Amino acids and peptides. XIII. Synthetic studies on N-terminal tripeptide amide analogs of fibrin alpha-chain

N-Terminal tripeptide analogs of fibrin alpha-chain were synthesized and their inhibitory effect on fibrinogen/thrombin clotting was examined. A new water-soluble active ester, 3-pyridinium ester, was used for the synthesis. Among the synthetic peptides, H-Gly-Pro-Arg-hexamethyleneimine exhibited the highest inhibitory effect on fibrinogen-thrombin clotting.     

An enkephalin-degrading aminopeptidase from rat brain catalyzes the hydrolysis of a neuropeptide, kyotorphin (L-Tyr-L-Arg).

We studied the hydrolysis of a neuropeptide kyotorphin (L-Tyr-L-Arg) by an enkephalin-degrading aminopeptidase purified from cytosol of rat brain in vitro. The purified enzyme was homogeneous as judged by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE), gel filtration and isoelectric focusing. The aminopeptidase with an apparent molecular weight (Mr) = 98000 catalyzed the hydrolysis of Leu- and Met-enkephalins with Km values of 125 and 142 microM, respectively. The enzyme activity was inhibited by bestatin, amastatin and puromycin but not by pepstatin, leupeptin and phenylmethanesulfonyl fluoride (PMSF). Kyotorphin was degraded by the aminopeptidase at pH 7.0, and the Vmax and Km values were 9.2 mumol/min/mg protein and 95 microM, respectively. The Km value for kyotorphin was compatible to those for Leu- and Met-enkephalins. Taken together, these results suggest a possible involvement of the enkephalin-degrading aminopeptidase in cytosolic degradation of kyotorphin in neuronal cells of rat brain.     

Analysis method of the angiotensin-I converting enzyme inhibitory activity based on micellar electrokinetic chromatography.

A micellar electrokinetic chromatography (MEKC) method was developed for estimating the angiotensin-I converting enzyme (ACE) inhibitory activity by separating the hippuric acid liberated in the ACE reaction mixture in the presence of an inhibitor, captopril. The hippuric acid was successfully separated and detected by MEKC with a 25 mM sodium dodecyl sulfate solution in a 25 mM phosphate-50 mM borate buffer at pH 7.0; the total analysis took about 5 min. A good linear relationship was observed between the inhibitor and the peak area of hippuric acid release. No significant difference in the ACE inhibitory activity (IC50) of captopril (an antihypertensive medicine) or autolyzed-mushrooms (functional foods) was observed between the conventional method and the MEKC method. The MEKC method was found to be a useful technique for a rapid assay of the ACE inhibitory activity.     

Micro-phase sorbent extraction for trace analysis via in situ sorbent formation: application to the spectrophotometric determination of nitrite in environmental waters.

A micro-phase sorbent in situ formation from an aqueous solution was proposed for the sensitive spectrophotometric determination of nitrite in environmental waters. Nitrite in a 10 mL sample solution was converted into a cationic azo dye by the reaction with 4-trifluoromethylanilinium ion and N-1-naphthylethylenediammonium ion in an acidic medium. Addition of dodecylbenzenesulfonate ion caused the formation of a suspension of ion associate in the solution. Centrifugation of the solution led to the isolation of a liquid organic phase that extracted the azo dye at the bottom of the centrifuge tube. The volume of the new phase was ca. 5 microL. After the aqueous phase was discarded, the organic phase was dissolved with 1 mL of 2-methoxyethanol to measure the absorbance. Nitrogen as NO2- at concentrations from 1.5 to 30 microg L(-1) was determined with sufficient precision. When 0.2 mL of 2-methoxyethanol was applied to dissolve the organic phase, 0.3-4.8 microg NO2--N L(-1) was determined. The recovery tests for nitrite added to some river water and seawater were satisfactory. This method is very simple and rapid. It takes only 30 min from the dye formation to the measurement of the absorbance.     

Chiral discrimination of a helically organized crown ether array parallel to the helix axis of polyisocyanate

The asymmetric polymerization of 4′‐isocyanatobenzo‐18‐crown‐6 with the lithium amide of (S)‐(2‐methoxymethyl)pyrrolidine successfully proceeded to afford end‐functionalized poly(4′‐isocyanatobenzo‐18‐crown‐6) with (S)‐(2‐methoxymethyl)pyrrolidine (polymer 2 ). In the circular dichroism (CD) spectrum of 2 , a clear positive Cotton effect was observed in the range of 240–350 nm corresponding to the absorption of the polymer backbone, indicating that 2 partially formed a one‐handed helical structure, which was preserved by the chirality of (S)‐(2‐methoxymethyl)pyrrolidine bonding to the terminal end in 2 . In the titration experiments for the CD intensity of 2 in the presence of D ‐ and L ‐Phe·HClO₄ (where Phe is phenylalanine), a small but remarkable difference was observed in the amount of the chiral guest needed for saturation of the CD intensity and in the saturated CD intensity, indicating that the extremely stable, one‐handed helical part should exist in the main chain of 2 , which was not inverted even when the unfavorable chiral guest for the predominant helical sense, L ‐Phe·HClO₄, was added. In addition, helical polymer 2 exhibited a chiral discrimination ability toward racemic guests; that is, the guests were extracted from the aqueous phase into the organic phase with enantiomeric excess. The driving force of the chiral discrimination ability of 2 should certainly be attributed to the one‐handed helical structure in 2 . © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 325–334, 2006     

Hydration Structure Around the Carboxyl Group Studied by Neutron Diffraction with 12C/13C and H/D Isotopic Substitution Methods

Time-of-Flight (TOF) neutron diffraction measurements have been carried out on aqueous 8 mol% sodium acetate solutions in D₂O. Scattering cross sections that were observed for sample solutions involving ¹²C/¹³C and H/D isotopically substituted acetate ions were used to derive the first-order difference functions, Δ_H(Q) and Δ_C(Q), and corresponding distribution functions, G _H(r;r) and G _C(r;r), which describe the environmental structure around the methyl and the carboxyl groups within the acetate ion, respectively. Structural parameters concerning the first hydration shell of the carboxyl group within the acetate ion were obtained through the least squares fit to the observed intermolecular difference function, Δ_C ^inter(Q). The nearest neighbor C _O...D _W1 (C_O: carboxyl carbon atom, D_W1: water deuterium atom) distance, r(C _O...D _W1 ), and the angle, ∠ C _O ...D _W1 -O _W (O _W : water oxygen atom), were determined to be 2.63(1) Å and 120(1)°, respectively. The coordination number, n(C _O ...D _W1 ), was obtained to be 4.0(1). These results are consistent with the hydration structure in which water molecules in the first hydration shell of the carboxyl group are hydrogen-bonded with oxygen atoms of the carboxyl group.     

Palladium-catalyzed oxidation of alcohols via their allyl carbonates under neutral conditions

Treatment of alkyl allyl carbonates derived from various alcohols with a palladium catalyst in MeCN affords ketones and aldehydes in high yields. This new method of oxidation of alcohols can be applied to various alcohols except simple primary alcohols.     

Cobalt-catalyzed cross-coupling reactions of alkyl halides with allylic and benzylic Grignard reagents and their application to tandem radical cyclization/cross-coupling reactions

Details of cobalt-catalyzed cross-coupling reactions of alkyl halides with allylic Grignard reagents are disclosed. A combination of cobalt(II) chloride and 1,2-bis(diphenylphosphino)ethane (DPPE) or 1,3-bis(diphenylphosphino)propane (DPPP) is suitable as a precatalyst and allows secondary and tertiary alkyl halides--as well as primary ones--to be employed as coupling partners for allyl Grignard reagents. The reaction offers a facile synthesis of quaternary carbon centers, which has practically never been possible with palladium, nickel, and copper catalysts. Benzyl, methallyl, and crotyl Grignard reagents can all couple with alkyl halides. The benzylation definitely requires DPPE or DPPP as a ligand. The reaction mechanism should include the generation of an alkyl radical from the parent alkyl halide. The mechanism can be interpreted in terms of a tandem radical cyclization/cross-coupling reaction. In addition, serendipitous tandem radical cyclization/cyclopropanation/carbonyl allylation of 5-alkoxy-6-halo-4-oxa-1-hexene derivatives is also described. The intermediacy of a carbon-centered radical results in the loss of the original stereochemistry of the parent alkyl halides, creating the potential for asymmetric cross-coupling of racemic alkyl halides.     

Homogeneous palladium catalyst suppressing Pd black formation in air oxidation of alcohols

In homogeneous catalyst systems, there is the persistent problem that metal aggregation and precipitation cause catalyst decomposition and considerable loss of catalytic activity. Pd black formation is a typical example. Pd catalysts are known to easily aggregate and form Pd black, although they realize a wide variety of useful reactions in organic synthesis. In order to overcome this intrinsic problem of homogeneous Pd catalysis, we explored a new class of Pd catalyst by adopting aerobic oxidation of alcohols as a probe reaction. Herein we report a new catalyst system that suppresses the Pd black formation even under air and with a high substrate to catalyst molar ratio (S/C: more than 1000) in oxidation of alcohols. The novel pyridine derivatives having a 2,3,4,5-tetraphenylphenyl substituent and its higher dendritic unit at the 3-position of the pyridine ring were found to be excellent ligands with Pd(OAc)2 in the palladium-catalyzed air (balloon) oxidation of alcohols in toluene at 80 degrees C. Comparison with structurally related pyridine ligands revealed that introduction of the 2,3,4,5-tetraphenylphenyl substituent at the 3-position of pyridine ring effectively suppresses the Pd black formation, maintaining the catalytic activity for a long time to give aldehydes or ketones as products in high yields.