Synthesis and chiroptical properties of a helical poly(phenylacetylene) bearing optically active pyrene pendants

A novel poly(phenylacetylene) derivative bearing optically active pyrene moieties as the pendant groups (poly-(R)-1) was prepared by the polymerization of the corresponding monomer (R)-1 in the presence of a rhodium catalyst, and its chiroptical property was investigated. Poly-(R)-1 exhibited an induced circular dichroism (ICD) in the polymer backbone region due to the predominantly one-handed helical conformation. The ICD pattern dramatically changed and was accompanied by inversion of the Cotton effect sign in response to a change in the temperature and solvent, indicating that poly-(R)-1 underwent a helix-helix transition in response to the external stimuli.     

Chiral stationary phase covalently bound with a chiral pseudo-18-crown-6 ether for enantiomer separation of amino compounds using a normal mobile phase

In order to apply the excellent chiral recognition ability of chiral pseudo-18-crown-6 ethers that we developed to chiral separation, we prepared a chiral stationary phase (CSP) by immobilizing a chiral pseudo-18-crown-6-type host on 3-aminopropyl silica gel. A chiral column was prepared by the slurry-packing method in a stainless steel HPLC column. A liquid chromatography system using this CSP combined with the detection by mass spectrometry was used for enantiomer separation of amino compounds. A normal mobile phase can be used on this CSP as opposed to conventional dynamic coating-type CSPs. Enantiomers of 18 common natural amino acids were efficiently separated. The chiral separation observed for amino acid methyl esters, amino alcohols, and lipophilic amines was fair using this HPLC system. In view of the correlation between the enantiomer selectivity observed in chromatography and the complexion in solution, the chiral recognition in host-guest interactions might contribute to this enantiomer separation.     

Synthesis of non-proteinogenic (D)- or (L)-amino acids by asymmetric hydrogenation

Summary Non-proteinogenic amino acids play an increasing role in oligopeptide chemistry. Their pharmacological and chemical properties, caused by D-configuration and unnatural residues, are more and more used for drug design. Different methods of asymmetric synthesis have been developed during the last decade to prepare unusual amino acids. One of them, the asymmetric hydrogenation of dehydroamino aids catalyzed by chiral rhodium (I) complexes, will be described. A series of examples, D- and L-configured, like naphthyl-, thienyl-, furyl-, and pyridylalanines, as well as phenylalanines substituted by chlorine, fluorine, p-nitro, p-methyl, p-trifluoromethyl, p-isopropyl, and p-tert-butyl have been prepared and characterized. Some analytical data like melting points and values of optical rotation are summarized in tables.Abbreviations (–)-DIOP (4R,5R)-4,5-Bis(diphenylphosphinomethyl)-2,2-dimethyl-1,3-dioxolane - (–)-BPPM (2S,4S)-N-tert-Butoxycarbonyl-4-diphenylphosphino-2-diphenylphosphinomethyl-pyrrolidine - Ph--glup Phenyl 4,6-O-(R)-benzylidene-2,3-O-bis(diphenylphosphino)--D-glucopyranoside - DuPHOS 1,2-bis-(phospholano)benzene - PROPRAPHOS 2,3-O,N-bis(diphenylphosphino)-1-(naphthoxy)-2-hydroxy-3-isopropylamino propane - PINDOPHOS 2,3-O,N-bis(diphenylphosphino)-1-(4-indolyloxy)-2-hydroxy-3-isopropylamino propane     

Supramolecular chirality induction in bis(zinc porphyrin) by amino acid derivatives: rationalization and applications of the ligand bulkiness effect

The achiral syn conformer (face-to-face) of the ethane-bridged bis(zinc porphyrin) (syn-ZnD) transforms into the corresponding chiral extended anti bis-ligated species (anti-ZnD.L2) in the presence of enantiopure ligands (L: amino acid derivatives). The mechanism of the supramolecular chirality induction is based on chiral ligand binding to zinc porphyrins and subsequent formation of either right- or left-handed screw structures in anti-ZnD.L2. The screw structure formation arises from steric interactions between the bulkiest substituent at the asymmetric carbon of the ligand and the peripheral ethyl groups of the neighboring porphyrin ring directed towards the covalent bridge. The sign and amplitude of the induced circular dichroism (CD) are dependent on the steric bulk of the substituents at the chiral center. The greater difference in size between the chiral center's substituents gives the stronger induced CD signal. Rationalization of the ligand bulkiness effect on chirality induction by amino acid derivatives, application of this supramolecular system for the determination of ligand absolute configuration, and relative bulkiness of the substituents at the asymmetric carbon are discussed.     

Chiral discrimination phenomena in mixed-ligand copper(II) complexes with amino acids and 1,3-dicarbonyl compounds

Circular dichroism (CD) spectra of individual mixed-ligand copper(II) complexes of 1,3-dicarbonyl compounds, (1S)- or (1R)-3-hydroxymethylene camphor, (1S)-3-trifluoroacetyl camphor, or (1R)-2-hydroxymethylene menthone, and α-amino acids, alanine, valine, proline, or their N-alkyl derivatives, were calculated from CD spectra of equilibrium solutions containing the above constituents in methanol or ethylene dichloride. Diastereomeric mixed-ligand complexes incorporating identical dicarbonyl but enantiomeric N-alkyl-α-amino acid ligands exhibit quasi-enantiomeric CD spectra. Unsubstituted amino acids, on the contrary, will make no decisive contributions to the net optical activity spectrum of the mixed-ligand complexes. Formation constants of diastereomeric mixed-ligand complexes have been calculated from data on disproportionation of the latter into corresponding equally paired complexes. Enantioselectivity was demonstrated to amount to up to 700 cal/mol. Possible steric structures of mixed-ligand complexes are discussed. © 1993 Wiley-Liss, Inc.     

Preparation and chiro-optical characterization of polyaniline doped with (+) or (-)-2-pyrrolidone-5-carboxylic acid (PCA)

Optically active polyaniline (PANI) salts were readily generated in solution via the enantioselective acid doping of neutral emeraldine base (EB) form of PANI with either (+) or (-)-2-pyrrolidone-5-carboxylic acid (PCA) in dimethylsulfoxide (DMSO) and dimethylformamide (DMF) solvents. Strong mirror imaged circular dichroism (CD) spectra were obtained for the deep green polymer solutions obtained with (+) or (-) PCA, suggesting that the acid doping is enantioselective, with one helical screw of the polymer chain being preferentially produced depending on the nature of enantiomer. It was observed that molar concentration of PCA as well as nature of solvent plays a very important role in the generation of optically active PANI. The generated optically active PANI did not show any loss of optical activity up to 200 h.     

Chiral recognition by the copper(II) complex of 6-deoxy-6-N-(2-methylaminopyridine)-β-cyclodextrin

A modified β-cyclodextrin bearing a 2-aminomethylpyridine binding site for copper(II) (6-deoxy-6-[N-(2-methylamino)pyridine)]-β-cyclodextrin, CDampy was synthesized by C₆-monofunctionalization. The acid-base properties of the new ligand in aqueous solution were investigated by potentiometry and calorimetry, and its conformations as a function of pH were studied by NMR and circular dichroism (c.d.). The formation of binary copper(II) complexes was studied by potentiometry, EPR, and c.d. The copper(II) complex was used as chiral selector for the HPLC enantiomeric separation of underivatized aromatic amino acids. Enantioselectivity in the overall stability constants of the ternary complexes with D- or L-Trp was detected by potentiometry, whereas the complexes of the Ala enantiomers did not show any difference in stability. These results were consistent with a preferred cis coordination of the amino group of the ligand and of the amino acid in the ternary complexes (“cis effect”), which leads to the inclusion of the aromatic side chain of D-Trp, but not of that of L-Trp. In Trp-containing ternary complexes, the two enantiomers showed differences in the fluorescence lifetime distribution, consistent with only one conformer of D-Trp and two conformers of L-Trp, and the latter were found to be more accessible to fluorescence quenching by acrylamide and KI. Chirality 9:341–349, 1997. © 1997 Wiley-Liss, Inc.     

Tat cell-penetrating peptide has the characteristics of a poly(proline) II helix in aqueous solution and in SDS micelles.

Tat cell-penetrating peptide (GRKKRRQRRRPPQG) is able to translocate and carry molecules across cell membranes. Using CD spectroscopy the conformation of this synthetic peptide was studied in aqueous and membrane-mimicking, micellar SDS solutions at different temperatures. The CD spectrum of the Tat cell-penetrating peptide in SDS micellar solution was virtually unchanged from that in aqueous solution, and at low temperature it was close to that of a poly(proline) II helix.     

The N-terminal intrinsically disordered region of Ncb5or docks with the cytochrome b5 core to form a helical motif that is of ancient origin

NADH cytochrome b₅ oxidoreductase (Ncb5or) is a cytosolic ferric reductase implicated in diabetes and neurological conditions. Ncb5or comprises cytochrome b₅ (b₅) and cytochrome b₅ reductase (b₅R) domains separated by a CHORD-Sgt1 (CS) linker domain. Ncb5or redox activity depends on proper inter-domain interactions to mediate electron transfer from NADH or NADPH via FAD to heme. While full-length human Ncb5or has proven resistant to crystallization, we have succeeded in obtaining high-resolution atomic structures of the b₅ domain and a construct containing the CS and b₅R domains (CS/b₅R). Ncb5or also contains an N-terminal intrinsically disordered region of 50 residues that has no homologs in other protein families in animals but features a distinctive, conserved L³⁴MDWIRL⁴⁰ motif also present in reduced lateral root formation (RLF) protein in rice and increased recombination center 21 in baker's yeast, all attaching to a b₅ domain. After unsuccessful attempts at crystallizing a human Ncb5or construct comprising the N-terminal region naturally fused to the b₅ domain, we were able to obtain a high-resolution atomic structure of a recombinant rice RLF construct corresponding to residues 25–129 of human Ncb5or (52% sequence identity; 74% similarity). The structure reveals Trp¹²⁰ (corresponding to invariant Trp³⁷ in Ncb5or) to be part of an 11-residue α-helix (S¹¹⁶QMDWLKLTRT¹²⁶) packing against two of the four helices in the b₅ domain that surround heme (α2 and α5). The Trp¹²⁰ side chain forms a network of interactions with the side chains of four highly conserved residues corresponding to Tyr⁸⁵ and Tyr⁸⁸ (α2), Cys¹²⁴ (α5), and Leu⁴⁷ in Ncb5or. Circular dichroism measurements of human Ncb5or fragments further support a key role of Trp³⁷ in nucleating the formation of the N-terminal helix, whose location in the N/b₅ module suggests a role in regulating the function of this multi-domain redox enzyme. This study revealed for the first time an ancient origin of a helical motif in the N/b₅ module as reflected by its existence in a class of cytochrome b₅ proteins from three kingdoms among eukaryotes.     

Equilibrium unfolding of the poly(glutamic acid)20 helix

The equilibrium structural ensemble of a 20-residue polyglutamic acid peptide (E(20)) was studied with FRET, circular dichroism, and molecular dynamics (MD) simulations. A FRET donor, o-aminobenzamide, and acceptor, 3-nitrotyrosine, were introduced at the N- and C-termini, respectively. Circular dichroism, steady state FRET, and time-resolved FRET measurements were employed to characterize the fraction helix and end-to-end distance under different pH conditions: pH 4 (60% alpha-helix), pH 6 (0% alpha-helix), and pH 9 (0% alpha-helix). At pH 4, the end-to-end distance was measured at 24 A and determined to be considerably less than the 31 A predicted for an alpha-helix of the same length. At pH 6 and 9, the end-to-end distance was measured at > 31 and 39 A respectively, both which are determined to be considerably greater than the 27 A predicted for a freely jointed random coil of the same length. To better understand the physical forces underlying the unusual helix-coil transition in this peptide, three theoretical MD models of E(20) were constructed: (1) a pure alpha-helix, (2) an alpha-helix with equivalent attractive intramolecular contacts, and (3) a weak alpha-helix with termini-weighted intramolecular contacts ("sticky ends"). Using MD simulations, the bent helix structure calculated from Model 3 was found to be the closest in agreement with the experimental data.     

X-ray studies on crystalline complexes involving amino acids and peptides: Part XVIII. Crystal structure of a new form of L-arginine D-glutamate and a comparative study of amino acid crystal structures containing molecules of the same and mixed chirality

The new form of L-arginine D-glutamate is monoclinic, P2₁, witha = 9.941(1),b = 4.668(2),c = 17.307(1) Å,β = 95.27(1)°, and Z = 2. In terms of composition, the new form differs from the old form in that the former is a monohydrate whereas the latter is a trihydrate. The structure has been solved by the direct methods and refined to R = 0.085 for 1012 observed reflections. The conformation of the arginine molecule is the same in both the forms whereas that of the glutamate ion is different. The change in the conformation of the glutamate ion is such that it facilitates extensive pseudosymmetry in the crystals. The molecules arrange themselves in double-layers stabilised by head-to-tail sequences involving main chains, in both the forms. However, considerable differences exist between the two forms in the interface, consisting of side chains and water molecules, between double-layers. A comparative study of the relationship between the crystal structures of L and DL amino acids on the one hand and that between the structures of LL and LD amino acid-amino acid complexes on the other, provides interesting insights into amino acid aggregation and the effect of chirality on it. The crystal structures of most hydrophobic amino acids are made up of double-layers and those of most hydrophilic amino acids contain single layers, irrespective of the chiralities of the amino acids involved. In most cases, the molecules tend to appropriately rearrange themselves to preserve the broad features of aggregation patterns when the chirality of half the molecules is reversed as in the structures of DL amino acids. The basic elements of aggregation in the LL and the LD complexes, are similar to those found in the crystals of L and DL amino acids. However, the differences between the LL and the LD complexes in the distribution of these elements are more pronounced than those between the distributions in the structures of L and DL amino acids.     

Factors governing 3(10)-helix vs alpha-helix formation in peptides: percentage of C(alpha)-tetrasubstituted alpha-amino acid residues and sequence dependence

As an additional step toward the dissection of the factors responsible for the onset of 3(10)-helix vs alpha-helix in peptides, in this paper we describe the results of a three-dimensional (3D) structural analysis by x-ray diffraction of the N(alpha)-acylated heptapeptide alkylamide mBrBz-L-Iva-L-(alphaMe)Val-L-Abu-L-(alphaMe)Val-L-(alphaMe)Phe-L-(alphaMe)Val-L-Iva-NHMe characterized by a single (L-Abu3) C(alpha)-trisubstituted and six C(alpha)-tetrasubstituted alpha-amino acids. We find that in the crystal state this peptide is folded in a mixed helical structure with short elements of 3(10)-helix at either terminus and a central region of alpha-helix. This finding, taken together with the published NMR and x-ray diffraction data on the all C(alpha)-methylated parent sequence and its L-Val2 analog (also the latter heptapeptide has a single C(alpha)-trisubstituted alpha-amino acid) strongly supports the view that one C(alpha)-trisubstituted alpha-amino acid inserted near the N-terminus of an N(alpha)-acylated heptapeptide alkylamide sequence may be enough to switch a regular 3(10)-helix into an essentially alpha-helical conformation. As a corollary of this work, the x-ray diffraction structure of the N(alpha)-protected, C-terminal tetrapeptide alkylamide Z-L-(alphaMe)Val-L-(alphaMe)Phe-L-(alphaMe)Val-L-Iva-NHMe, also reported here, is clearly indicative of the preference of this fully C(alpha)-methylated, short peptide for the 3(10)-helix. As the same terminally blocked sequence is mixed 3(10)/alpha-helical in the L-Abu3 heptapeptide amide but regular 3(10)-helical in the tetrapeptide amide and in the parent heptapeptide amide, these results point to an evident plasticity even of a fully C(alpha)-methylated short peptide.     

Preparation and application of a new doubly tethered chiral stationary phase containing N-CH3 amide linkage based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid

A new doubly tethered chiral stationary phase (CSP 5) based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was developed by attaching the second tethering group to silica gel through a carbon atom of the first tethering group of the corresponding singly tethered CSP (CSP 2) containing an N-CH3 tertiary amide linkage, which was previously developed in our laboratory, in order to enhance the CSP stability without the loss of chiral recognition efficiency. The new CSP was quite effective in the resolution of various racemic alpha-amino acids, amines, and amino alcohols, and the chiral recognition efficiency of the new CSP was even greater than that of the corresponding singly tethered CSP especially in terms of the resolution factors (RS). The stability of the new CSP was greater than that of the corresponding singly tethered CSP. The chromatographic resolution behaviors of the new CSP were generally consistent with those of the corresponding singly tethered CSP.     

Nonaqueous synthesis of a selectively modified, highly anionic sulfopropyl ether derivative of cyclomaltoheptaose (beta-cyclodextrin) in the presence of 18-crown-6

A highly anionic cyclomaltooligosaccharide (cyclodextrin, CD) derivative containing sulfopropyl functional groups on the primary face of the CD was synthesized. Heptakis(2,3-di-O-methyl)cyclomaltoheptaose [heptakis(2,3-di-O-methyl)-beta-cyclodextrin] was reacted with 1,3-propane sultone and potassium hydride (KH) in anhydrous tetrahydrofuran in the presence of 18-crown-6 to yield highly substituted potassium heptakis(2,3-di-O-methyl-6-O-sulfopropyl)cyclomaltoheptaose [heptakis(KSPDM)-beta-CD] with an average degree of substitution (DSCE) of 6.9 as determined by inverse detection capillary electrophoresis (CE). The principal species in the product is the fully substituted heptakis(KSPDM)-beta-CD. Complete NMR assignments of the hydrogen and carbon atoms are made using a combination of gCOSY and gHSQC. In the absence of 18-crown-6, the reaction generates a mixture of multiply charged derivatives with average DSCE of 4.1. The possible roles of the crown ether in the reaction are discussed. The ROESY NMR spectrum of the inclusion complex that forms between heptakis(KSPDM)-beta-CD and 2-naphthoic acid in D2O reveals that 2-naphthoic acid inserts with the carboxyl group toward the derivatized primary rim of the cyclodextrin.     

Macromolecularization of a tripeptide analogue of the Cu(II) binding site of human serum albumin: 2. Conformational and binding properties towards Cu(II) and Ni(II)ions of Gly-Gly-His derivatives of poly(l-lysine)

The conformational and binding properties towards Cu(II) and Ni(II) ions of Gly-Gly-His derivatives of poly(l-lysine) have been investigated mainly using circular dichroism (c.d.) spectroscopy. These derivatized polymers can be considered macromolecular analogues of the Cu(II) and Ni(II) binding site of human serum albumin. It has been shown that modification up to 53% of the ε-amino groups of lysine side chains by covalent binding of the tripeptide unit Gly-Gly-His does not induce appreciable alteration of the α-helix forming tendency of the polylysine backbone. The derivatized polymers exhibit strong affinity towards Cu(II) and Ni(II) ions. At neutral pH, complexes are formed in which each tripeptide chelating unit is linked to one metal ion. The spectral characteristics in the visible absorption region are consistent with a square planar geometry of the complexes, with deprotonated peptide groups and one imidazole nitrogen in the coordination sphere of the ion. C.d. measurements in the far u.v. indicate that complex formation in the side chains causes an increase of ordered structure of the peptide backbone at neutral pH. This fact is interpreted in terms of a reduced electrostatic repulsion among side chains due to charge neutralization in the tripeptide units linked to metal ions.     

β‐amino acid substitution to investigate the recognition of angiotensin II (AngII) by angiotensin converting enzyme 2 (ACE2)

In spite of the important role of angiotensin converting enzyme 2 (ACE2) in the cardiovascular system, little is known about the substrate structural requirements of the AngII–ACE2 interaction. Here we investigate how changes in angiotensin II (AngII) structure affect binding and cleavage by ACE2. A series of C3 β‐amino acid AngII analogs were generated and their secondary structure, ACE2 inhibition, and proteolytic stability assessed by circular dichroism (CD), quenched fluorescence substrate (QFS) assay, and LC‐MS analysis, respectively. The β‐amino acid‐substituted AngII analogs showed differences in secondary structure, ACE2 binding and proteolytic stability. In particular, three different subsets of structure‐activity profiles were observed corresponding to substitutions in the N‐terminus, the central region and the C‐terminal region of AngII. The results show that β‐substitution can dramatically alter the structure of AngII and changes in structure correlated with ACE2 inhibition and/or substrate cleavage. β‐amino acid substitution in the N‐terminal region of AngII caused little change in structure or substrate cleavage, while substitution in the central region of AngII lead to increased β‐turn structure and enhanced substrate cleavage. β‐amino acid substitution in the C‐terminal region significantly diminished both secondary structure and proteolytic processing by ACE2. The β‐AngII analogs with enhanced or decreased proteolytic stability have potential application for therapeutic intervention in cardiovascular disease. Copyright © 2010 John Wiley & Sons, Ltd.     

Stereoselective acylation of a racemic amine with C(alpha)-methyl phenylglycine-based dipeptide 5(4H)-oxazolones

Reactions of a racemic amine with chiral, N(alpha)-acetylated, C(alpha)-methyl l-phenylglycine-based dipeptide 5(4H)-oxazolones proceed diastereoselectively to give predominantly dipeptide alkylamides comprising d-alpha-phenylethylamine. Diastereoselectivity is remarkably sensitive to solvent polarity and reaction temperature but not significantly to the nature of the C(alpha)-tetrasubstituted alpha-amino acid at position 1 of the dipeptide. The beta-turn 3D structures of the aminolysis products were established in CDCl(3) solution by FT-IR absorption and in one case in the crystal state by X-ray diffraction as well.     

Disruption of the beta-sheet structure of a protected pentapeptide, related to the beta-amyloid sequence 17-21, induced by a single, helicogenic C(alpha)-tetrasubstituted alpha-amino acid.

Fifteen years ago it was shown that an alpha-aminoisobutyric acid (Aib) residue is significantly more effective than an L-Pro or a D-amino acid residue in inducing beta-sheet disruption in short model peptides. As this secondary structure element is known to play a crucial role in the neuropathology of Alzheimer's disease, it was decided to check the effect of Aib (and other selected, helix inducer, C(alpha)-tetrasubstituted alpha-amino acids) on the beta-sheet conformation adopted by a protected pentapeptide related to the sequence 17-21 of the beta-amyloid peptide. By use of FT-IR absorption and 1H NMR techniques it was found that the strong self-association characterizing the pentapeptide molecules in weakly polar organic solvents is completely abolished by replacing a single residue with Aib or one of its congeners.     

Difficulties in location and acceptance of phloem sap combined with reduced concentration of phloem amino acids explain lowered performance of the aphid Rhopalosiphum padi on nitrogen deficient barley (Hordeum vulgare) seedlings

Effects of nitrogen deficiency in hydroponically grown barley seedlings (Hordeum vulgare L.) on the development and reproduction of the aphid Rhopalosiphum padi (L.) (Hemiptera: Aphididae) were investigated.Plant growth was significantly reduced in seedlings grown without nitrogen. Aphid intrinsic rate of increase (r _m) was also significantly lower on these plants compared with that on plants grown with 8 mol m^–3 nitrogen. Phloem sap was collected from seedling stems by aphid stylectomy and amino acids quantified by HPLC. There was a significant reduction in the concentration of non-essential amino acids as a group, but not of essential amino acids. Electrical penetration graphs (EPG) indicated that aphids reached the phloem more quickly and fed for longer on plants grown with nitrogen. This is the first reported study in which this combination of techniques has been used to understand the interactions of an aphid and plant under different environmental conditions.     

Spectroscopic and structural investigations on [(DD18C6)H2]I8 formed in the reaction of N,N-dibenzylated diazacrown ether (DD18C6) with iodine

Photometric titration measurements indicated in the reaction of diazacrown ether N,N^′-dibenzyl-1,4,10,13-tetraoxo-7,16-diazacyclooctadecane (DD18C6, 2) with iodine in chloroform that a complex was formed in the molar ratio DD18C6:I₂=1:4. This complex was also prepared on a preparative scale as dark brown compound and characterized by microanalysis, UV-Vis, IR, and Raman spectroscopy. By X-ray diffraction analysis the solid-state structure of the complex was shown to be [(DD18C6)H₂]I₈ ([3]I₈) consisting of a doubly protonated macrocycle and an octaiodide dianion (I₈ ²⁻) in the typical (nearly planar) Z-shaped geometry. The macrocycle is C_i symmetric and the protonated nitrogen atoms adopt an endo-endo orientation that is stabilized by the three-center hydrogen bonds N-H(?O)₂, where the oxygen atoms of the macrocycle act as hydrogen acceptors. The orientation of the phenyl groups of the benzyl sidearms are turned above and below the macrocycle. Quantum chemical calculations on the DFT level of theory of the non-protonated and the doubly protonated macrocycle (DD18C6, 2c) and [(DD18C6)H₂]²⁺ (3c), respectively, were performed and discussed for 3c in terms of conformational strain of the macrocycle, the strength of the intramolecular N-H?O hydrogen bonds and cation-π interactions.