Absolute configuration determination of chiral molecules in the solution state using vibrational circular dichroism

Advances in the measurement, calculation, and application of vibrational circular dichroism (VCD) for the determination of absolute configuration are described. The purpose of the review is to provide an up-to-date perspective on the capability of VCD to solve problems of absolute stereochemistry for chiral molecules primarily in the solution state. The scope of the article covers the experimental methods needed for the accurate measurement of VCD spectra and the theoretical steps required to systematically deduce absolute configuration. Determination of absolute configuration of a molecule by VCD requires knowledge of its conformation or conformational distribution, and hence VCD analysis necessarily provides solution-state conformation information, in many cases available by no other method, as an additional benefit. Comparisons of the advantages and limitations of VCD relative to other available chiroptical methods of analysis are also presented.     

Magnesium and biological activity of oxytocin analogues modified on aromatic ring of amino acid in position 2.

For the purpose of evaluating substitution effects in the ortho, meta or para positions of the aromatic ring of tyrosine or phenylalanine in position 2 of oxytocin on uterotonic activity in vitro in the presence and absence of magnesium ions, six new analogues of oxytocin ([D- and L-m-methylphenylalanine2]oxytocin, [D- and L-m-methoxyphenylalanine2]oxytocin and [D- and L-o-methyltyrosine2]-oxytocin) were synthesized and several previously described analogues resynthesized. For the phenylalanine series, it is found that, in the absence of magnesium ions, substitution of the ortho and meta positions leads to loss of intrinsic activity (the analogues are antagonists) in contrast to the para position. In the tyrosine series, only methyl substitution in the meta position has this effect (substitution of ortho position only attenuates the agonistic biological activity). Addition of Mg ions restores to a certain degree the agonistic activity in the case of the o-methylphenylalanine analogue and enhances the agonistic activity of o-methyltyrosine oxytocin. All other analogues keep the original qualities as in the absence of Mg. Molecular modelling calculations of the structure of the above analogues was carried out to help explain these findings of the molecular level.     

A conformational analysis of mono and dialkyl ethers of 2,2′-dihydroxy-1,1′-binaphthalene by circular dichroism spectroscopy and cholesteric induction in nematic liquid crystals

The coupling of the analysis of the absorption and circular dichroism (CD) spectra with that of the cholesteric mesophases induced in nematic liquid crystals indicated some interesting conformational features of bridged and nonbridged mono- and dialkylethers of optically active 2,2′-dihydroxy-1,1′-binaphthalene. Bridged derivatives are characterized by relatively small dihedral angles. Simple monoalkyl ethers are characterized by larger dihedral angles but they all assume an s-cis conformation, owing to the existence of intramolecular hydrogen bonds. Nonbridged dialkylethers prefer even larger dihedral angles and, depending on the bulkiness of the alkyl groups, the s-trans conformation can be found. Interestingly, the conformation of dialkylethers is strongly dependent on the structure of the liquid crystal solvent, because the intramolecular hydrogen bond is not possible there. © 1995 Wiley-Liss, Inc.     

Solution conformational study of nociceptin an(d its 1-13 and 1-11 fragments using circular dichroism and two-dimensional NMR in conjunction with theoretical conformational analysis.

Conformational studies of nociceptin (NC-NH2), its fully active fragment, NC(1-13)-NH2, and two significantly less potent fragments, NC(1-13)-OH and NC(1-11)-OH, were conducted in water and TFE solutions by the employment of circular dichroism, and in DMSO-d6 by 2DNMR spectroscopy in conjunction with theoretical conformational analysis. The conformations of all thepeptides studied were calculated taking two approaches. The first assumes multiconformational equilibrium of the peptide studied, which is characterized by a set of conformations (and their statistical weight values)obtained from a global conformational analysis using three methods: the electrostatically driven Monte-Carlo (EDMC) with the ECEPP/3 force field, the simulated annealing (SA) protocols in the AMBER and CHARMM force fields. The second approach incorporates the interproton distance and dihedral angle constraints into the starting conformation. Calculations were performed using the distance geometry and SA protocol in the CHARMM force field implemented in the X-PLOR program. The CD experiments indicated that for the active peptides, hydrophobic solvents induced a significantly higher (compared with those remaining)content order, probably a helical structure. Unfortunately, as a result of the conformational flexibility of thepeptides, the analysis of conformations obtained with both approaches and different force fields did not alllow the selection of any structural elements of the NC peptides that might be connected with their bioactivity. The only common element found in most conformations of the active peptides was a helical character of fragment 8-13, which allowed the side chains of basic amino acid residues to be exposed to the outside of the molecule and probably to interact with the ORL1 receptor.     

Determination of the absolute configurations of synthetic daunorubicin analogues using vibrational circular dichroism spectroscopy and density functional theory

The absolute configurations of three synthesized anthracycline analogues have been determined using vibrational circular dichroism (VCD) spectroscopy and the density functional theory (DFT) calculations. The experimental VCD spectra of the three compounds have been measured for the first time in the film state, prepared from their CDCl₃ solutions. Conformational searches for the monomers and some dimers of the three compounds have been performed at the DFT level using the B3LYP functional and the 6‐311G** and 6‐311++G** basis sets. The corresponding vibrational absorption and VCD spectra have been calculated. The good agreement between the experimental and the calculated spectra allows one to assign the absolute configurations of the three compounds with high confidence. In addition, the dominant conformers of the three compounds have also been identified. Chirality, 2010. © 2010 Wiley‐Liss, Inc.     

Morphine, beta-endorphin and [D-Ala2] Met-enkephalin inhibit oxytocin release by acetylcholine and suckling

Morphine inhibits suckling-induced oxytocin (OT) release in lactating mice. Since beta-endorphin and enkephalins have several actions in common with morphine, the action of these opioid peptides on OT release was investigated. In anesthetized lactating rats, OT release was achieved by intraventricular injection of acetylcholine (ACh) or by the physiological stimulus of suckling. The amount of OT released was estimated by comparing milk-ejection responses to these stimuli to those following known amounts of intravenous (IV) OT. Both beta-endorphin and [D-Ala2]Met-enkephalin inhibited ACh-induced and suckling-induced OT release. Naloxone antagonized opiate inhibition in both cases.     

Vibrational circular dichroism and IR spectral analysis as a test of theoretical conformational modeling for a cyclic hexapeptide

A model cyclohexapeptide, cyclo-(Phe-(D)Pro-Gly-Arg-Gly-Asp) was synthesized and its IR and VCD spectra were used as a test of density functional theory (DFT) level predictions of spectral intensities for a peptide with a nonrepeating but partially constricted conformation. Peptide structure and flexibility was estimated by molecular dynamics (MD) simulations and the spectra were simulated using full quantum mechanical (QM) approaches for the complete peptide and for simplified models with truncated side chains. After simulated annealing, the backbone conformation of the ring structure is relatively stable, consisting of a normal beta-turn and a tight loop (no H-bond) which does not vary over short trajectories. Only in quite long MD runs at high temperatures do other conformations appear. MD simulations were carried out for the cyclic peptide in water and in TFE, which match experimental solvents, as well as with and without protonation of the Asp carboxyl group. DFT spectral simulations were made using the annealed structure and were extended to include basis set variation, to determine an optimal computational approach, and solvent simulation with a polarized continuum model (PCM). Stepwise full DFT simulation of spectra was done for various sequences with the same backbone geometry but based on (1) solely Gly residues, (2) Ala substitution except Gly and Pro, and (3) complete sequences with side chains. Additionally, a selection of structures was used to compute IR and VCD spectra with the optimal method to determine structural variation effects. The side chains, especially the Asp-COOH and Arg-NH(2) transitions, had an impact on the computed amide frequencies, IR intensities and VCD pattern. Since experimentally these groups would have little chirality, due to conformational variation, they do not impact the observed VCD spectra. Correcting for frequency shifts, the Ala model for the cyclopeptide gives the clearest representation of the amide VCD. The experimental sign pattern for the amide I' band in D(2)O and also the sharper, more intense amide I VCD band in TFE was seen to some degree in one conformer with Type II' turns, but the data favor a mix of structures.     

Examination of structural characteristics of the potent oxytocin antagonists [dPen1,Pen6]-OT and [dPen1,Pen6, 5-tBuPro7]-OT by NMR, Raman, CD spectroscopy and molecular modeling.

The synthesis and biological evaluation of penicillamine(6)-5-tert-butylproline(7)-oxytocin analogs and comparison with their proline(7)-oxytocin counterparts has led to the discovery of two potent oxytocin (OT) antagonists: [dPen(1),Pen(6)]-oxytocin (1, pA(2) = 8.22, EC(50) = 6.0 nM) and [dPen(1),Pen(6),5-tBuPro(7)]-oxytocin (2, pA(2) = 8.19, EC(50) = 6.5 nM). In an attempt to understand the conformational requirements for their biological activity, spectroscopic analyses of 1 and 2 were performed using (1)H NMR, laser Raman and CD techniques. In H(2)O, oxytocin analogs 1 and 2 exhibited cis-isomer populations of 7% and 35%, respectively. Measurement of the amide proton temperature coefficients revealed solvent shielded hydrogens for Gln(4) and Pen(6) in the major trans-conformer of 1 as well as for Gln(4) in the minor cis-conformer of 2. Few long-distance NOEs were observed, suggesting conformational averaging for analogs 1 and 2 in water; moreover, a lower barrier (16.6 +/- 0.2 kcal/mol) for isomerization of the amide N-terminal to 5-tBuPro(7) relative to OT was calculated from measuring the coalescence temperature of the Gly(9) backbone NH signals in the NMR spectra of 2. Observed bands in the Raman spectra of 1 and 2 correspond to C(beta)-S-S-C(beta) dihedral angles of +110-115 degrees and +/-90 degrees , respectively. In water, acetonitrile and methanol, the CD spectra for 1 exhibited a positive maximum around 236-239 nm; in trifluoroethanol, the spectra shifted and a negative maximum was observed at 240 nm. The CD spectra of 2 were unaffected by solvent changes and exhibited a negative maximum at 236-239 nm. The CD and Raman data both suggested that a conformation having a right-handed screw sense about the disulfide and a chi(CS-SC) dihedral angle value close to 115 degrees was favored for analog 1 in water, methanol and acetonitrile, but not trifluoroethanol, where a +/-90 degrees angle was favored. Analog 2 was more resilient to conformational change about the disulfide, and adopted a preferred disulfide geometry corresponding to a +/-90 degrees chi(CS-SC) dihedral angle. Monte Carlo conformational analysis of analogs 1 and 2 using distance restraints derived from NMR spectroscopy revealed two prominent conformational minima for analog 1 with disulfide geometries around +114 degrees and +116 degrees . Similar analysis of analog 2 revealed one conformational minimum with a disulfide geometry around +104 degrees . In sum, the conformation about the disulfide in [dPen(1),Pen(6)]-OT (1) was shown to be contingent on environment and in TFE, adopted a geometry similar to that of [dPen(1),Pen(6),5-tBuPro(7)]-OT (2) which appeared to be stabilized by hydrophobic interactions between the 5-tBuPro(7) (5R)-tert-butyl group, the Leu(8) isopropyl sidechain and the Pen(6)beta-methyl substituents. In light of the conformational rigidity of 2 about the disulfide bond, and the similar geometry adopted by 1 in TFE, a S-S dihedral angle close to +110 degrees may be a prerequisite for their binding at the receptor.     

pH-induced conformational changes in chromatin subunits measured by circular dichroism and immunochemical reactivity

Changes in the conformational state of chromatin core particles from chicken erythrocytes were studied by both immunochemical and biophysical methods as a function of pH and ionic strength. When the pH of core particles in a solution of ionic strength 3, 60 or 220 mM was lowered from pH 7.5, a sharp transition in the circular dichroism spectrum of DNA monitored between 320 and 260 nm was observed at pH 6.65. This change in DNA ellipticity was totally reversible. Binding to core particles of antibodies specific for histones H2B, H2A, H3 and for the IRGERA (synthetic C-terminal) peptide of H3 was used to follow changes in histone antigenicity. Binding was studied in the pH range 7.5-5.35, and at ionic strength of 60 and 220 mM. A change in reactivity of some histone epitopes was observed around pH 6.2–6.5. However, the changes observed by circular dichroism and antibody binding pertain to different components of chromatin subunits and they probably reflect independent phenomena. The alteration in accessibility of these determinants at the surface of core particles was completely reversible and was dependent on ionic strength. The conformation changes in core particles occurring near physiological ionic strength and pH may reflect dynamic changes in chromatin structure that possess functional significance.     

The influence of 1-aminocyclopentane-1-carboxylic acid at position 2 or 3 of AVP and its analogues on their pharmacological properties.

This study describes the synthesis and some pharmacological properties of eight new analogues of arginine vasopressin (AVP) substituted at position 2 or 3 with cycloleucine (1-aminocyclopentane-1-carboxylic acid, Apc). All new peptides were tested for their pressor, antidiuretic and uterotonic in vitro potency. The Apc3 modification resulted in an almost complete loss of potency in all three tests, which is interpreted as a loss of interaction with all three neurohypophyseal hormone receptors. On the other hand, the Apc2 modification resulted in compounds having differently modified activities (high antidiuretic potency, low and graded pressor activity and either no activity or low oxytocin antagonizing activity in the uterotonic in vitro test) thus selectively altering the interaction with the receptors similar to that of 1-aminocyclohexane-1-carboxylic acid (Acc). The results obtained may be helpful for designing new analogues of arginine vasopressin.     

Detection by circular dichroism of conformational transitions in pH and thermosensitive copolymers based on N-isopropylacrylamide and N-methacryloyl-L-leucine

Circular dichroism (CD) spectra in the region of 210-250 nm allow visualization of intrachain phase transition of pH- and thermosensitive polyelectrolytes. Indeed, in 0.001 M citrate and acetate buffers, at pH 4.0-5.5, aqueous solutions of a poly(N-isopropylacrylamide-co-N-methacryloyl-L-leucine) (NIPAAm-MALEU) copolymer containing 90.9 mol% of NIPAAm residues exhibit a well-defined sigmoidal increase in the CD signal at 220 nm with increasing temperature. This phenomenon is suggestive of a highly cooperative transition which occurs at lower temperatures compared to that observed by cloud point measurements. The change in the CD signal is less sharp at higher pH, indicating varying cooperativity with pH. For pH 6.0 and higher, no such phenomena are observed.     

Quantification of isomeric equilibria formed by metal ion complexes of 8-[2-(phosphonomethoxy)ethyl]-8-azaadenine (8,8aPMEA) and 9-[2-(phosphonomethoxy)ethyl]-8-azaadenine (9,8aPMEA). Derivatives of the antiviral nucleotide analogue 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA)

The acidity constants of the two-fold protonated acyclic 9-[2-(phosphonomethoxy)ethyl]-8-azaadenine, H2(9,8aPMEA)(+)(-), and its 8-isomer, 8-[2-(phosphonomethoxy)ethyl]-8-azaadenine, H2(8,8aPMEA)(+)(-), both abbreviated as H2(PA)(+)(-), as well as the stability constants of their M(H;PA)+ and M(PA) complexes with the metal ions M2+=Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+ or Cd2+, have been determined by potentiometric pH titrations in aqueous solution at I=0.1 M (NaNO3) and 25 degrees C. Application of previously determined straight-line plots of log K(M)M(R-PO3) versus pK(H)H(R-PO3)for simple phosph(on)ate ligands, R-PO3(2-), where R represents a residue without an affinity for metal ions, proves that for all M(PA) complexes a larger stability is observed than is expected for a sole phosphonate coordination of the metal ion. This increased stability is attributed to the formation of five-membered chelates involving the ether oxygen present in the aliphatic residue (-CH2-O-CH2-PO3(2-)) of the ligands. The formation degrees of these chelates were calculated; they vary between about 13% for Ca(8,8aPMEA) and 71% for Cu(8,8aPMEA). The adenine residue has no influence on complex stability except in the Cu(9,8aPMEA) and Zn(9,8aPMEA) systems, where an additional stability increase attributable to the adenine residue is observed and equilibria between four different isomers exist. This means (1) an open isomer with a sole phosphonate coordination, M(PA)op, where PA(2-)=9,8aPMEA2-, (2) an isomer with a five-membered chelate involving the ether oxygen, M(PA)cl/O, (3) an isomer which contains five- and seven-membered chelates formed by coordination of the phosphonate group, the ether oxygen and the N3 site of the adenine residue, M(PA)cl/O/N3, and finally (4) a macrochelated isomer involving N7, M(PA)cl/N7. For Cu(9,8aPMEA) the formation degrees are 15, 30, 48 and 7% for Cu(PA)op, Cu(PA)cl/O, Cu(PA)cl/O/N3 and Cu(PA)cl/N7, respectively; this proves that the macrochelate involving N7 is a minority species. The situation for the Cu(PMEA) system, where PMEA2- represents the parent compound, i.e. the dianion of 9-[2-(phosphonomethoxy)ethyl]adenine, is quite similar. The relationship between the antiviral activity of acyclic nucleoside phosphonates and the structures of the various complexes is discussed and an explanation is offered why 9,8aPMEA is biologically active but 8,8aPMEA is not.     

Conformational properties of 2',5' linked Rp- and Sp-phosphorothioate oligoadenylates studied by circular dichroism and NMR

2',5'-Linked oligoadenylates (2-5A) are involved in the antiviral action of interferon. The 2-5A binds and activates 2-5A dependent RNase (RNase L), which degrades viral mRNA, resulting in the inhibition of protein synthesis. 2',5'-Linked phosphorothioate oligoadenylates with an Rp configuration bind to and activate the RNase L. On the other hand, 2',5' phosphorothioate oligoadenylate with an Sp configuration weakly binds to the RNase L and is devoid of the RNase L activation ability. Comparative circular dichroism (CD) and NMR studies are carried out to characterize the difference in properties between the two configurations of the 2',5' phosphorothioate oligoadenylates. 2',5' Rp-Phosphorothioate oligoadenylates showed CD spectra similar to those of the corresponding native 2',5' oligoadenylates, while the 2',5' Sp-phosphorothioate oligoadenylates exhibited a weaker CD band compared to the former two, indicating the weaker base-stacking interaction of the 2',5' Sp-phosphorothioate oligoadenylates. The temperature-dependent change in the CD revealed that 2',5' phosphorothioate oligoadenylates showed larger DeltaH(0) and DeltaS(0) values for the thermal transition of the conformation than the corresponding native 2',5' oligoadenylates. The NMR spectral assignment was accomplished by several NMR measuring techniques. The 2'-H of the ribose ring linked to the 2',5' Sp-phosphorothioate showed a higher field chemical shift of the proton NMR than that linked to the corresponding 2',5' Rp-phosphorothioate. 2',5' Rp- and Sp-phosphorothioate oligoadenylates possess a sugar conformation similar to that of the corresponding native 2',5' oligoadenylates.     

Interactions of vasopressin and oxytocin receptors with vasopressin analogues substituted in position 2 with 3,3′‐diphenylalanine – a molecular docking study

Vasopressin and oxytocin receptors belong to the superfamily of G protein‐coupled receptors and play an important role in many physiological functions. They are also involved in a number of pathological conditions being important drug targets. In this work, four vasopressin analogues substituted at position 2 with 3,3′‐diphenylalanine have been docked into partially flexible vasopressin and oxytocin receptors. The bulky residue at position 2 acts as a structural restraint much stronger in the oxytocin receptor (OTR) than in the vasopressin V2 receptor (V2R), resulting in a different location of the analogues in these receptors. This explains the different, either agonistic or antagonistic, activities of the analogues in V2R and OTR, respectively. In all complexes, the conserved polar residues serve as anchor points for the ligand both in OTR and V2R. Strong interactions of the C‐terminus of analogue II ([Mpa¹,d ‐Dpa²,Val⁴,d ‐Arg⁸]VP) with extracellular loop 3 may be responsible for its highest activity at V2R. It also appears that V2R adapts more readily to the docking analogues by conformational changes in the aromatic side chains triggering receptor activation. A weak activity at V1a vasopressin receptor appears to be caused by weak receptor–ligand interactions. Results of this study may facilitate a rational design of new analogues with the highest activity/selectivity at vasopressin and OTRs. Copyright © 2013 European Peptide Society and John Wiley & Sons, Ltd.     

Identification of MK-5710 ((8aS)-8a-methyl-1,3-dioxo-2-[(1S,2R)-2-phenylcyclo- propyl]-N-(1-phenyl-1H-pyrazol-5-yl)hexahydro-imidazo[1,5-a]pyrazine-7(1H)-carboxamide), a potent smoothened antagonist for use in Hedgehog pathway dependent malignancies, part 2

The Hedgehog (Hh-) signaling pathway is a key developmental pathway which gets reactivated in many human tumors, and smoothened (Smo) antagonists are emerging as novel agents for the treatment of malignancies dependent on the Hh-pathway, with the most advanced compounds demonstrating encouraging results in initial clinical trials. A novel series of potent bicyclic hydantoin Smo antagonists was reported in the preceding article, these have been resolved, and optimized to identify potent homochiral derivatives with clean off-target profiles and good pharmacokinetic properties in preclinical species. While showing in vivo efficacy in mouse allograft models, unsubstituted bicyclic tetrahydroimidazo[1,5-a]pyrazine-1,3(2H,5H)-diones were shown to epimerize in plasma. Alkylation of the C-8 position blocks this epimerization, resulting in the identification of MK-5710 (47) which was selected for further development.     

Identification of MK-5710 ((8aS)-8a-methyl-1,3-dioxo-2-[(1S,2R)-2-phenylcyclopropyl]-N-(1-phenyl-1H-pyrazol-5-yl)hexahydroimid azo[1,5-a]pyrazine-7(1H)-carboxamide), a potent smoothened antagonist for use in Hedgehog pathway dependent malignancies, part 1

The Hedgehog (Hh-) signaling pathway is a key developmental pathway which controls patterning, growth and cell migration in most tissues, but evidence has accumulated showing that many human tumors aberrantly reactivate this pathway. Smoothened antagonists offer opportunities for the treatment of malignancies dependent on the Hh pathway, and the most advanced clinical candidates are demonstrating encourage initial results. A novel series of [6,5]-bicyclic tetrahydroimidazo[1,5-a]pyrazine-1,3(2H,5H)-dione smoothened antagonists has been identified, and the series has been extensively explored to ascertain the key detriments for activity, demonstrating that the trans-2-phenylcyclopropyl and hydantoin ring systems are critical for potency, while a variety of urea substituents can be tolerated. The combination of these optimal groups gives smoothened antagonists with activity in the low nanomolar range.     

Phase equilibrium in poly(rA).poly(rU) complexes with Cd2+ and Mg2+ ions, studied by ultraviolet, infrared, and vibrational circular dichroism spectroscopy

Ultraviolet (UV) and infrared (IR) absorption and vibrational circular dichroism (VCD) spectroscopy were used to study conformational transitions in the double-stranded poly(rA). poly(rU) and its components-single-stranded poly(rA) and poly(rU) in buffer solution (pH 6.5) with 0.1M Na+ and different Mg2+ and Cd2+ (10(-6) to 10(-2) M) concentrations. Transitions were induced by elevated temperature that changed from 10 up to 96 degrees C. IR absorption and VCD spectra in the base-stretching region were obtained for duplex, triplex, and single-stranded forms of poly(rA) . poly(rU) at [Mg2+],[Cd2+]/[P] = 0.3. For single-stranded polynucleotides, the kind of conformational transition (ordering --> disordering --> compaction, aggregation) is conditioned by the dominating type of Me2+-polymer complex that in turn depends on the ion concentration range. The phase diagram obtained for poly(rA) . poly(rU) has a triple point ([Cd2+] approximately 10(-4)M) at which the helix-coil (2 --> 1) transition is replaced with a disproportion transition 2AU --> A2U + poly(rA) (2 --> 3) and the subsequent destruction of the triple helix (3 --> 1). The 2 --> 1 transitions occur in the narrow temperature interval of 2 degrees -5 degrees . Unlike 2 --> 1 and 3 --> 1 melting, the disproportion 2 --> 3 transition is a slightly cooperative one and observed over a wide temperature range. At [Me2+] approximately 10(-3) M, the temperature interval of A2U stability is not less than 20 degrees C. In the case of Cd2+, it increases with the rise of ion concentration due to the decrease of T(m) (2-->3). The T(m) (3-->1) value is practically unchanged up to [Cd2+] approximately 10(-3)M. Differences between diagrams for Mg(2+) and Cd2+ result from the various kinds of ion binding to poly(rA).poly-(rU) and poly(rA).     

Study of the conformational properties of C-terminal fragments of histones H1, H5, and beta-endorphin by circular dichroism]

Circular dichroism technique has been used for investigating the conformation of histone H1 and H5 C-terminal fragments and beta-endorphin. It has been shown that in aqueous solution these polypeptides adopt preferably the left-handed helical conformation of the poly-L-proline II type. The linear temperature dependence of the CD value during solution heating was found to be broken in the temperature interval between 50 and 55 degrees C. It was supposed to occur due to the conformation destruction.     

Electronic circular dichroism of monomethyl [O, O, O]-phosphate and [O, O, O]-thiophosphate revisited

Phosphoryl-transfer reactions have long been of interest due to their importance in maintaining numerous cellular functions. A phosphoryl-transfer reaction results in two possible stereochemical outcomes: either retention or inversion of configuration at the transferred phosphorus atom. When the product is phosphate, isotopically-labeled [¹⁶O, ¹⁷O, ¹⁸O]-phosphate derivatives can be used to distinguish these outcomes; one oxygen must be replaced by sulfur or esterified to achieve isotopic chirality. Conventionally, stereochemical analysis of isotopically chiral phosphate has been based on ³¹P NMR spectroscopy and involves complex chemical or enzymatic transformations. An attractive alternative would be direct determination of the enantiomeric excess using chiroptical spectroscopy. (S)-Methyl-[¹⁶O, ¹⁷O, ¹⁸O]-phosphate (MePi^∗), 7 and enantiomeric [¹⁶O, ¹⁷O, ¹⁸O]-thiophosphate (TPi^∗), 10, were previously reported to exhibit weak electronic circular dichroism (ECD), although with 10 the result was considered to be uncertain. We have now re-examined the possibility that excesses of 7 and 10 enantiomers can be detected by ECD spectrometry, using both experimental and theoretical approaches. 7 and both the (R) and (S) enantiomers of 10 (10a, 10b) were synthesized by the ‘Oxford route’ and characterized by ¹H, ³¹P and ¹⁷O NMR, and by MS analysis. Weak ECD could be found for 7, with suboptimal S/N. No significant ECD could be detected for the 10 enantiomers.Time-dependent DFT (TDDFT) calculations of the electronic excitation energies and rotational strengths of the same three enantiomers were carried out using the functional B3LYP and the basis set 6-311G^∗∗. The isotopically-perturbed geometries were predicted using the anharmonic vibrational frequency calculational code in GAUSSIAN 03. In the case of 10, calculations were also carried out for the hexahydrated complex to investigate the influence of the aqueous solvent. The predicted excitation wavelengths are greater than the observed wavelengths, a not unusual result of TDDFT calculations. The predicted anisotropy ratios are 2.9 × 10⁻⁵ for 7, −5.3 × 10⁻⁶ for 10a/b, and 1.7 × 10⁻⁶ for 10a/b⋅(H₂O)₆. For 7 the predicted anisotropy ratio approximates that observed in this work, 4.5 × 10⁻⁵ at 208 nm. For 10a/b, the upper limits of the experimental anisotropy ratios (<5 × 10⁻⁶ at 225 nm, pH 9; <5 × 10⁻⁶ at 236 nm, pH 12) are comparable to the predicted magnitude of the value for 10a/b. The lower predicted value for 10a/b · (H₂O)₆ suggests that the aqueous environment affects the ECD significantly. Altogether, the TDDFT calculations together with a stereochemical analysis based on NMR and the MS data support the conclusion that the experimental ECD results for MePi^∗ and TPi^∗ may be reliable in order of magnitude.     

Synthesis of Carbocyclic 1-[4-(Hydroxymethyl)cyclopent-2-enyl]-1,2,4-triazole-3-carboxamide and Its Derivatives

Abstract

The synthesis of carbocyclic 1-[4-(hydroxymethyl)cyclopent-2-enyl]-1,2,4-triazole-3-carboxamide (6a) and its derivatives was achieved from triol 10 in excellent overall yield. This route involves a Pd(0)-catalyzed coupling reaction as a key step.