José C. Aponte

Thin layer chromatography in the separation of unsaturated organic compounds using silver-thiolate chromatographic material

We report the use of silver-thiolate chromatographic material (AgTCM) as a stable material for use in TLC. The AgTCM stationary phase operates under the same principles as silver-ion chromatography, separating compounds by degree of unsaturation; however, the AgTCM stationary phase shows considerable advantages over Ag-TLC in terms of light stability and shelf lifetime. We demonstrate the light stability of the AgTCM-TLC and its application for separations based on the degrees of unsaturation using fatty acid methyl esters (FAMEs) and polycyclic aromatic hydrocarbons (PAHs).

Separation of unsaturated organic compounds using silver–thiolate chromatographic material

Separation of organic compounds containing various numbers of double bonds (DB) can be readily achieved by using silver ion impregnated silica gel, often called silver-ion or argentation chromatography. However, the practical application of silver-ion liquid chromatography in analytical and preparative separations has been limited by the concerns about the stability and mobility of silver ions and the widespread use of reversed phase high performance liquid chromatography. Silver covalently anchored onto the thiol moiety of mercaptopropyl modified silica gel has been tested for the separation of polycyclic aromatic hydrocarbons by ring numbers, but has never been shown to separate mixtures of alkenes having different number of double bonds. We report here that silver-thiolate chromatographic material (AgTCM; including, but not limited to, silver(I) mercaptopropyl silica gel) is also highly efficient in liquid chromatographic separation of alkane/alkenes differing by one double bond. AgTCM displays exceptionally high selectivity for unsaturated compounds and high stability under extended heat and light exposure, while silver is virtually immobile during solvent elution. Compared to ionic silver, silver-thiolate interacts with double bonds less strongly, allowing AgTCM to efficiently separate olefins using less polar (and often less viscous and lower cost) solvents. The interaction energy between silver and ethylene is calculated using established computational methods and the results are in full agreement with our experimental results. Importantly, the exceptional stability of AgTCM gives rise to much higher compound recovery than conventional silver-ion silica gel during the chromatographic elution. Our results pave the way for the development of novel covalently bonded, transition metal-containing chromatographic materials.

Purification of omega-3 polyunsaturated fatty acids from fish oil using silver-thiolate chromatographic material and high performance liquid chromatography.

Omega-3 polyunsaturated fatty acids (ω-3 PUFAs) have become increasingly popular in dietary supplements worldwide and global demand for higher purity ω-3 PUFAs in clinical applications has been rising rapidly in the recent years. Traditional methods for isolating ω-3 PUFAs however, generally require multiple, cumbersome steps to obtain high purity (>95%) products. In this study, we report an efficient liquid chromatography method for purifying individual omega-3 fatty acid ethyl esters (FAEEs), eicosapentaenoic acid (EPA, C20:5) and docosahexaenoic acid (DHA, C22:6), at >95% purity using a silver(I)-mercaptopropyl stationary phase, otherwise known as silver-thiolate chromatographic material (AgTCM). We demonstrate the key variables controlling separation of FAEEs with a silver(I)-thiolate stationary phase and examine important similarities and differences between silver-thiolate and silver-ion chromatography in regards to the separation mechanism taking place. We demonstrate the separation behavior of FAEEs under various conditions such as mobile phase composition, flow rate and sample injection amount. Purification of EPA and DHA was carried out on an analytical and semi-preparative scale and the resulting purity was determined by GC-FID. We report high attainable purity for EPA (>95%) and DHA (>99%) from a single isocratic separation of fish oil FAEEs within 5-10min using a mobile phase of heptane/acetone (95:5, % v/v).

Efficient liquid chromatographic analysis of mono-, di-, and triglycerols using silver thiolate stationary phase

We show that the characterization of mono-, di- and triglycerols can be readily accomplished by high performance liquid chromatography (HPLC) with silver(I)-mercaptopropyl modified silica gel, or silver thiolate chromatographic material (AgTCM), which can be used with evaporative light scattering detection (ELSD) or atmospheric pressure chemical ionization mass spectroscopy (APCI-MS). Separation of triglycerols varying by degrees of unsaturation and cis/trans configuration in common oil samples can be achieved using a simple linear gradient of hexane and acetone. In addition to double bonds, AgTCM also displays major selectivity for compounds with different levels of polarity, allowing for efficient separation between mono-, di- and triglycerols. In comparison to conventional reversed phase columns, AgTCM produces simple chromatograms for rapid assessment of degrees of unsaturation and the amount of trans fats in triglycerides, which are central issues to food quality determination. In comparison to previous silver-ion based HPLC separations, AgTCM-HPLC based column offers greatly enhanced stability, inertness, durability, and reproducibility allowing routine coupling of the HPLC with a mass spectrometer for detection.

Cytotoxic and anti-infective phenolic compounds isolated from Mikania decora and Cremastosperma microcarpum

An anticancer-bioassay guided isolation of the ethanol extract and fractions of two plants from the Peruvian rainforest, Mikania decora and Cremastosperma microcarpum, led to the characterization of one abundant diterpene, ent-pimara-8(14),15-dien-19-oic acid (1), three thymol derivatives, 10-acetoxy-8,9-dehydro-6-methoxythymol butyrate (2), 10-acetoxy-8,9-epoxy-6-methoxythymol isobutyrate (3), and acetylschizoginol (4), as well as one neolignan, (±)-trans-dehydrodiisoeugenol (5). Only the latter was isolated from C. microcarpum. These compounds exhibited significant cytotoxic activity against a panel of human tumor cell lines. Compounds 3 and 4 were also investigated for their in vitro antileishmanial and trypanocidal activity against Leishmania amazonensis axenic amastigotes and Trypanosoma cruzi trypomastigotes.

The unique liquid chromatographic properties of Group 11 transition metals for the separation of unsaturated organic compounds

Silver(I) and copper(I) are known to form reversible complexes with π bonds, which have been exploited in LC for separating unsaturated organic compounds. Prominent examples include the use of AgNO3-impregnated silica gel in LC, and the use of copper(I) salts for selective extraction of alkenes from hydrocarbon mixtures. The Dewar-Chatt-Duncanson model is often invoked to explain the interaction between Ag(I) and Cu(I) and π bonds. However, it is unclear if such a reversible interaction is directly related to their d(10) outer electronic configurations. Particularly, Au(I) has not been reported to separate olefins with different numbers of double bonds in LC. Also, there has not been a systematic comparison of the liquid chromatographic properties of other d(10) transition metal salts (e.g., Zn(II), Cd(II)), making it difficult to fully understand the observed reversible interactions of Ag(I) and Cu(I) with π bonds. We demonstrate for the first time that silica gel impregnated with all three Group 11 transition metals with 1+ oxidation state strongly and similarly retain olefin compounds in LC, while transition metals from Groups 10 and 12 do not. We also tested a range of functionalized silica gels to improve the stability of Cu(I) and Au(I) ions on the surface of the silica.