Implementing stepwise solvent elution in sequential injection chromatography for fluorimetric determination of intracellular free amino acids in the microalgae Tetraselmis gracilis

The concept of sequential injection chromatography (SIC) was exploited to automate the fluorimetric determination of amino acids after pre-column derivatization with o-phthaldialdehyde (OPA) in presence of 2-mercaptoethanol (2MCE) using a reverse phase monolithic C₁₈ stationary phase. The method is low-priced and based on five steps of isocratic elutions. The first step employs the mixture methanol: tetrahydrofuran: 10 mmol L⁻¹ phosphate buffer (pH 7.2) at the volumetric ratio of 8:1:91; the other steps use methanol: 10 mmol L⁻¹ phosphate buffer (pH 7.2) at volumetric ratios of 20:80, 35:65, 50:50 and 65:35. At a flow rate of 10 μL s⁻¹ a 25 mm long-column was able to separate aspartic acid (Asp), glutamic acid (Glu), asparagine (Asn), serine (Ser), glutamine (Gln), glycine (Gly), threonine (Thr), citruline (Ctr), arginine (Arg), alanine (Ala), tyrosine (Tyr), phenylalanine (Phe), ornithine (Orn) and lysine (Lys) with resolution >1.2 as well as methionine (Met) and valine (Val) with resolution of 0.6. Under these conditions isoleucine (Ile) and leucine (Leu) co-eluted. The entire cycle of amino acids derivatization, chromatographic separation and column conditioning at the end of separation lasted 25 min. At a flow rate of 40 μL s⁻¹ such time was reduced to 10 min at the cost of resolution worsening for the pairs Ctr/Arg and Orn/Lys. The detection limits varied from 0.092 μmol L⁻¹ for Tyr to 0.51 μmol L⁻¹ for Orn. The method was successfully applied to the determination of intracellular free amino acids in the green alga Tetraselmis gracilis during a period of seven days of cultivation. Samples spiked with known amounts of amino acids resulted in recoveries between 94 and 112%.     

Total Synthesis,Absolute Configuration,and Biological Activity of Xyloallenoide A

The novel natural product xyloallenoide A, isolated from the marine mangrove endophytic fungus from the South China Sea, and its diastereoisomer xyloallenoide A1, which contain N‐methyl‐substituted amino acids, were synthesized. The absolute configurations of the amino acid units of xyloallenoide A were finally confirmed to be L ‐Lys, Me‐D ‐Val, and Me‐L ‐Ala. This report represents a practical and attractive alternative for the synthesis of N‐methyl‐substituted cyclotripeptides. In the preliminary bioassay, synthetic xyloallenoide A showed marginal activities against KB (IC₅₀=9.6 μM ) and KBv200 cells (IC₅₀=10.3 μM ), and xyloallenoide A1 was inactive against KB and KBv200 cells.     

GC-MS Studies on Derivatization of Creatinine and Creatine by BSTFA and Their Measurement in Human Urine

In consideration of its relatively constant urinary excretion rate, creatinine (2-amino-1-methyl-5H-imidazol-4-one, MW 113.1) in urine is a useful endogenous biochemical parameter to correct the urinary excretion rate of numerous endogenous and exogenous substances. Reliable measurement of creatinine by gas chromatography (GC)-based methods requires derivatization of its amine and keto groups. Creatinine exists in equilibrium with its open form creatine (methylguanidoacetic acid, MW 131.1), which has a guanidine and a carboxylic group. Trimethylsilylation and trifluoroacetylation of creatinine and creatine are the oldest reported derivatization methods for their GC-mass spectrometry (MS) analysis in human serum using flame- or electron-ionization. We performed GC-MS studies on the derivatization of creatinine (d₀-creatinine), [methylo-²H₃]creatinine (d₃-creatinine, internal standard) and creatine (d₀-creatine) with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) using standard derivatization conditions (60 min, 60 °C), yet in the absence of any base. Reaction products were characterized both in the negative-ion chemical ionization (NICI) and in the positive-ion chemical ionization (PICI) mode. Creatinine and creatine reacted with BSTFA to form several derivatives. Their early eluting N,N,O-tris(trimethylsilyl) derivatives (8.9 min) were found to be useful for the precise and accurate measurement of the sum of creatinine and creatine in human urine (10 µL, up to 20 mM) by selected-ion monitoring (SIM) of m/z 271 (d₀-creatinine/d₀-creatine) and m/z 274 (d₃-creatinine) in the NICI mode. In the PICI mode, SIM of m/z 256, m/z 259, m/z 272 and m/z 275 was performed. BSTFA derivatization of d₀-creatine from a freshly prepared solution in distilled water resulted in formation of two lMate-eluting derivatives (14.08 min, 14.72 min), presumably creatinyl-creatinine, with the creatininyl residue existing in its enol form (14.08 min) and keto form (14.72 min). Our results suggest that BSTFA derivatization does not allow specific analysis of creatine and creatinine by GC-MS. Preliminary analyses suggest that pentafluoropropionic anhydride (PFPA) is also not useful for the measurement of creatinine in the presence of creatine. Both BSTFA and PFPA facilitate the conversion of creatine to creatinine. Specific measurement of creatinine in urine is possible by using pentafluorobenzyl bromide in aqueous acetone.     

Mixed-Ligand Complexation of Zinc and Cobalt(II) Complexonates with Amino Acids in an Aqueous Solution

The formation of mixed-ligand complexes in the M(II)–Nta and Ida–L systems (M = Co, Zn; L = His, Orn, Lys, Gly, Im, en), where Ida and Nta are the residues of iminodiacetic and nitrilotriacetic acids, was studied by pH-metry, calorimetry, and NMR spectroscopy. The thermodynamic parameters (logK, Δ _r G⁰, Δ _r H, Δ _r S) of formation for these complexes were determined at 298.15 K and an ionic strength I = 0.5 (KNO3). The most probable pattern of coordination between a complexone and an amino acid in mixed-ligand complexes was revealed.     

Stable-Isotope Dilution GC–MS Measurement of Metformin in Human Serum and Urine after Derivatization with Pentafluoropropionic Anhydride and Its Application in Becker Muscular Dystrophy Patients Administered with Metformin,l-Citrulline,or Their Combination

Metformin (N,N-dimethylguanylguanidine) is one of the most prescribed drugs with pleiotropic, exerted in part by not fully elucidated mechanisms of action. We developed and validated a gas chromatography–mass spectrometry (GC–MS) method for the quantitative analysis of metformin (metformin-d₀) in 10-µL aliquots of human serum and urine using N,N-[dimethylo-²H₆]guanylguanidine (metformin-d₆) as the internal standard. The method involves evaporation of the samples to dryness, derivatization with pentafluoropropionic (PFP) anhydride in ethyl acetate (30 min, 65 °C), and extraction into toluene. The negative-ion chemical ionization GC–MS spectra of the PFP derivatives contain a single intense ion with mass-to-charge (m/z) ratios of m/z 383 for metformin-d₀ and m/z 389 for metformin-d₆. Our results suggest that all amine/imine groups of metformin-d₀ and metformin-d₆ are converted to their N,N,N-tripentafluoropropionyl derivatives, which cyclize to form a symmetric triazine derivative, of which the non-ring amine group is amidated. Quantification was performed by selected-ion monitoring (SIM) of m/z 383 and m/z 389. Upon validation, the method was applied to determine serum and urine metformin concentrations in 19 patients with Becker muscular dystrophy (BMD). Serum and urine samples were collected at baseline (Visit I), after six weeks of supplementation (Visit II) with metformin (3 × 500 mg/d; metformin group; n = 10) or l-citrulline (3 × 1500 mg/d; citrulline group; n = 9) followed by a six-week supplementation with 3 × 500 mg/d of metformin plus 3 × 1500 mg/d l-citrulline. At Visit I, the metformin concentration in the serum and urine was very low in both groups. The metformin concentrations in the serum and urine of the patients who first took metformin (MET group) were higher at Visit II and Visit III. The metformin concentration in the serum and urine samples of the patients who first took l-citrulline (CITR group) were higher at Visit III. The serum and urine concentrations of metformin were insignificantly lower in the CITR group at Visit III. The mean fractional excretion (FE) rate of metformin was 307% (Visit II) and 322% (Visit III) in the MET group, and 290% in the CITR group (Visit III). This observation suggests the accumulation of metformin in the kidney and its secretion in the urine. The GC–MS is suitable to measure reliably circulating and excretory metformin in clinical settings.     

Behaviour of α-functional organotransition metals. Synthesis,characterisation and some reactions of 1-acetoxyalkyl derivatives of iron(II) and molybdenum(II)

Acetoxyalkyl metal derivatives M(C₅H₅)(CO)_n[CHROC(O)Me] [M = Fe, n = 2; M = Mo, n = 3; R = H, Me] are readily prepared by reaction of bromoalkylacetates with the appropriate cyclopentadienylcarbonylmetallate anion. The complexes are characterised by their NMR (¹H and ¹³C) and IR parameters and by mass spectrometry. The acetoxyethyl species are thermally labile via β-hydrogen transfer. Treatment of acetoxymethyl complexes with protic acids leads to carbon-oxygen cleavage and release of acetic acid; HCl affords chloromethyl complexes, carboxylic acids yield new carboxylatomethyl derivatives, HBF₄ leads to decomposition. The metalloesters are resistant to hydrolysis, transesterification and carboxylate displacement by nucleophiles (HO^?, MeO^?, H₂N^? Et₂N^?). Migratory insertion of CO could not be induced.     

Synthesis of N,N-dimethyl-2,4-dinitro-5-fluorobenzylamine and its reactions with amino acids and peptides

A practical synthesis is described for N,N-dimethyl-2,4-dinitro-5-fluorobenzylamine (DMDNFB) and its -d₆ analog as an alternative Sanger's reagent (DNFB), for purposes of amino acid derivatization detectable by positive mode electrospray ionization mass spectrometry. DMDNFB is comparable to DNFB in its efficiency to derivatize amino acids and peptides. Various DMDNP (d₀/d₆) derivatives of (modified) lysine were synthesized to evaluate the potential use of isotope-edited LC-ESI-MS as a tool for structural definition of the posttranslational modification of protein-based lysines.     

Development of a sensitive fluorescent derivatization reagent 1,2-benzo-3,4-dihydrocarbazole-9-ethyl chloroformate (BCEOC) and its application for determination of amino acids from seeds and bryophyte plants using high-performance liquid chromatography with fluorescence detection and identification with electrospray ionization mass spectrometry

A pre-column derivatization method for the sensitive determination of amino acids and peptides using the tagging reagent 1,2-benzo-3,4-dihydrocarbazole-9-ethyl chloroformate (BCEOC) followed by high-performance liquid chromatography with fluorescence detection has been developed. Identification of derivatives was carried out by liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS/MS). The chromophore of 2-(9-carbazole)-ethyl chloroformate (CEOC) reagent was replaced by 1,2-benzo-3,4-dihydrocarbazole functional group, which resulted in a sensitive fluorescence derivatizing reagent BCEOC. BCEOC can easily and quickly label peptides and amino acids. Derivatives are stable enough to be efficiently analyzed by high-performance liquid chromatography. The derivatives showed an intense protonated molecular ion corresponding m/z (M + H)⁺ under electrospray ionization (ESI) positive-ion mode with an exception being Tyr detected at negative mode. The collision-induced dissociation of protonated molecular ion formed a product at m/z 246.2 corresponding to the cleavage of CO bond of BCEOC molecule. Studies on derivatization demonstrate excellent derivative yields over the pH 9.0-10.0. Maximal yields close to 100% are observed with a 3-4-fold molar reagent excess. Derivatives exhibit strong fluorescence and extracted derivatization solution with n-hexane/ethyl acetate (10:1, v/v) allows for the direct injection with no significant interference from the major fluorescent reagent degradation by-products, such as 1,2-benzo-3,4-dihydrocarbazole-9-ethanol (BDC-OH) (a major by-product), mono-1,2-benzo-3,4-dihydrocarbazole-9-ethyl carbonate (BCEOC-OH) and bis-(1,2-benzo-3,4-dihydrocarbazole-9-ethyl) carbonate (BCEOC)₂. In addition, the detection responses for BCEOC derivatives are compared to those obtained with previously synthesized 2-(9-carbazole)-ethyl chloroformate (CEOC) in our laboratory. The ratios AC_BCEOC/AC_CEOC = 2.05-6.51 for fluorescence responses are observed (here, AC is relative fluorescence response). Separation of the derivatized peptides and amino acids had been optimized on Hypersil BDS C₁₈ column. Detection limits were calculated from 1.0 pmol injection at a signal-to-noise ratio of 3, and were 6.3 (Lys)-177.6 (His) fmol. The mean interday accuracy ranged from 92 to 106% for fluorescence detection with mean %CV < 7.5. The mean interday precision for all standards was <10% of the expected concentration. Excellent linear responses were observed with coefficients of >0.9999. Good compositional data could be obtained from the analysis of derivatized protein hydrolysates containing as little as 50.5 ng of sample. Therefore, the facile BCEOC derivatization coupled with mass spectrometry allowed the development of a highly sensitive and specific method for the quantitative analysis of trace levels of amino acids and peptides from biological and natural environmental samples.     

Behavior and characteristics of biogenic amines, ornithine and lysine derivatized with the o-phthalaldehyde-ethanethiol-fluorenylmethyl chloroformate reagent

The stability and characteristics of the ornithine (Orn), lysine (Lys), putrescine (Put), cadaverine (Cad), 1,7-diaminoheptane (Diah), spermidine (Spd) and spermine (Spm) derivatives obtained with the o-phthalaldehyde (OPA)-ethanethiol (ET)-fluorenylmethyl chloroformate (FMOC) reagent has been investigated. The stoichiometry of the introduced, two-step derivatization process has been followed by photodiode array (DAD) and fluorescence (FL) detections, simultaneously, while the composition of derivatives was confirmed by on-line HPLC-electrospray ionization (ESI) MS measurements. Depending on the composition of the OPA reagents, in addition to the secondary amino group-containing Spd and Spm, under common aqueous conditions also Orn and Lys do react with FMOC resulting in derivatives of various compositions. Applying the OPA-ET reagent of increasing methanol (Met) content (38-80%, v/v) the formation of the FMOC group containing Orn and Lys derivatives could be considerably decreased. Optimum elution condition (18 min, including equilibration) was developed for the simultaneous quantitation of Orn, Lys, Put, Cad, Diah, Spd and Spm, in the presence of the rest of protein amino acids. The practical utility of the method was demonstrated by the analysis of mouse tissues. Average reproducibility of quantitations, characterized with the relative standard deviation percentages of fluorescence intensities and UV responses, in order of listing, proved to be 2.1% and 2.1%, respectively.     

Liquid chromatographic fluorescence determination of amino acids in plasma and urine after derivatization with phanquinone

Phanquinone (4,7-phenanthroline-5,6-dione) has been investigated as a pre-column derivatization fluorogenic reagent for liquid chromatographic determination of primary amino acids in biological samples. The derivatization reaction was carried out at 68 degrees C both in the presence of aqueous phosphate buffer (pH 8) for 30 min and without buffer for 60 min to allow the determination of basic amino acids (Orn, Lys, Arg). The resulting derivatives were separated under reversed-phase HPLC and detected at lambda(em) = 460 nm with lambda(ex) = 400 nm. The proposed method was validated and applied to the determination of a variety of amino acids directly in urine and after deproteinization with 5-sulfosalicylic acid in plasma samples. The detection and quantitation limits were found in the range 10-450 and 35-1400 fmol, respectively.     

Solvothermal Preparation of a Lanthanide Metal-Organic Framework for Highly Sensitive Discrimination of Nitrofurantoin and l-Tyrosine

Metal-organic frameworks (MOFs) have been rapidly developed for their broad applications in many different chemistry and materials fields. In this work, a multi-dentate building block 5-(4-(tetrazol-5-yl)phenyl)-isophthalic acid (H₃L) containing tetrazole and carbolxylate moieties was employed for the synthesis of a two-dimensional (2D) lanthanide MOF [La(HL)(DMF)₂(NO₃)] (DMF = N,N-dimethylformamide) (1) under solvothermal condition. The fluorescent sensing application of 1 was investigated. 1 exhibits high sensitivity recognition for antibiotic nitrofurantoin (K_sv: 3.0 × 10³ M⁻¹ and detection limit: 17.0 μM) and amino acid l-tyrosine (K_sv: 1.4 × 10⁴ M⁻¹ and detection limit: 3.6 μM). This work provides a feasible detection platform of 2D MOFs for highly sensitive discrimination of antibiotics and amino acids.     

Analysis of catecholamines and related compounds in one whole metabolic pathway with high performance liquid chromatography based on derivatization

The simultaneous determination of Tyr, catecholamines and their metabolites in one whole metabolic pathway using high performance liquid chromatography coupled with fluorescence detection (HPLC–FLD) based on pre-column derivatization has been achieved successfully. 1,3,5,7-Tetramethyl-8-(N-hydroxysuccinimidyl butyric ester)-difluoroboradiaza-s-indacene (TMBB-Su), a highly reactive fluorescent reagent synthesized in our previous work, was used for the labeling of tyrosine (Tyr), l-3,4-dihydroxyphenylalanine (l-DOPA), dopamine (DA), norepinephrine (NE), epinephrine (E) and metanephrine (MN). Derivatization conditions including reagent concentration, buffer, reaction temperature and reaction time were also investigated to improve the derivatization efficiency and thus the sensitivity of the detection. The separation of the derivatives was obtained on C₁₈ column with the mobile phase of 20 mM pH 3.5 citric acid (H₃Cit)–sodium hydrogen phosphate (Na₂HPO₄) buffer and methanol. Good linearities with correlation coefficients square (R²) greater than 0.998 in the corresponding concentration ranges were observed and the detection limits (S/N = 3) were found in the range from 0.10 to 0.40 nM (l-DOPA: 1.45 nM). The proposed method has been applied to the detection of catecholamines and related compounds in mice liver and brain samples without tedious extraction or purification procedure, which exhibits excellent selectivity and sensitivity in the analysis of complex samples. This work provides an alternative approach in the metabolic research of catecholamines and is helpful for the study of catecholamine metabolism.     

Synthesis of carborane analogues of γ-aminobutanoic acid

General method for preparation of high yields of novel N-protected carborane amino acid derivatives, 3-acylamino-1-carboxymethyl-2-R-o-carboranes (R = H, Me, Ph), is reported. The synthesis starts from readily available 3-amino-o-carboranes and includes the protection of amino group, introduction of carboxymethyl function to the carbon atom of polyhedron via the metallation of carborane CH bond with sodium amide in liquid ammonia, and treatment of corresponding sodium carboranes with sodium bromoacetate. Deprotection of N-acylated carborane amino acids is studied in acidic media. Depending on the procedure employed, closo- or nido-carborane amino acids were obtained.     

New N-nitrosoamines. 1. Synthesis and nitrosative cleavage of the Mannich bases derived from nitrogen-containing heterocycles and primary aliphatic amines

The reactions of isatin, benzotriazole, and succinimide with formaldehyde and methylamine yield monoamines RCH₂N(Me)CH₂R and methylenediamines RCH₂N(Me)CH₂N(Me)CH₂R. The use of ethylenediamine as the amino component affords N,N"-disubstituted imidazolidines, while the reactions with 3-aminopropan-1-ol give N-substituted tetrahydro-1,3-oxazines. RCH₂NBuⁱ ₂ was obtained from succinimide, formaldehyde, and diisobutylamine. Nitrosative cleavage of the amines obtained was studied; it was shown that monoamines and methylenediamines give N-nitrosoamines RCH₂N(NO)Me, which were structurally characterized by X-ray diffraction analysis. RCH₂NBuⁱ ₂ affords diisobutylnitrosamine, while imidazolidines transform into dinitroso compounds RCH₂N(NO)CH₂CH₂N(NO)CH₂R.     

Measurements of the solubilities of derivatized amino acids in supercritical carbon dioxide

The solubilities of phenylalanine and tyrosine in supercritical carbon dioxide (SCCO₂) were measured after derivatization as the N-acetyl amino acid ethyl ester, N-carbobenzoxy amino acid and N-acetyl amino acid. Using an SCCO₂ flow system, a measuring method of the saturated solubilities of the derivatized amino acids was established in which the contact height of the extraction cell, i.e. a packed column, is increased till the concentration of a derivatized amino acid at the exit of the cell reaches a plateau. The solubilities of N-acetyl phenylalanine ethyl ester (APEE) exceeded 0.001 mole fraction, which is higher than those of caffeine produced in industrial SCCO₂ processes. A possible way of separating the amino acid mixtures using polarity differences in different amino acid side chains was demonstrated using the solubility data of the N-acetyl-amino acid ethyl esters in SCCO₂, as the solubilities of APEE are higher than those of N-acetyl tyrosine ethyl ester by two orders of magnitude.     

Trichloro- and methyldichlorogermyl monochelates and dibromo- and dichlorogermyl bischelates derived from N,N-disubstituted amides of 2-hydroxycarboxylic acids

The reactions of GeCl₄, GeBr₄, and MeGeCl₃ with O-trimethylsilyl derivatives of N,N-disubstituted amides of 2-hydroxycarboxylic acids afforded pentacoordinate and hexacoordinate neutral (O,O)-mono- and (O,O)-bischelates. The reactions of glycolic acid derivatives with GeX₄ produced bischelates X₂Ge[OCH₂C(O)NR²R³]₂ 7a,c,d (X = Cl, R² = R³ = Me (a), (CH₂)₅ (c), (CH₂CH₂)₂O (d)) and 8a (X = Br). By contrast, the reactions of lactic and mandelic acid derivatives with GeCl₄ and MeGeCl₃ gave monochelates Cl₃Ge[OCH(R¹)C(O)NR²R³] (S)-9a–c (R¹ = Me) and Cl₂MeGe[OCH(R¹)C(O)NR²R³] 10a (R¹ = H), (S)-11a,b (R¹ = Me), and (S)-12a (R¹ = Ph) (R²R³ = (CH₂)₄ (b)), respectively. According to the X-ray diffraction data, the Ge atom in bischelates 7c,d and 8a has a coordination number 6, and its coordination polyhedron can be described as a slightly distorted octahedron. In monochelates (S)-9a-c, 10a, (S)-11a,b, and (S)-12a, the Ge atom has a coordination number 5, and its coordination polyhedron can be described as a trigonal bipyramid with two halogen atoms or one halogen atom and one ethereal oxygen atom in equatorial positions and the halogen atom and the amide oxygen atom in the axial positions. The bonds in the axial positions are somewhat longer than the corresponding bonds in tetracoordinate Ge compounds.     

Screening of amino acids in dried blood spots by stable isotope derivatization-liquid chromatography-electrospray ionization mass spectrometry

Direct infusion mass spectrometry (DIMS) is a powerful technique in clinical diagnosis for screening neonatal amino acid metabolic disorders from dried blood spots (DBS). However, DIMS sometimes generated false-positive results for analysis of amino acids. In this work, we utilized a stable isotope derivatization method, combining with liquid chromatography tandem mass spectrometry (SID-LC-MS), to improve the specificity for screening amino acids in DBS specimens. A pair of isotope reagents, p-(dimethylamino)phenyl isothiocyanate (DMAP-NCS) and 4-isothiocyanato-N,N-bis(methyl-[²H₂])aniline ([²H₄]DMAP-NCS), was synthesized and used to label amino acids in DBS specimens. The [²H₄]DMAP-NCS labelled amino acid standards were used as internal standards to compensate the matrix effect. This method was validated by measuring linearity, recovery and accuracy. The results showed that the developed SID-LC-MS method can be used for sensitive and selective determination of 12 diagnostically important amino acids in DBS specimens.     

The enthalpic interaction coefficients of N,N′-hexamethylenebisacetamide and N-methylformamide with four types of amino acids in aqueous sucrose solutions at 298.15 K

The mixing enthalpies of N,N′-hexamethylenebisacetamide (HMBA) and N-methylformamide (NMF) with glycine, l-alanine, l-serine, and l-valine in aqueous sucrose solutions have been determined by using mixing-flow isothermal microcalorimetry at the temperature of 298.15 K along with their dilution enthalpies, respectively. Based on the obtained results, the heterotactic enthalpic interaction coefficients (h_xy, h_xxy, and h_xyy) have been obtained according to McMillan–Mayer’s theory with the sucrose molality from 0 to 1.5 mol · kg⁻¹. The fitted results indicate that the values of h_xy between HMBA or NMF and the four investigated amino acids in aqueous sucrose solutions are all positive. Meanwhile, the values of h_xy reach the corresponding maximum at different sucrose molalities except that the values of h_xy between NMF and glycine decrease monotonically with the increasing molality of sucrose. Furthermore, the order for the value of h_xy of the four amino acids with HMBA or NMF are h_xy (l-valine) > h_xy (l-alanine) > h_xy (l-serine) > h_xy (glycine) in pure water or in aqueous solution with the same molality of sucrose. The values of h_xy between HMBA and the four amino acids are much larger than that between NMF and the same amino acids with the same molality of sucrose. All the variations of the heterotactic enthalpic pairwise interaction coefficients in the quaternary systems can be interpreted with the help of the solute–solute and solute–solvent interactions theory.     

A convenient synthesis of activated enantiomerically pure 2-ethynylaziridines

2,3-cis- and 2,3-trans-N-Arylsulfonyl-2-ethynylaziridines with high enantiomeric purity have been synthesized. N-Protected amino aldehydes synthesized from natural α-amino acids were successively treated with Ph₃PC(Br)CO₂Me, DIBAL, MsCl–Et₃N, NaH in DMSO, and tert-BuOK in THF to yield 2-ethynylaziridines in good to high yields.     

Structure of aminonitrones and electronic effect of substituents on their acid-base properties

A series of para-substituted aromatic aminonitrones p-RC₆H₄C(NH₂)=N⁺(Me)O^– (R = NMe₂, H, Br, Cl, CF₃) have been prepared. Acidity constants of the conjugate acids RC₆H₄C(NH₂)N⁺(Me)OH at 25°C in a EtOH–H₂O mixture (5: 95) have been determined by potentiometric titration. A linear correlation between log (k_R/k_H) and σ_para values has been revealed, and a ρ²⁹⁸(σ_para) parameter has been determined as of 0.635.