Enantioselective Recognition of Aspartic Acids by Chiral Ligand Exchange Potentiometry

Enantioselective resolution is realized by combining potentiometry with ligand exchange (CE) in a new method called chiral ligand exchange potentiometry (CLEP). A chiral selector, N‐carbobenzoxy‐L ‐aspartic acid (N‐CBZ‐L‐Asp), preferentially recognizes D ‐aspartic acid (D‐Asp) and undergoes ligand exchange with the enantiomeric labile coordination complexes of [Cu(II)(D‐Asp)₂] or [Cu(II)(L‐Asp)₂] to form a diastereoisomeric complex [(D‐Asp)Cu(II)(N‐CBZ‐L‐Asp)] (a) or [(L‐Asp)Cu(II)(N‐CBZ‐L‐Asp)] (b). Considerable stereoselectivity occurs in the formation of these diastereoisomeric complexes, and their net charges were ?2 (a) and 0 (b), respectively, resulting in different Nernst factor (electrode slope), thus enabling chiral D‐Asp to be distinguished by potentiometry without any pre‐ or postseparation processes.     

Development of a New Class of Inhibitors for the Malarial Aspartic Protease Plasmepsin II Based on a Central 7‐Azabicyclo[2.2.1]heptane Scaffold

Plasmepsin II (PMII), a malarial aspartic protease involved in the catabolism of hemoglobin in parasites of the genus Plasmodium, and renin, a human aspartic protease, share 35% sequence identity in their mature chains. Structures of 4‐arylpiperidine inhibitors complexed to human renin were reported by Roche recently. The major conformational changes, compared to a structure of renin, with a peptidomimetic inhibitor were identified and subsequently modeled in a structure of PMII (Fig. 1). This distorted structure of PMII served as active‐site model for a novel class of PMII inhibitors, according to a structure‐based de novo design approach (Fig. 2). These newly designed inhibitors feature a rigid 7‐azabicyclo[2.2.1]heptane scaffold, which, in its protonated form, is assumed to undergo ionic H‐bonding with the two catalytic Asp residues at the active site of PMII. Two substituents depart from the scaffold for occupancy of either the S1/S3 or S2′‐pocket and the hydrophobic flap pocket, newly created by the conformational changes in PMII. The inhibitors synthesized starting from N‐Boc‐protected 7‐azabicyclo[2.2.1]hept‐2‐ene ( 6 ; Schemes 1–5) displayed up to single‐digit micromolar activity (IC₅₀ values) toward PMII and good selectivity towards renin. The clear structure? activity relationship (SAR; Table) provides strong validation of the proposed conformational changes in PMII and the occupancy of the resulting hydrophobic flap pocket by our new inhibitors.     

Engineered C–N Lyase: Enantioselective Synthesis of Chiral Synthons for Artificial Dipeptide Sweeteners

Aspartic acid derivatives with branched N‐alkyl or N‐arylalkyl substituents are valuable precursors to artificial dipeptide sweeteners such as neotame and advantame. The development of a biocatalyst to synthesize these compounds in a single asymmetric step is an as yet unmet challenge. Reported here is an enantioselective biocatalytic synthesis of various difficult N‐substituted aspartic acids, including N‐(3,3‐dimethylbutyl)‐l ‐aspartic acid and N‐[3‐(3‐hydroxy‐4‐methoxyphenyl)propyl]‐l ‐aspartic acid, precursors to neotame and advantame, respectively, using an engineered variant of ethylenediamine‐N,N′‐disuccinic acid (EDDS) lyase from Chelativorans sp. BNC1. This engineered C–N lyase (mutant D290M/Y320M) displayed a remarkable 1140‐fold increase in activity for the selective hydroamination of fumarate compared to that of the wild‐type enzyme. These results present new opportunities to develop practical multienzymatic processes for the more sustainable and step‐economic synthesis of an important class of food additives.     

Sesquiterpenoids and Phenylpropane Derivatives from Sonchus uliginosus

Six new compounds were isolated from the whole plant of Sonchus uliginosus, including three eudesmane‐type sesquiterpenoids (1β,6α)‐1,6,14‐trihydroxyeudesm‐3‐en‐12‐oic acid γ‐lactone ( 1 ), (1β,6α)‐1,6,14‐trihydroxyeudesma‐3,11(13)‐dien‐12‐oic acid γ‐lactone ( 2 ), and (1β,6α)‐1,6‐dihydroxy‐14‐O‐[(4‐hydroxyphenyl)acetyl]eudesma‐3,11(13)‐dien‐12‐oic acid γ‐lactone ( 3 ), and three phenylpropane derivatives, 4‐hydroxy‐γ,3,5‐trimethoxybenzenepropanol ( 6 ), γ,3,4,5‐tetramethoxybenzenepropanol ( 7 ), and γ,3,4,5‐tetramethoxybenzenepropanol acetate ( 8 ), together with the two known compounds 4 and 5 . The new structures were elucidated by means of spectroscopic methods, such as IR, EI‐MS, HR‐ESI‐MS, 1D‐ and 2D‐NMR, and by comparison of the spectroscopic data with those reported for structurally related compounds.     

A Practical One‐Pot Synthesis of New S‐Glycosyl Amino Acid Building Blocks for Combinatorial Neoglycopeptide Synthesis

S‐Glycosyl L‐aspartic acid building blocks were synthesized starting from 1‐thiosugars by reaction with 5‐aminopentanol and suitably protected L‐aspartic acid pentafluorophenyl ester in a one‐pot procedure under Mitsunobu conditions using 1,1′‐azodicarbonyl dipiperidine and trimethyl phosphine. The method allowed for the preparation of S‐glycosyl amino acid building blocks in one step without protection of the amino function for the Mitsunobu condensation. Alternatively, the title compounds were prepared by a stepwise approach via 5‐aminopentyl 1‐thioglycosides.     

Palmitoleate (=(9Z)‐Hexadeca‐9‐enoate) Esters of Oleanane Triterpenoids from the Golden Flowers of Tagetes erecta: Isolation and Autoxidation Products

Six oleanane‐type triterpenoid esters were isolated from the golden flowers of Tagetes erecta. Spectral studies characterized their structures as 3‐O‐[(9Z)‐hexadec‐9‐enoyl]erythrodiol ( 1 ), 11α,12α:13β,28‐diepoxyoleanan‐3β‐yl (9Z)‐hexadec‐9‐enoate ( 2 ), 13β,28‐epoxyolean‐11‐en‐3β‐yl (9Z)‐hexadec‐9‐enoate ( 3 ), 28‐hydroxy‐11‐oxoolean‐12‐en‐3β‐yl (9Z)‐hexadec‐9‐enoate ( 4 ), 3‐O‐[(9Z‐hexadec‐9‐enoyl]‐β‐amyrin ( 5 ), and 11‐oxoolean‐12‐en‐3β‐yl (9Z)‐hexadec‐9‐enoate ( 6 ). Compounds 1 – 4 and 6 are new natural products, while the known 5 was isolated for the first time from the genus Tagetes, from which only one triterpenoid has earlier been obtained. Aerial oxidation (autoxidation) converted amyrin 1 into 2 – 4 and transformed amyrin 5 into 6 . The configuration of 1 – 6 and an autoxidation mechanism (Scheme) involving the formation of the intermediate 11α‐hydroxyolean‐12‐ene derivatives 1b and 5b on thermal decomposition of the labile 11α‐OOH derivatives 1a and 5a , respectively, under neutral conditions are discussed. For the first time, the reactivity of the allylic H? C(11) bond of triterpenoids of type 1 and 5 toward aerial oxidation was observed. The long‐chain ester group at C(3) of 1 and 5 may be responsible for their labile nature, as β‐amyrin ( 7 ), erythrodiol ( 8 ), and ursolic acid were found to be inert toward autoxidation.     

Synthesis of New Furo[3,4‐b]quinolin‐1(3H)‐one Scaffolds Derived from γ‐Lactone‐Fused Quinolin‐4(1H)‐ones

In the context of our aim of discovering new antitumor drugs among synthetic γ‐lactone‐ and γ‐lactam‐fused 1‐methylquinolin‐4(1H)‐ones, we developed a rapid access to 5‐methyl‐1,3‐dioxolo[4,5‐g]furo[3,4‐b]quinoline‐8,9(5H,6H)‐dione ( 9 ) exploiting the γ‐lactone‐fused chloroquinoline 10 previously synthesized in our laboratory (Scheme 1). We also elaborated efficient synthetic methods allowing for a rapid access to two nonclassical bioisosteres of 9 , i.e., a deoxy and a carba analogue. The deoxy analogue 11 was prepared in two steps from the γ‐lactone‐fused quinoline 13 which was also the synthetic precursor of 10 (Scheme 1). The carba analogue 6,9‐dihydro‐5‐methyl‐9‐methylene‐1,3‐dioxolo[4,5‐g]furo[3,4‐b]quinolin‐8(5H)‐one ( 12 ) was easily prepared by HCl elimination from the 9‐(chloromethyl)dioxolofuroquinoline 15 , which was obtained via a three‐component one‐pot reaction from N‐methyl‐3,4‐(methylenedioxy)aniline (=N‐methyl‐1,3‐benzodioxol‐5‐amine; 16 ), commercially available chloroacetaldehyde, and tetronic acid ( 17 ) (Scheme 2).     

pH‐ and β‐cyclodextrin‐responsive micelles based on polyaspartamide derivatives as drug carrier

A novel kind of graft polymer poly(aspartic acid)‐ethanediamine‐g‐adamantane/methyloxy polyethylene glycol (Pasp‐EDA‐g‐Ad/mPEG) was designed and synthesized for drug delivery in this study. The chemical structure of the prepared polymer was confirmed by proton NMR. The obtained polymer can self‐assemble into micelles which were stable under a physiological environment and displayed pH‐ and β‐cyclodextrin (β‐CD)‐responsive behaviors because of the acid‐labile benzoic imine linkage and hydrophobic adamantine groups in the side chains of the polymer. The doxorubicin (Dox)‐loaded micelles showed a slow release under physiological conditions and a rapid release after exposure to weakly acidic or β‐CD environment. The in vitro cytotoxicity results suggested that the polymer was good at biocompatibility and could remain Dox biologically active. Hence, the Pasp‐EDA‐g‐Ad/mPEG micelles may be applied as promising controlled drug delivery system for hydrophobic antitumor drugs. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1387–1395     

A New Class of Inhibitors for the Malarial Aspartic Protease Plasmepsin II Based on a Central 11‐Azatricyclo[6.2.1.02,7]undeca‐2,4,6‐triene Scaffold

A new class of nonpeptidic inhibitors of the malarial aspartic protease plasmepsin II (PMII) with up to single‐digit micromolar activities (IC₅₀ values) was developed by structure‐based de novo design. The active‐site matrix used in the design was based on an X‐ray crystal structure of PMII, onto which the major conformational changes seen in the structure of renin upon complexation of 4‐arylpiperidines – including the unlocking of a new hydrophobic (flap) pocket – were modeled. The sequence identity of 35% between mature renin and PMII had prompted us to hypothesize that an induced‐fit adaptation around the active site as observed in renin might also be effective in PMII. The new inhibitors contain a central 11‐azatricyclo[6.2.1.0^2,7]undeca‐2(7),3,5‐triene core, which, in protonated form, undergoes ionic H‐bonding with the two catalytic Asp residues at the active site of PMII (Figs. 1 and 2). This tricyclic scaffold is readily prepared by a Diels? Alder reaction between an activated pyrrole and a benzyne species generated in situ (Scheme 1). Two substituents with naphthyl or 1,3‐benzothiazole moieties are attached to the central core (Schemes 1–4) for accommodation in the hydrophobic flap and S1/S3 (or S2′, depending on the optical antipode of the inhibitor) pockets at the active site of the enzyme. The most‐potent inhibitors (±)‐ 19a – 19c (IC₅₀ 3–5 μM ) and (±)‐ 23b (2 μM ) (Table) bear an additional Cl‐atom on the 1,3‐benzothiazole moiety to fully fill the rear of the flap pocket. Optimization of the linker between the tricyclic scaffold and the 1,3‐benzothiazole moiety, based on detailed conformational analysis (Figs. 3 and 4), led to a further small increase in inhibitory strength. The new compounds were also tested against other aspartic proteases. They were found to be quite selective against renin, while the selectivity against cathepsin D and E, two other human aspartic proteases, is rather poor (Table). The detailed SARs established in this investigation provide a valuable basis for the design of the next generations of more‐potent and ‐selective PMII inhibitors with potential application in a new antimalarial therapy.     

Adenanthusone,a New Ursane Type Nortriterpenoid from Isodon adenanthus

A new ursane‐type nortriterpenoid, adenanthusone (=(11α,12α)‐4‐demethyl‐11,12‐epoxy‐3,13‐dihydroxy‐2‐oxoursa‐3,20(30)‐diene‐28‐oic acid γ‐lactone; 1 ) was isolated from Isodon adenanthus. Its structure was determined by NMR spectra and X‐ray crystallographic diffraction analysis. The biogenetic implication of the nortriterpene is discussed.     

Synthesis of Biodegradable Amphoteric Poly[(aspartic acid)‐co‐lysine] by Thermal Polycondensation

A new class of biodegradable polyampholytes, poly[(aspartic acid)‐co‐lysine], were synthesized by thermal polycondensation of aspartic acid and lysine under reduced pressure and subsequent hydrolysis. Polymerization conditions were optimized to yield maximal water‐soluble poly(succinimide‐co‐lysine) with high molecular weight (160°C/3.5 h). The succinimide/lysine ratio in the polyampholytes could be adjusted by their feed ratio. Characterization of the poly(succinimide‐co‐lysine) by ¹H NMR revealed that ω‐amine and carboxylic groups in lysine participated in the polymerization, leaving α‐amino groups as pendant cationic moieties.     

Chiral separation of N-methyl-dl-aspartic acid in rat brain tissue as N-ethoxycarbonylated (S)-(+)-2-octyl ester derivatives by GC-MS

A selective and sensitive analytical method was developed for enantiomeric separation and determination of N‐methyl‐DL‐aspartic acid (NMA). The method involved the conversion of each enantiomer into N‐ethoxycarbonylated (S)‐(+)‐2‐octyl ester derivative for the direct separation by gas chromatography–mass spectrometry (GC‐MS). The diastereomeric derivatives showed characteristic mass spectral properties for analysis by selected ion monitoring mode (SIM) and enabling enantioseparation on an achiral capillary column. Two enantiomers were baseline separated, and the detection limits for N‐methyl‐L‐aspartic acid (NMLA) and N‐methyl‐D‐aspartic acid (NMDA) were 0.07 and 0.03 ng/g, respectively. When applied to rat brain tissues for absolute configuration of NMA, only NMDA was determined, while NMLA was monitored as lower than the limit of detection. Copyright © 2012 John Wiley & Sons, Ltd.     

Formation of Periodic γ‐Turns in α/β‐Hybrid Peptides: DFT and NMR Experimental Evidence

Hybrid peptidic oligomers comprising natural and unnatural amino acid residues that can exhibit biomolecular folding and hydrogen‐bonding mimicry have attracted considerable interest in recent years. While a variety of hybrid peptidic helices have been reported in the literature, other secondary structural patterns such as γ‐turns and ribbons have not been well explored so far. The present work reports the design of novel periodic γ‐turns in the oligomers of 1:1 natural‐α/unnatural trans‐β‐norborenene (TNAA) amino acid residues. Through DFT, NMR, and MD studies, it is convincingly shown that, in the mixed conformational pool, the heterogeneous backbone of the hybrid peptides preferentially adopt periodic 8‐membered (pseudo γ‐turn)/7‐membered (inverse γ‐turn) hydrogen bonds in both polar and non‐polar solvent media. It is observed that the stereochemistry and local conformational preference of the β‐amino acid building blocks have a profound influence on accessing the specific secondary fold. These findings may be of significant relevance for the development of molecular scaffolds that facilitate desired positioning of functional side‐chains.     

Influence of β‐Alanine on Hairpin Polyamide Orientation in the DNA Minor Groove

Antiparallel polyamides containing 1H‐pyrrole, 1H‐imidazole, and 3‐hydroxy‐1H‐pyrrole amino acids display a preference for minor‐groove binding oriented N? C with respect to the 5′‐3′ direction of the DNA helix. We find that replacement of a central Py/Py pair with a β/β pair within a ten‐ring hairpin relaxes the orientation preference and, for some DNA sequences, causes the polyamide to prefer the opposite C? N orientation. Substitution of the achiral γ‐aminobutanoic acid (γ) with either (R)(or S)‐2‐(acetylamino)‐4‐aminobutanoic acid moderates the orientation preference of the 2‐β‐2‐hairpin.     

Determination of kynurenic acid levels in rat brain microdialysis samples and changes in kynurenic acid levels induced by N‐acetylaspartic acid

The levels of kynurenic acid, an endogenous antagonist of α₇ nicotinic acetylcholine and N‐methyl‐D ‐aspartate receptors, were measured in microdialysis samples obtained from the prefrontal cortices of rats using column‐switching high‐performance liquid chromatography with fluorescence detection. When the perfusate was constantly infused at a rate of 1.0 μ/min, the in vitro recovery of kynurenic acid through the dialysis membrane was approximately 20.4%, and the precision was within 1.31%. Endogenous kynurenic acid in the microdialysis sample was clearly detected using column‐switching high‐performance liquid chromatography. As an application study, N‐acetyl‐L ‐aspartic acid, an endogenous metabolite and precursor of N‐acetyl‐L ‐aspartyl‐L ‐glutamic acid, which is an agonist of metabotropic glutamate receptors, was infused for 120 min through the microdialysis probe. The kynurenic acid level significantly increased during the infusion of N‐acetyl‐L ‐aspartic acid, suggesting that kynurenic acid might have some association with N‐acetyl‐L ‐aspartic acid in vivo. Copyright © 2009 John Wiley & Sons, Ltd.     

Design and Synthesis of p/m-[p-（un）Substituted Phenylsulfonamido]phenyl β-Diketo Acids and Quinoxalone Derivatives

Diketo acid derivatives are potent and selective HIV-1 integrase inhibitors. To investigate the detailed synthesis of those derivatives, a series of p/m-[p-（un）substituted phenylsulfonamido]phenyl β-diketo acid derivatives have been designed and synthesized. The quinoxalone derivatives as the potential bioisosteres of the biologically labile β-diketoacid pharmacophores have also been synthesized from reactions of the corresponding diketo acids with o-phenylenediamine. The structures of all diketo acid （ester） and quinoxalone derivatives were confirmed by 1^H NMR, 13^C NMR, IR, HRMS and/or MS （ESI）. X-ray crystallographic analysis of 11b demonstrates a similar arrangement of the side chain of quinoxalone derivatives with the parent diketoacids due to the intramolecular hydrogen bond （O…H-N） and the sp^2 hybridization configuration of the two nitrogen atoms of the quinoxalone ring.     

Determination of GABA, glutamate and carbamathione in brain microdialysis samples by capillary electrophoresis with fluorescence detection

Disulfiram has been used as a deterrent in the treatment of alcohol abuse for almost 60 years. Our laboratory has shown that a disulfiram metabolite, S‐(N,N‐diethylcarbamoyl) glutathione (carbamathione), is formed from disulfiram and appears in the brain after the administration of disulfiram. Carbamathione does not inhibit aldehyde dehydrogenase but has been shown to be a partial non‐competitive inhibitor of the N‐methyl‐D ‐aspartic acid glutamate (Glu) receptor. In light of disulfiram's apparent clinical effectiveness in cocaine dependence, and carbamathione's effect on the N‐methyl‐D ‐aspartic acid receptor, the effect of carbamathione on brain Glu and γ‐aminobutyric acid (GABA) needs to be further examined. A CE‐LIF method based on derivatization with napthalene‐2,3‐dicarboxyaldehyde to simultaneously detect both neurotransmitter amino acids and carbamathione in brain microdialysis samples is described. The separation of Glu, GABA and carbamathione was carried out using a 50 mmol/L boric acid buffer (pH 9.6) on a 75 cm×50 μm id fused‐silica capillary (60 cm effective) at +27.5 kV voltage with a run time of 11 min. The detection limits for Glu, GABA and carbamathione were 6, 10 and 15 nmol/L, respectively. This method was used to monitor carbamathione and the amino acid neurotransmitters in brain microdialysis samples from the nucleus accumbens after the administration of an intravenous dose of the drug (200 mg/kg) and revealed a carbamathione‐induced change in GABA and Glu levels. This method demonstrates a simple, rapid and accurate measurement of two amino acid neurotransmitters and carbamathione for in vivo monitoring in the brain using microdialysis sampling.     

Synthesis and helical conformation of poly(N‐propargylamides) carrying L‐aspartic acid in the side chain

Aspartic acid‐based novel poly(N‐propargylamides), i.e., poly[N‐(α‐tert‐butoxycarbonyl)‐L ‐aspartic acid β‐benzyl ester N′‐propargylamide] [poly( 1 )] and poly[N‐(α‐tert‐butoxycarbonyl)‐L ‐aspartic acid α‐benzyl ester N′‐propargylamide] [poly( 2 )] with moderate molecular weights were synthesized by the polymerization of the corresponding monomers 1 and 2 catalyzed with (nbd)Rh⁺[η⁶‐C₆H₅B^?(C₆H₅)₃] in CHCl₃ at 30 °C for 2 h in high yields. The chiroptical studies revealed that poly( 1 ) took a helical structure in DMF, while poly( 2 ) did not in DMF but did in CH₂Cl₂, CHCl₃, and toluene. The helicity of poly( 1 ) and poly( 2 ) could be tuned by temperature and solvents. Poly( 2 ) underwent solvent‐driven switch of helical sense, accompanying the change of the tightness. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5168–5176, 2005     

Cyclic lipoundecapeptide amphisin from Pseudomonas sp. strain DSS73

The crystal structure of the lipoundecapeptide amphisin, presented here as the tetrahydrate, C₆₆H₁₁₄N₁₂O₂₀·4H₂O, originating from non‐ribosomal biosynthesis by Pseudomonas sp. strain DSS73, has been solved to a resolution of 0.65 Å. The primary structure of amphisin is β‐hydroxy­decanoyl‐d ‐Leu‐d ‐Asp‐d ‐allo‐Thr‐d ‐Leu‐d ‐Leu‐d ‐Ser‐l ‐Leu‐d ‐Gln‐l ‐Leu‐l ‐Ile‐l ‐Asp (Leu is leucine, Asp is aspartic acid, Thr is threonine, Ser is serine, Gln is glut­amine and Ile is isoleucine). The peptide is a lactone, linking Thr4 O_γ to the C‐terminal. The stereochemistry of the β‐hydroxy acid is R. The peptide is a close analogue of the cyclic lipopeptides tensin and pholipeptin produced by Pseudomonas fluorescens. The structure of amphisin is mainly helical (3₁₀‐helix), with the cyclic peptide wrapping around a hydrogen‐bonded water mol­ecule. This lipopeptide is amphiphilic and has biosurfactant and antifungal properties.     

Conformation and structure of bis­(glycyl‐l‐aspartic acid) oxalate 0.4‐hydrate

The title bis­(glycyl‐l ‐aspartic acid) oxalate complex {systematic name: bis­[2‐(2‐ammonio­acetamido)butane­dioic acid] oxalate 0.4‐hydrate}, 2C₆H₁₁N₂O₅⁺·C₂O₄²⁻·4H₂O, crystallizes in a triclinic space group with the planar peptide unit in a trans conformation. The asymmetric unit consists of two glycyl‐l ‐aspartic acid mol­ecules with positively charged amino groups and neutral carboxyl groups, and an oxalate dianion. The twist around the C—Cα bond indicates that both the peptide mol­ecules adopt extended conformations, while the twist around the N—Cα bond shows that one has a folded and the other a semi‐extended state. The present complex can be described as an inclusion compound with the dipeptide mol­ecule as the host and the oxalate anion as the guest. The usual head‐to‐tail sequence of aggregation is not observed in this complex, as is also the case with the glycyl‐l ‐aspartic acid dihydrate mol­ecule. The study of aggregation and inter­action patterns in binary systems is the first step towards understanding more complex phenomena. This further leads to results that are of general interest in bimolecular aggregation.