(8)。两个新化合物,即雪胆皂苷A(2)和雪胆皂苷B(9)。     

Design, synthesis and the effect of 1,2,3-triazole sialylmimetic neoglycoconjugates on Trypanosoma cruzi and its cell surface trans-sialidase

This work describes the synthesis of a series of sialylmimetic neoglycoconjugates represented by 1,4-disubstituted 1,2,3-triazole-sialic acid derivatives containing galactose modified at either C-1 or C-6 positions, glucose or gulose at C-3 position, and by the amino acid derivative 1,2,3-triazole fused threonine-3-O-galactose as potential TcTS inhibitors and anti-trypanosomal agents. This series was obtained by Cu(I)-catalysed azide-alkyne cycloaddition reaction ('click chemistry') between the azido-functionalized sugars 1-N(3)-Gal (commercial), 6-N(3)-Gal, 3-N(3)-Glc and 3-N(3)-Gul with the corresponding alkyne-based 2-propynyl-sialic acid, as well as by click chemistry reaction between the amino acid N(3)-ThrOBn with 3-O-propynyl-GalOMe. The 1,2,3-triazole linked sialic acid-6-O-galactose and the sialic acid-galactopyranoside showed high Trypanosoma cruzitrans-sialidase (TcTS) inhibitory activity at 1.0mM (approx. 90%), whilst only the former displayed relevant trypanocidal activity (IC(50) 260μM). These results highlight the 1,2,3-triazole linked sialic acid-6-O-galactose as a prototype for further design of new neoglycoconjugates against Chagas' disease.     

l-Ascorbic Acid Biosynthesis in Higher Plants from l-Gulono-1, 4-lactone and l-Galactono-1, 4-lactone

Detached bean (Phaseolus vulgaris) and strawberry (Fragaria) fruits fed l-gulono-1,4-lactone or l-galactono-1,4-lactone convert this compound, in part, to l-ascorbic acid. When l-galactono-1,4-lactone is given as a 0.25% solution to detached bean shoots, the ascorbic acid content is tripled in less than 10 hours. l-Gulono-1,4-lactone is only 5 to 10% as effective as its epimer. Experiments with specifically labeled l-gulono-1,4-lactone and l-galactono-1,4-lactone prove that conversion is direct. Ascorbic acid is labeled at the same carbon as its precursor.A method is described for preparation of l-galactono-1,4-lactone-2-(14)C from myo-inositol-2-(14)C. This method can be extended to the preparation of l-ascorbic acid-2-(14)C on the basis of results obtained in the present study.     

NODAGATOC,a new chelator-coupled somatostatin analogue labeled with [67/68Ga] and [111In] for SPECT,PET, and targeted therapeutic applications of somatostatin receptor (hsst2) expressing tumors

A monoreactive NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid) derived prochelator (1-(1-carboxy-3-carbo-tert-butoxypropyl)-4,7-(carbo-tert-butoxymethyl)-1,4,7-triazacyclononane (NODAGA(tBu)(3))) was synthesized in five steps with an overall yield of 21%. It is useful for the coupling to the N-terminus of peptides on solid phase and in solution; it was coupled to [Tyr3]-octreotide (TOC) on solid phase, and the resulting peptide, NODAGA-Tyr3-octreotide (NODAGATOC), was labeled with the radiometals 111In and 67Ga in high yields and good specific activities. [67Ga]- and [111In]-NODAGA-Tyr3-octreotide appear to be useful to visualize primary tumors and metastases which express somatostatin receptors subtype 2 (sstr2), such as neuroendocrine tumors, because of their high affinity to this receptor subtype with IC(50) = 3.5 +/- 1.6 nM and 1.7 +/- 0.2 nM, respectively. NODAGATOC could be used as a SPECT and PET tracer, when labeled with 111In, 67Ga, or 68Ga, and even for therapeutic applications. Surprisingly, [111In]-NODAGATOC shows 2 times higher binding affinity to sstr2, but also a factor of 4 higher affinity to sstr5 compared to [67Ga]-NODAGATOC. [67Ga]-NODAGATOC is very stable in serum and rat liver homogenate. There is no difference in the rate of internalization into AR4-2J rat pancreatic tumor cells; both radioligands are highly internalized, at 4 h a 3 times higher uptake compared to [111In]-DOTA-Tyr3-octreotide ([111In]-DOTATOC) was found. The biodistribution of [67Ga]-NODAGATOC in AR4-2J tumor bearing nude mice is very favorable at short times after injection; there is fast excretion from all nontarget organs except the kidneys and high uptake in sst receptor rich organs and in the AR4-2J tumor. Again it is superior to [111In]-DOTATOC in this respect. The results indicate an improved biological behavior which is likely due to the fact that an additional spacer group separates the chelate from the pharmacophoric part of the somatostatin analogue.     

A new strategy to combat Alzheimer's disease. Combining radical-scavenging potential with metal-protein-attenuating ability in one molecule

Oxidative stress and excessive redox metals have been implicated in the pathogenesis of Alzheimer's disease (AD), which leads to the tentative employment of radical scavengers and metal chelators in clinical therapy of AD. The preliminary successes of both therapy strategies inspire us to propose that better clinical effects can be expected for a compound combining radical-scavenging potential with metal-protein-attenuating ability. Based on theoretical investigation, we indicate that two novel metal chelators (1-(benzimidazole-2-ylmethyl)-1,4,7-triazacyclononane and 1,4-bis(benzimidazole-2-ylmethyl)-1,4,7-triazacyclonone), especially the latter, are promising to fulfill this new strategy.     

Targeting radiopharmaceuticals: Comparative biodistribution studies of gallium and indium complexes of multidentate ligands

New multidentate ligands with structures similar to N,N′-bis[2-hydroxybenzyl]ethylenediamme-N,N′-diacetic acid (HBED) and N,N′-bis(pyridoxyl)ethylenediamine-N,N′-diacetic acid (PLED) were synthesized. The in vitro lipophilicity, electrophoretic behavior, and the in vivo biodistribution were studied for the ¹¹¹In- and ^67,68Ga-complexes of N,N′-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine-N,N′-diacetic acid (Me₄HBED); N,N′-bis(5-t-butyl-2-hydroxy-3-methylbenzyl)ethylenediamine-N,N′-diacetic acid (t-butyl HBED); N,N′-bis[2-hydroxy-5-sulfobenzyl)ethylenediamine-N,N′-diacetic acid (SHBED); N,N′-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine-N-(2-hydroxyethyl)-N′-acetic acid (HBMA); and N,N′-bis(5-deoxypyridoxyl)ethylenediamine-N,N′-diacetic acid (DPLED). The biodistribution of the radiometal complexes were carried out in rats and an imaging study was performed in a non-human primate. The rapid clearance of the lipophilic complexes from blood and through the hepatobiliary system was easily demonstrated; as well, the hydrophilic complexes were cleared rapidly through the urinary tract. Positron emission tomographic images were generated from a study in a primate after administration of ⁶⁸Ga-t-butyl HBED. These images well demonstrate the efficient liver accumulation and rapid hepatobiliary clearance (< 1 h) and well differentiate images of the liver and gall bladder.     

Constituents of Aristolochia cinnabarina

Nine compounds, aristolochic acid (1), aristolochic acid- Ⅱ (2), aristolochic acid- Ⅲ a (3), aristoloside (4), aristolactam-N-β-D-glucoside (5), aristolactam-β-D-glucoside (6), tuberosinone (7), tuberosinone N-β-D-glucoside (8) and 6-O-p-coumaroyl-D-glucopyranose (9), were isolated from methanol extracts of fresh roots of Aristolochia cinnabarina. Among them, compound 9 was reported in Aristolochiaceae for the first time.     

Transferrin-based radiolabeled probe predicts the sensitivity of human renal cancer cell lines to ferroptosis inducer erastin

Ferroptosis induction has been recognized as a novel cancer therapeutic strategy. To effectively apply ferroptosis-targeting cancer therapy to individual patients, a diagnostic indicator for selecting this therapeutic strategy from a number of molecular targeting drugs is needed. However, to date, methods that can predict the efficacy of ferroptosis-targeting treatment have not been established yet. In this study, we focused on the iron metabolic pathway to develop a nuclear imaging technique for diagnosing the susceptibility of cancer cells to ferroptosis. As a nuclear probe, human transferrin (Tf) was labeled with Gallium-68 (⁶⁸Ga) using 2-(p-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) as a chelator (⁶⁸Ga-NOTA-Tf). Western blot assay and clonogenic survival assay with human renal cancer cell lines A498 and 786-O revealed that the protein expression level of transferrin receptor1 (TfR1) and sensitivity to a ferroptosis inducer, erastin, were correlated. A cellular uptake assay with ⁶⁸Ga-NOTA-Tf revealed that the cancer cells sensitive to erastin highly internalized the ⁶⁸Ga-NOTA-Tf. Furthermore, treatment with the TfR1 inhibitor ferristatin II reduced the cellular uptake of ⁶⁸Ga-NOTA-Tf, indicating that the intracellular uptake of the probe was mediated by TfR1. These results suggest that ⁶⁸Ga-NOTA-Tf can be useful in predicting the sensitivity of cancer cells to ferroptosis inducers.     

Synthesis of the specific monosulfates of cholic acid.

The three isomeric cholic acid-monosulfates were synthetized and characterized. Cholic acid-3-sulfate was obtained by reacting cholic acid for 2 min with chlorosulfonic acid in pyridine and chromatography of the resulting bile salt mixture on Sephadex LH-20. The 7- and the 12-monosulfate were prepared by sulfation of the corresponding monohydroxy-diacetates followed by removal of the acetyl groups by alkaline hydrolysis and purification by chromatography on Sephadex LH-20. On TLC in n-butanol-acetic acid-water (10:1:1, v/v) the Rf values were 0.59 for cholic acid-3-sulfate, 0.52 for cholic acid-7-sulfate and 0.48 for cholic acid-12-sulfate. The time required for complete solvolysis at 37 degrees C in acid methanol-acetone (1:9) was 3 h for cholic acid-3-sulfate, 12 h for the 12-monosulfate and 18 h for the 7-monosulfate.     

Effects of electronic structure on chiral induction in outer-sphere electron transfer reactions of complexes with the C3 symmetric ligand, 1,4,7-triazacyclononane-1,4,7-tris[2′(R)-2′-propionate](-3)

The ligand 1,4,7-triazacyclononane-1,4,7-tris[2′(R)-2′-propionate](-3)((R)-tacntp³⁻), binds stereospecifically to transition metal ions. The structures of the complexes [Cr((R)-tacntp)]·NaBr and [Fe((R)-tacntp)]·H₂O have been determined by X-ray crystallography. Both complexes have the Λ-configuration but the conformation of the chelate rings in Λ-[Cr((R)-tacntp)] is (λ,λ,λ) with a geometry close to octahedral while in Λ-[Fe((R)-tacntp)] it is (δ,δ,δ) and the geometry is closer to that of a trigonal prism. Chiral induction in the electron transfer reactions of Λ-[Co((R)-tacntp)], Λ-[Fe((R)-tacntp)] and Λ-[Mn((R)-tacntp)] with [Co((RR,SS)-chxn)₃]²⁺ has been investigated. All three reactions are outer-sphere and four isomeric [Co((RR,SS)-chxn)₃]³⁺ products are identified in each case. The oxidants Λ-[Fe((R)-tacntp)] and Λ-[Mn((R)-tacntp)] show very similar selectivities, quite different from those of Λ-[Co((R)-tacntp)]. Reasons for this behavior are discussed.