Synthesis,conformation, and activity of HCO-Met-ΔZ Leu-Phe-Ome,an active analogue of chemotactic N-formyltripeptides

In order to induce a β-turn conformation into the chemotactic linear tripeptide N-formyl-L -methionyl-L -leucyl-L -phenylalanine (fMLP), the new analogue N-formyl-L -methionyl-Δ^Zleucyl-L -phenylalanine methyl ester [ Δ^ZLeu]²f MLP-OMe ( 1 ) has been synthesized. The conformational and biochemical consequences of this chemical modification have been determined. Analogue 1 has been synthesized by using N-carboxy-(Z)-α,β-didehydroleucine anhydride as key compound to introduce the unsaturated residue at the central position of the tripeptide 1 . The x-ray analysis shows that 1 adopts in the crystal a type II β-turn conformation in which the new residue occupies the (i + 2) position, and an intramolecular H bond is formed between the formylic oxygen and the Phe NH. ¹H-nmr analysis based on nuclear Overhauser effect measurements suggests that the same folded conformation is preferred in CDCl₃ solution; this finding is also supported by molecular dynamics simulation. The biological activity of 1 has been determined on human neutrophils (polymorphonuclear leukocytes) and compared to that shown by f MLP-OMe. Chemotactic activity, granule enzyme release, and superoxide anion production have been determined. Analogue 1 is practically inactive as chemoattractant, highly active in the superoxide generation, and similar to the parent in the lysozyme release. The conformational restriction imposed on the backbone by the presence of the unsaturated residue is discussed in relation with the observed bioselectivity. © 1993 John Wiley & Sons, Inc.     

2D 1H and 31P NMR Spectra and Distorted A-DNA-Like Duplex Structure of a Phosphorodithioate Oligonucleotide

Abstract

Assignment of the ¹H and ³¹P NMR spectra of a phosphorodithioate modified oligonucleotide decamer duplex, d(CGCTTpS^? ₂AAGCG)₂ (10-mer-S; a site of dithioate substitution is designated with the symbols pS^? ₂), was achieved by two-dimensional homonuclear TOCSY, NOES Y and ¹H-³¹P Pure Absorption phase Constant time (PAC) heteronuclear correlation spectroscopy. In contrast to the parent palindromic decamer sequence (1) which has been shown to exist entirely in the duplex B-DNA conformation under comparable conditions (100 mM KCI), the dithiophosphate analogue forms a hairpin loop. However, the duplex form of the dithioate oligonucleotide can be stabilized at lower temperatures, higher salt and strand concentration. The solution structure of the decamer duplex was calculated by an iterative hybrid relaxation matrix method (MORASS) combined with 2D NOESY-distance restrained molecular dynamics. These backbone modified compounds, potentially attractive antisense oligonucleotide agents, are often assumed to possess similar structure as the parent nucleic acid complex. Importantly, the refined structure of the phosphorodithioate duplex shows a significant deviation from the parent unmodified, phosphoryl duplex. An overall bend and unwinding in the phosphorodithioate duplex is observed. The structural distortion of the phosphorodithioate duplex was confirmed by comparison of helicoidal parameters and groove dimensions. Especially, the helical twists of the phosphorodithioate decamer deviate significantly from the parent phosphoryl decamer. The minor groove width of phosphorodithioate duplex 10-mer-S varies between 8.4 and 13.3 Å which is much wider than those of the parent phosphoryl decamer d(CGCTTAAGCG)₂ (4.2～9.4Å). The larger minor groove width of 10-mer-S duplex contributes to the unwinding of the backbone and indicates that the duplex has an overall A-DNA-like conformation in the region surrounding the dithiophosphate modification.     

Synthesis and diuretic activities of pseudoproline‐containing analogues of the insect kinin core pentapeptide

C‐2 dimethylated/unmethylated thiazolidine‐4‐carboxylic acid and C‐2 dimethylated oxazolidine‐4‐carboxylic acid were introduced into the insect kinin core pentapeptide in place of Pro³, yielding three new analogues. NMR analysis revealed that the peptide bond of Phe²‐pseudoproline (ΨPro)³ is practically 100% in cis conformation in the case of dimethylated pseudoproline‐containing analogues, about 50% cis for the thiazolidine‐4‐carboxylic acid analogue and about 33% cis for the parent Pro³ peptide. The diuretic activities are consistent with the population of cis conformation of the Phe²‐ΨPro³/Pro³ peptide bonds, and the results confirm a cis Phe‐Pro bond as bioactive conformation. Copyright © 2011 European Peptide Society and John Wiley & Sons, Ltd.     

Conformational restriction of Tyr and Phe side chains in opioid peptides: Information about preferred and bioactive side-chain topology

The side chain of Tyr and Phe was fixed into the gauche (−) or gauche (+) conformation by using the Tic or Htc structures, and into the trans conformation by using an aminobenzazepine-type (Aba) structure. When incorporated into dermorphin or deltorphin II, the Tic and Htc analogues all showed a large decrease in both μ and δ affinities and activities. Fixation of Phe³ in the trans rotamer resulted in a large increase in δ affinity in the dermorphin analogue, whereas in the [Aba³-Gly⁴] deltorphin II analogue, good δ affinity is maintained despite the removal of the Glu side chain. Whereas several authors propose a gauche (−) preferred conformation for the Phe³ side chain, these results suggest a trans conformation at the δ receptor. The use of these conformationally constrained residues for evaluating the preferred solution conformation in the flexible N-terminal tripeptide Tyr-D-Ala-Phe is illustrated. The ¹H-nmr parameters—chemical shift, temperature dependence, and nuclear Overhauser effects to the D-Ala² methyl protons in the different analogues—provide direct evidence to confirm the proposed sandwich conformation in the native peptides. © 1996 John Wiley & Sons, Inc.     

Folded Structures in Protonated Reduced Dipeptides

Reduced dipeptides with the general formula RCO-Xaa- rXbb-N⁺HR′R′′ (rXbb, reduced analogue of residue Xbb: NH-C^α HR¹ -C_r H₂) are shown to adopt a folded conformation in solution and in the solid state. The protonated reduced amide bond is an active proton donor capable of interacting with a peptide carbonyl to give a strong hydrogen bond topologically equivalent to the i+2 or i+3? i interaction. The resulting conformation is similar to the γ- or β-turn structure found in peptides and proteins.     

Gramicidin S analogs with a D-Ala, Gly, or L-Ala residue in place of the D-Phe residue: molecular conformations and interactions with phospholipid membrane

The proton nmr and CD spectra of gramicidin S (GS) cyclic-(Val^1,1′-Orn^2,2′-Leu^3,3′-D-Phe^4,4′-Pro^5,5′)₂ and of GS analogs—namely, [D-Ala^4,4′]-GS, [Gly^4,4′]-GS, and [L-Ala^4,4′]-GS—were analyzed. The molecular conformation of [D-Ala^4,4′]-GS is similar to that of GS, with the trans form about the D-Ala-Pro peptide bond. The molecular conformation of [Gly^4,4′]-GS depends on the solvent composition of dimethylsulfoxide-d₆/trifluoroethanol (DMSO)-d₆/TFE and DMSO-d₆/H₂O as well as the solute concentration. In DMSO-d₆ solution, [Gly^4,4′]-GS forms the GS-type conformation of the monomer at lower concentration. At higher concentration, the GS-type conformer is converted to the other one that forms molecular aggregates. The cis form about the X-Pro peptide bonds is found for [Gly^4,4′]-GS and [L-Ala^4,4′]-GS in DMSO-d₆ and for [L-Ala^4,4′]-GS in TFE solution. The large temperature dependences of α-proton chemical shifts of [L-Ala^4,4′]-GS in DMSO-d₆ solution indicate that the conformer equilibrium changes with temperature. The GS-type conformation is not formed in [L-Ala^4,4′]-GS. The two active peptide analogs, [D-Ala^4,4′]-GS and [Gly^4,4′]-GS, interact with the phospholipid membrane, taking the GS-type conformation. By contrast, an inactive analog, [L-Ala^4,4′]-GS, does not interact with phospholipid membrane. The activities of GS analogs are found to correlate to the formation of the GS-type conformation upon binding with phospholipid membrane.     

Structure of an 11-mer DNA Duplex Containing the Carbocyclic Nucleotide Analog: 2′-Deoxyaristeromycin

Abstract

2′-deoxyaristeromycin (dAr) is a nucleoside analogue that is resistant to the action of DNA glycosylases. High-resolution NMR spectroscopy and molecular dynamics simulations were used to determine the three-dimensional structure of an 11-mer DNA containing a single dAr?T base pair at its center. Analysis of the spectra revealed the existence of a right-handed duplex in solution, stabilized by Watson-Crick hydrogen bonding and base-stacking interactions. The carbocyclic sugar adopted a C1′-exo conformation and sugars of the 3′-flanking base pair had puckers in the O4′—endo range. The dAr?T base pair was mildly propeller twisted, and the dAr analogue showed a positive roll with the 3′-flanking base. Our findings indicate that the observed resistance of dAr-containing oligodeoxynucleotides to the catalytic action of DNA glycosylases relates to its electronic properties rather than structure, and validate the use of dAr and related carbocyclic nucleoside analogues for biological and structure/function relationship studies.     

GLYCOGEN AND GLYCOGEN PHOSPHORYLASE IN THE CEREBRAL CORTEX OF MICE UNDER THE INFLUENCE OF METHIONINE SULPHOXIMINE

Abstract— Glucose and glycogen levels in the mouse cerebral cortex in vivo were studied after recovery from methionine sulphoximine seizures. The animals appeared normal 24 h after methionine sulphoximine administration but both glucose and glycogen still persisted at higher levels 72 h after injection (by 64 and 275 per cent, respectively). When seizures were prevented by methionine, the increase in glucose and glycogen at the longer time intervals was significantly smaller than in animals treated with methionine sulphoximine only; glucose reached normal values at 48 or 72 h; the accumulation of glycogen was reduced by about three to five times, but after 72 h the levels were still significantly higher than in control animals (67 or 32 per cent increase, depending on the administered dose of methionine). In contrast to the considerable accumulation of glycogen after administration of methionine sulphoximine in vivo, it had no effect on the level of glycogen in brain cortex slices in vitro. After 3 h incubation in the absence of methionine sulphoximine, glycogen was resynthesized to a level of about 4 μmol/g wet tissue and this value was not significantly affected by the presence of various concentrations of methionine sulphoximine in the incubation medium (10^-5 to 10^-2 M). The total (a+b forms) phosphorylase activity of mouse cerebral cortex in vivo after methionine sulphoximine administration was not affected. The fraction of active phosphorylase was reduced by about 50 per cent at the time of seizures. When seizures were prevented by methionine, the decrease in active phosphorylase was also completely prevented. In the preconvulsive period (1-2 h) and after recovery from the seizures (48 h after methionine sulphoximine administration) active phosphorylase was normal. The possible mechanisms involved in the increased accumulation of glycogen after methionine sulphoximine administration are discussed.     

Modified chemotactic peptides: Synthesis,conformation, and activity of HCO-Thp-Ac6c-Phe-OMe

HCO-Thp-Ac₆c-Phe-OMe (3) has been synthesized as a new analogue of the prototypical chemotactic agent HCO-Met-Leu-Phe-OMe (fMLP-OMe). Compound 3 contains 4-aminotetrahydrothiopyran-4-carboxylic acid (Thp) and 1-aminocyclohexane-1-carboxylic acid (Ac₆c) as achiral, conformationally restricted mimics of Met and Leu, respectively. In the crystal, the formyltripeptide adopts an helical conformation at the Thp and Ac₆c residues, of the type α_R and α_L, respectively, whereas the C-terminal phenylalanine is quasi-extended. A system of two consecutive γ-turns, centered at the first two residues, better explains the nmr data as compared with an alternative β-turn structure. The conformation of the new analogue 3 is compared with those of two related peptides containing Thp as N-terminal residue. The biological activity of 3 has been determined on human neutrophils and compared to that of the previously studied model [Ac₆c²] fMLP-OMe. While the above analogue is highly active in the superoxide anion production, the new tripeptide 3 is practically unable to elicit any of the tested biological activities. © 1996 John Wiley & Sons, Inc.     

Development of a model for the δ-opioid receptor pharmacophore. 4. Residue 3 dehydrophenylalanine analogues of Tyr-c[D-Cys-Phe-D-Pen]OH (JOM-13) confirm required gauche orientation of aromatic side chain

We have previously proposed a model for the δ-opioid receptor binding conformation of the high affinity tetrapeptide Tyr-c[D -Cys-Phe-D -Pen] OH (JOM-13) based on experimental and theoretical conformational analysis of this peptide and a correlation of conformational preferences of further conformationally restricted analogues of this tetrapeptide with their receptor binding affinities. A key element of this model is the requirement that the Phe³ side chain exist in the x¹ = −60° conformation. Conformational calculations on the residue 3 dehydrophenylalanine analogues of JOM-13 suggest that while the dehydro(Z) phenylalanine analogue can be superimposed easily with the proposed binding conformer of JOM-13, the dehydro(E)phenylalanine analogue cannot. These results lead to the prediction that the dehydro(Z)-phenylalanine analogue should display similar δ-receptor binding affinity as JOM-13 while the dehydro(E)phenylalanine analogue is expected to bind less avidly. Synthesis and subsequent opioid receptor binding analysis of the dehydrophenylalanine analogues of JOM-13 confirm these predictions, lending support to the δ-pharmacophore model. © 1996 John Wiley & Sons, Inc.     

Purification and properties of an intracellular leucine aminopeptidase from the fungus,Penicillium citrinum strain IFO 6352

An intracellular leucine aminopeptidase (LAP) fromPenicillium citrinum (IFO 6352) was purified to homogeneity using three successive purification steps. The enzyme has a native molecular mass of 63 kDa using HPLC gel filtration analysis and a molecular mass of 65 kDa when using SDS-polyacrylamide gel electrophoresis. This monomeric aminopeptidase showed maximum enzyme activity at pH 8.5. An optimum temperature was 45–50°C whenl-Leu-p-nitroanilide (pNA) was the substrate, and enzyme activity drastically decreased above 60°C. The Michaelis-Menten constants forl-Leu-pNA andl-Met-pNA were 2.7 mM and 1.8 mM, respectively. When the enzyme reacted with biosynthetic methionyl human growth hormone, it showed high specificity for N-terminal methionine residue and recognized a stop sequence (Xaa-Pro). The aminopeptidase was inactivated by EDTA or 1,10-phenanthroline, indicating that it is a metallo-exoprotease. Enzyme activity was restored to 90% of maximal activity by addition of Co²⁺ ions. The activity of EDTA-treated enzyme was restored by addition of Zn²⁺, but reconstitution with Ca²⁺, Mg²⁺ or Mn²⁺ restored some enzyme activity. It is likely that Co²⁺ ions play an important role in the catalysis or stability of thePenicillium citrinum aminopeptidase, as zinc plays a similar function in other leucine aminopeptidases.     

Synthesis,biological activity,and conformational analysis of [pGlu6, N-MePhe8, Aib9] substance P (6–11): A selective agonist for the NK-3 receptor

A highly potent and selective agonist to the tachykinin NK-3 receptor, [pGlu⁶, N-MePhe⁸, Aib⁹] substance P (6–11) ( I ), was synthesized via the solid phase method. The ED₅₀ of I was 4n M in the guinea pig ileum in the absence of atropine (NK-1 + NK-3 receptors) and this agonist was 5000-fold less potent in the presence of atropine (NK-1 receptor). The analogue was virtually inactive in the rat vas deferens (NK-2 receptor). A detailed analysis of the solution conformation of this analogue in DMSO-d₆ and in a DMSO-d₆ H₂O cryornixture was carried out by a combination of ¹H-nmr 2D techniques (DQF-COSY, TOCSY, NOESY and ROESY) and model building based on empirical energy calculations. Peptide I exists as a mixture of isomers containing cis and trans Phe-N-MePhe peptide bonds. The main isomer, containing a cis Phe-N-MePhe peptide bond, shows a preferred folded conformation characterized by a type VI β-turn with Phe and N-MePhe in the i + 1 and i + 2 positions. The turn is followed by a helical segment extending to the C-terminal. This conformation is compared to previously reported conformations of other selective tachykinin agonists and may be a promising lead for the design of novel NK-3 agonists with additional conformational constraints. © 1993 John Wiley & Sons, Inc.     

Conformational preferences of dopamine analogues for inhibition of norepinephrine N-methyltransferase. Conformationally defined adrenergic agents. 7

A series of analogues of dopamine (DA) with varying degrees of conformational flexibility have been examined as potential substrates or competitive inhibitors of the enzyme norepinephrine N-methyltransferase (NMT). A conformationally defined (rigid) analogue of the fully extended conformation of DA, 2-amino-6, 7-dihydroxybenzonorbornene hydrobromide ($\text{3}$; 6, 7-D2HX) proved to be a better substrate than the non-catechol parent 2-aminobenzonorbornene ($\text{4}$; 2HX). However, analogues $\text{3}$ and $\text{4}$ displayed equivalent competitive inhibitory activity toward phenylethanolamine (PEA). Neither 6, 7-ADTN (5), a DA analogue in the 2-aminotetralin (2AT) system, nor 6, 7-DTHIQ ($\text{7}$), a DA analogue in the tetrahydroisoquinoline (THIQ) system, showed substrate activity; 6, 7-ADTN was a poorer competitive inhibitor than the parent 2AT but 6, 7-DTHIQ was a better competitive inhibitor than its parent, THIQ ($\text{8}$). A tricyclic conformationally defined analogue $\text{9}$ of 6, 7-ADTN was devoid of either substrate or inhibitory activity. From these results it may be concluded that a fully extended side chain conformation is required for NMT substrate activity, and the better substrate activity for 6, 7-D2HX compared to $\text{4}$ is consistent with a proper catechol orientation for interaction with the norepinephrine (NE) binding site of NMT.     

Development of potent anti-infective agents from Silurana tropicalis: Conformational analysis of the amphipathic,alpha-helical antimicrobial peptide XT-7 and its non-haemolytic analogue [G4K]XT-7

Peptide XT-7 (GLLGP⁵LLKIA¹⁰AKVGS¹⁵NLL.NH₂) is a cationic, leucine-rich peptide, first isolated from skin secretions of the frog, Silurana tropicalis (Pipidae). The peptide shows potent, broad-spectrum antimicrobial activity but its therapeutic potential is limited by haemolytic activity (LC₅₀ = 140 µM). The analogue [G4K]XT-7, however, retains potent antimicrobial activity but is non-haemolytic (LC₅₀ > 500 µM). In order to elucidate the molecular basis for this difference in properties, the three dimensional structures of XT-7 and the analogue have been investigated by proton NMR spectroscopy and molecular modelling. In aqueous solution, both peptides lack secondary structure. In a 2,2,2-trifluoroethanol (TFE-d₃)-H₂O mixed solvent system, XT-7 is characterised by a right handed α-helical conformation between residues Leu³ and Leu¹⁷ whereas [G4K]XT-7 adopts a more restricted α-helical conformation between residues Leu⁶ and Leu¹⁷. A similar conformation for XT-7 in 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) micellular media was observed with a helical segment between Leu³ and Leu¹⁷. However, differences in side chain orientations restricting the hydrophilic residues to a smaller patch resulted in an increased hydrophobic surface relative to the conformation in TFE-H₂O. Molecular modelling of the structures obtained in our study demonstrates the amphipathic character of the helical segments. It is proposed that the marked decrease in haemolytic activity produced by the substitution Gly⁴ → Lys in XT-7 arises from a decrease in both helicity and hydrophobicity. These studies may facilitate the development of potent but non-toxic anti-infective agents based upon the structure of XT-7.     

Synthesis,conformation, and biological activity of two fMLP-OMe analogues containing the new 2-[2′-(methylthio) ethyl] methionine residue

The new C^α-tetrasubstituted α-amino acid residue 2-[2′-(methylthio) ethyl]methionine (Dmt) has been introduced into the reference chemotactic tripeptide HCO-Met-Leu-Phe-OMe (fMLP-OMe) in place of the leucine or methionine, respectively. The biological activity of the new analogues [Dmt²] fMLP-OMe (2) and [Dmt¹] fMLP-OMe (3) has been determined; whereas 2 is active toward human neutrophils, stimulating directed migration, superoxide anion generation, and lysozyme release, 3 results practically inactive in all tested assays. A conformational analysis on 2 and 3 has been performed in solution by using ir absorption and ¹H-nmr. The conformation of 2 was also examined in the crystal by x-ray diffraction methods. Both 2 and 3 adopt fully extended conformation in correspondence with the Dmt residue. Biological and conformational results are discussed and compared with related previously studied models. © 1997 John Wiley & Sons, Inc. Biopoly 42: 415–426, 1997     

Kinetics of Binding of Multisubstrate Analogue Inhibitor (2-Amino-9-[2-(Phosphonomethoxy)Ethyl]-6-Sulfanylpurine) with Trimeric Purine Nucleoside Phosphorylase

Complex formation of multisubstrate analogue inhibitor—2-amino-9-[2-(phosphonomethoxy)ethyl]-6-sulfanylpurine (PME-6-thio-Gua) with trimeric purine nucleoside phosphorylase from Cellulomonas sp. was investigated using a stopped-flow spectrofluorimetric approach. Results obtained indicate that, in contrast to binding of guanine, i.e., the transition-state conformation trapping ligand, for which binding at each active site is followed by the enzyme conformational change, association of the ground-state analogue PME-6-thio-Gua is a one-step process.     

4-Aminotetrolic Acid : New Conformational-restricted Analogue of γ-Aminobutyric Acid

RECENT studies strongly support a role for γ-aminobutyric acid (GABA) as an inhibitory transmitter at certain synapses in the mammalian central nervous system. Structure activity correlations of many GABA analogues implicate both the intramolecular distance between the zwitterionic centres and the rotational freedom of the molecule as important factors governing the synaptic activity of these substances¹. The following observations provide pertinent information about the active conformation(s) of GABA recognized by the receptor. (1) Muscimol, an isoxazole isolated from Amanita muscaria, seems to function as a GABA analogue as its inhibitory action on central neurones is comparable with that of GABA both in potency² and with respect to antagonism by bicuculline³. Molecular orbital calculations suggest that GABA and muscimol can assume similar conformations as zwitterions with the charged centres (N⁺ and 0^?) at least 5 and, more likely, 6 Å apart⁴. (2) The selective GABA antagonist bicuculline exhibits some degree of structural similarity with particular conformations of GABA and muscimol³. (3) X-ray crystallography indicates that GABA exists in a partially folded conformation in the solid state^5,6. (4) A model of the GABA receptor proposes that GABA adopts a folded conformation with a distance of less than 4.4 Å between the charged centres⁷. Observations (1) and (2) suggest extended conformations for GABA, while (3) and (4) suggest folded conformations.     

Effects of conformational constraint in 2- and 8-cycloleucine analogues of oxytocin and [1-penicillamine] oxytocin examined by circular dichroism and biosassay

The analogues of oxytocin and [1-penicillamine]oxytocin, containing a cycloleucine (Cle) residue in position 2 or 8, were investigated by means of circular dichroism measurements in different solvents, and the results examined in terms of their biological activities. A cycloleucine residue in position 2 substantially reduces the free conformational space of the hormone 20-membered ring moiety (including the disulfide group), and stabilizes a conformation which is close to one of the possible conformations of oxytocin and involves a -turn. In position 8, the Cle residue affects the conformation of the Tyr² side chain, apparently forcing it away from the space above the 20-membered disulfide ring. However, it does not appear that the Cle residue has any significant effect on the overall backbone conformation of the hormone. The steric effect of the penicillamine residue in position 1 on the conformation of the disulfide group and Tyr² side chain from previous investigations is further confirmed. The synthesis and biological potency of [1-penicillamine, 8-cycloleucine]oxytocin is described. This analogue exhibits a strong inhibitory effect on the uterotonic activity of oxytocinin vitro. It also inhibited the vasopressor response to vasopressin.     

Conformation of parathyroid hormone antagonists by CD,nmr, and molecular dynamics simulations

The conformation of two highly potent parathyroid hormone (PTH) antagonists was investigated in water/2, 2, 2-trifluoroethanol mixtures. The two peptides are derived from the sequence (7-34) of PTH and of PTH-related protein (PTHrP) and have a D -Trp replacing Gly in position 12. In the analogue derived from PTHrP, Lys¹¹ was replaced by Leu to remove the residual agonist activity. The study was conducted by CD and two-dimensional proton magnetic resonance spectroscopy, and the nuclear Overhauser effects found were utilized in restrained distance geometry and molecular dynamics simulations. Both peptides adopt a helical C-terminal conformation, which seems more stable in the case of the PTHrP analogue. A type II′ β-turn centered around D -Trp¹² and Lys¹³ is present inboth structures. © 1995 John Wiley & Sons, Inc.     

Discrimination in resolving systems II: Ephedrine-substituted mandelic acids

Binary diastereomeric (-) (1R,2S)-ephedrine salts of various mandelic acids obtained from 95% ethanol show considerable differences in solubility. Structures and some properties of the less-soluble (L) and more-soluble (M) solid phases of (-)-ephedrine with unsubstituted mandelic acid, 2-, 3-, and 4-monosubstituted halo (F, Cl, Br) mandelic acids, and 3- and 4-methylmandelic acids have been determined. Salts were found to be binary, without solvent of crystallization, and composed of double-layered arrays of alternating anions and cations linked by H-bonds normal to the layers. H-bonding links charged donors and acceptors usually along a crystallographic 2-fold screw axis. A striking discrimination is evident in that the (2R)-mandelate salts typically display a compact four-atom chain as the H-bonding repeating unit [⁺N—H…O(—C^?—O)…H-N′, C₂¹(4)] while the (2S)-mandelate salts adopt a more dimensionally variable six-atom chain repeating unit [⁺N—H…O—C^?—O…H—N′, C₂²(6)]. Two distinct packing schemes display the shorter H-bonding chain of the (2R)-mandelates which always occurs with ephedrinium ions in the fully extended conformation. Slightly greater packing efficiency and H-bonding energies of the (2R)-mandelate salts correlates with increased fusion points, lower solubilities (95% ethanol), and higher heats of fusion relative to the phase adopted by their diastereoisomers. In contrast, (2S)-mandelate salts exhibit considerably more structural variability involving all three major ephedrinium conformations, and at least four distinct packing motifs. Mandelates with larger 3′-substituents (Cl, Br, methyl) show similar property discriminations, but these occur with an opposing trend, that is, between phases in which the less-soluble salts contain (2S)-mandelates. Salts with 2-bromomandelate do not show property disparities and their structures are dissimilar to the other phases. © 1995 Wiley-Liss, Inc.