Cis Equilibrium Research Articles

London Dispersion Stabilizes Chloro-Substituted cis-Double Bonds

AbstractWe present a combined experimental and computational study on the thermodynamic stability of cis- and trans-alkenes substituted with dispersion energy donor (DED) groups. To investigate the role of noncovalent interactions on equilibrium of cis- and trans-alkenes we utilized hydrochlorination reactions. While the general assumption is that increasing steric bulk favors the trans-alkene, we observe an equilibrium shift towards the more crowded cis-alkene with increasing substituent size. With the aim to quantify noncovalent interactions, we performed a double mutant cycle to experimentally gauge the attractive potential of bulky substituents. Additionally, we utilized local energy decomposition analysis at the DLPNO-CCSD(T)/def2-TZVP level of theory. We found LD interactions and Pauli exchange repulsion to be the most dominant components to influence cis- and trans-alkene equilibria.

Cis- and trans molybdenum oxo complexes of a prochiral tetradentate aminophenolate ligand: Synthesis, characterization and oxotransfer activity

Reaction of [MoO2Cl2(dmso)2] with the tetradentate O2N2 donor ligand papy [H2papy = N-(2-hydroxybenzyl)-N-(2-picolyl)glycine] leads to formation of the dioxomolybdenum(VI) complex [MoO2(papy)] (1) as a mixture of cis and trans isomers. Recrystallization from methanol furnishes solid cis-1, whereas the use of a dichloromethane-hexane mixture allows for the isolation of the trans-1 isomer. Both isomers have been structurally characterized by X-ray crystallography and the energy difference between the isomeric pair has been investigated by electronic structure calculations. Optimization of two configurational isomers in the gas phase predicts the trans isomer to lie 2.5 kcal/mol lower in energy (ΔG) than the cis isomer, which is inconsistent with the solution NMR data in d3-MeCN that exhibit a Keq of ca. 3 at 298 K for the trans ⇌ cis equilibrium. The DFT-computed energy difference is significantly improved (Keq = 5.4) by the inclusion of the MeCN solvent using the polarization continuum model (PCM). Density functional calculations reveal that the isomerization proceeds via a Ray-Dutt twist mechanism with a barrier of 14.5 kcal/mol, which is in accordance with the 1H NMR spectral data and the rapid equilibration of these isomers in solution. The catalytic reactivity of [MoO2(papy)] in the epoxidation of cis-cyclooctene is described, as well as its ability to effect oxo transfer from DMSO to PPh3.

Conformational Features of Thioamide-Containing Dipeptoids and Peptoid-Peptide Hybrids-Computational and Experimental Approaches.

The effects of thioamide incorporation into N, N-dimethyl-2-( N-methylacetamido)acetamide and N-methyl-2-( N-methylacetamido)acetamide as the simplest models of a dipeptoid structure and a peptoid-peptide hybrid are discussed. The solvent-modulated conformational features of model compounds were examined by computations enhanced by natural bond orbital (NBO) analysis and experimentally by kinetic and equilibrium measurements using NMR spectroscopy. The computations supported by NBO analysis showed that intrinsic stability of the predominant trans isomer (αD and C7β forms) of the dipeptoid model results from an indirect n → π* interaction, occurring between the carbonyl oxygen lone pair ( n) and the π* orbital of the adjacent amide carbonyl through the C-H antibond (σ*). The direct n → π* interaction constitutes a negligible contribution to trans stabilization. The N-terminal thioxo substitution increases this indirect electron delocalization, making the αD isomer prevalent. The nX → σN'C-H* interaction is an additional source of stability of the trans-C7β form relevant for the underivatized dipeptoid model and its C-terminal thioamide counterpart. In the peptoid-peptide hybrid, the trans preference is perturbed by subtle differences in the H-bond donor-acceptor abilities between the thioxo and oxo groups. The cis isomer becomes more populated with an increase in the strength of polarity and the hydrogen bonding acceptor ability of the solvent molecules. While thioxo substitution slightly shifts the trans- cis equilibrium in polar solvents, it effectively allows for increasing or decreasing the barrier to trans- cis rotation with respect to underivatized model compounds depending on N- vs C-terminal thioamide backbone substitution.

Direct Observation of Aggregation‐Induced Backbone Conformational Changes in Tau Peptides

AbstractIn tau proteins, the hexapeptides in the R2 and R3 repeats are known to initiate tau fibril formation, which causes a class of neurodegenerative diseases called the taupathies. We show that in R3, in addition to the presence of the hexapeptides, the correct turn conformation upstream to it is also essential for producing prion‐like fibrils that are capable of propagation. A time‐dependent NMR aggregation assay of a slow fibril forming R3‐S316P peptide revealed a trans to cis equilibrium shift in the peptide‐bond conformation preceding P316 during the growth phase of the aggregation process. S316 was identified as the key residue in the turn that confers templating capacity on R3 fibrils to accelerate the aggregation of the R3‐S316P peptide. These results on the specific interactions and conformational changes responsible for tau aggregation could prove useful for developing an efficient therapeutic intervention in Alzheimer's disease.

Direct Observation of Aggregation-Induced Backbone Conformational Changes in Tau Peptides.

In tau proteins, the hexapeptides in the R2 and R3 repeats are known to initiate tau fibril formation, which causes a class of neurodegenerative diseases called the taupathies. We show that in R3, in addition to the presence of the hexapeptides, the correct turn conformation upstream to it is also essential for producing prion-like fibrils that are capable of propagation. A time-dependent NMR aggregation assay of a slow fibril forming R3-S316P peptide revealed a trans to cis equilibrium shift in the peptide-bond conformation preceding P316 during the growth phase of the aggregation process. S316 was identified as the key residue in the turn that confers templating capacity on R3 fibrils to accelerate the aggregation of the R3-S316P peptide. These results on the specific interactions and conformational changes responsible for tau aggregation could prove useful for developing an efficient therapeutic intervention in Alzheimer's disease.

Peptoid-Substituted Hybrid Antimicrobial Peptide Derived from Papiliocin and Magainin 2 with Enhanced Bacterial Selectivity and Anti-inflammatory Activity.

Antimicrobial peptides (AMPs) are important components of the host innate immune system. Papiliocin is a 37-residue AMP purified from larvae of the swallowtail butterfly Papilio xuthus. Magainin 2 is a 23-residue AMP purified from the skin of the African clawed frog Xenopus laevis. We designed an 18-residue hybrid peptide (PapMA) incorporating N-terminal residues 1-8 of papiliocin and N-terminal residues 4-12 of magainin 2, joined by a proline (Pro) hinge. PapMA showed high antimicrobial activity but was cytotoxic to mammalian cells. To decrease PapMA cytotoxicity, we designed a lysine (Lys) peptoid analogue, PapMA-k, which retained high antimicrobial activity but displayed cytotoxicity lower than that of PapMA. Fluorescent dye leakage experiments and confocal microscopy showed that PapMA targeted bacterial cell membranes whereas PapMA-k penetrated bacterial cell membranes. Nuclear magnetic resonance experiments revealed that PapMA contained an N-terminal α-helix from Lys(3) to Lys(7) and a C-terminal α-helix from Lys(10) to Lys(17), with a Pro(9) hinge between them. PapMA-k also had two α-helical structures in the same region connected with a flexible hinge residue at Nlys(9), which existed in a dynamic equilibrium of cis and trans conformers. Using lipopolysaccharide-stimulated RAW264.7 macrophages, the anti-inflammatory activity of PapMA and PapMA-k was confirmed by inhibition of nitric oxide and inflammatory cytokine production. In addition, treatment with PapMA and PapMA-k decreased the level of ultraviolet irradiation-induced expression of genes encoding matrix metalloproteinase-1 (MMP-1), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) in human keratinocyte HaCaT cells. Thus, PapMA and PapMA-k are potent peptide antibiotics with antimicrobial and anti-inflammatory activity, with PapMA-k displaying enhanced bacterial selectivity.

Evidence of Sulfur Mustard Exposure in Human Plasma by LC-ESI-MS-MS Detection of the Albumin-Derived Alkylated HETE-CP Dipeptide and Chromatographic Investigation of Its Cis/Trans Isomerism.

Sulfur mustard (SM) is a chemical warfare agent that causes painful blisters and chemically modifies endogenous biomacromolecules by alkylation to hydroxyethylthioethyl (HETE) adducts representing valuable long-term markers for post-exposure analysis. The albumin adduct formed in human plasma in vitro (HETE bound to the side chain of cysteine 34) was isolated and cleaved by current lots of pronase primarily generating the internal modified dipeptide (HETE-cysteine-proline, HETE-CP) instead of the formerly reported HETE-CPF tripeptide. The analyte was detected by liquid chromatography-electrospray ionization tandem-mass spectrometry (LC-ESI-MS-MS). In principle, HETE-CP undergoes a dynamic on-column equilibrium of cis-trans isomerism thus requiring separation at 50°C to obtain one narrow peak. Accordingly, we developed both a novel longer lasting but more sensitive microbore (1 mm i.d., flow 30 µL/min, cycle time 60 min, LOD 50 nM) and a faster, less sensitive narrowbore (2.1 mm i.d., 200 µL/min, cycle time 16 min, LOD 100 nM, both on Atlantis T3 material at 50°C) LC-ESI-MS-MS method suitable for verification analysis. The corresponding tri- and tetrapeptide, Q(HETE)-CPF were monitored simultaneously. HETE-CP peak areas were directly proportional to SM concentrations added to plasma in vitro (0.05-100 µM). Albumin adducts formed by deuterated SM (d8-SM) served as internal standard.

Multivalent ligands of CD22 for targeting of B cells

Multivalent ligands of CD22 for active targeting of B cells. Mary O’Reilly, Wei Hsu Chen, Gladys Completo, Ying Zeng, Satoshi Futakawa and Cory Rillahan and James C. Paulson, Departments of Chemical Physiology and Molecular Biology, The Scripps Research Institute, 10550 N. Torrey Pines Road, La Jolla, CA, 92037The siglecs comprise 13 members of the immunoglobulin superfamily that recognize sialic acid containing glycans, and are differentially expressed on leukocytes and glial cells. The natural ligands of siglecs typically occur on the same cell (in cis) and/or on adjacent cells (in trans). Cis ligands mask the binding of multivalent synthetic sialoside ligands and are thought to regulate the activity of siglecs as modulators of cell signaling. However, synthetic ligands of sufficient avidity can compete with cis ligands, demonstrating a dynamic equilibrium of cis and trans ligand probes. We have explored the relationship between valency, affinity and geometry for achieving avidity sufficient to compete with cis ligands of CD22 (Siglec-2) in situ. A notable achievement is a hetero-bifunctional ligand approach to create multivalent ligands using antibodies as a protein scaffold. The ligand is comprised of a CD22 ligand, 9-biphenylcarboxyl-NeuAcα2-6Galβ1-4GlcNAc (BPCNeuAc), coupled to an antigen, 4-hydroxy-3-nitrophenylacetic acid (NP) (BPCNeuAc-NP). The BPCNeuAc-NP ligand is able to efficiently assemble complexes of anti-NP antibodies with CD22 on both asialo- and native B cells. Surprisingly, assembly of the tertiary complex occurs with anti-NP IgM (n=10), IgA (n=4) and IgG (n=2). The results suggest that spacing of ligands using an antibody optimizes the contribution of geometry to achieve high avidity with low valency. Other multivalent configurations also show promise for targeting B cells. In particular, liposomes bearing BPCNeuAc ligands bind avidly to B cells and are endocytosed. Thus, BPCNeuAc-lipososomes may prove effective in delivery of therapeutic agents to B cells (Supported by NIH grants GM60938, AI50143 ).

Multivalent ligands of CD22 for targeting of B cells

AbstractMultivalent ligands of CD22 for active targeting of B cells. Mary O’Reilly, Wei Hsu Chen, Gladys Completo, Ying Zeng, Satoshi Futakawa and Cory Rillahan and James C. Paulson, Departments of Chemical Physiology and Molecular Biology, The Scripps Research Institute, 10550 N. Torrey Pines Road, La Jolla, CA, 92037The siglecs comprise 13 members of the immunoglobulin superfamily that recognize sialic acid containing glycans, and are differentially expressed on leukocytes and glial cells. The natural ligands of siglecs typically occur on the same cell (in cis) and/or on adjacent cells (in trans). Cis ligands mask the binding of multivalent synthetic sialoside ligands and are thought to regulate the activity of siglecs as modulators of cell signaling. However, synthetic ligands of sufficient avidity can compete with cis ligands, demonstrating a dynamic equilibrium of cis and trans ligand probes. We have explored the relationship between valency, affinity and geometry for achieving avidity sufficient to compete with cis ligands of CD22 (Siglec-2) in situ. A notable achievement is a hetero-bifunctional ligand approach to create multivalent ligands using antibodies as a protein scaffold. The ligand is comprised of a CD22 ligand, 9-biphenylcarboxyl-NeuAcα2-6Galβ1-4GlcNAc (BPCNeuAc), coupled to an antigen, 4-hydroxy-3-nitrophenylacetic acid (NP) (BPCNeuAc-NP). The BPCNeuAc-NP ligand is able to efficiently assemble complexes of anti-NP antibodies with CD22 on both asialo- and native B cells. Surprisingly, assembly of the tertiary complex occurs with anti-NP IgM (n=10), IgA (n=4) and IgG (n=2). The results suggest that spacing of ligands using an antibody optimizes the contribution of geometry to achieve high avidity with low valency. Other multivalent configurations also show promise for targeting B cells. In particular, liposomes bearing BPCNeuAc ligands bind avidly to B cells and are endocytosed. Thus, BPCNeuAc-lipososomes may prove effective in delivery of therapeutic agents to B cells (Supported by NIH grants GM60938, AI50143 ).

Crystal Structures and Solution Behavior of Paramagnetic, Trinuclear, Mixed‐Valent Cobalt Complexes with Salen‐Type Ligands

AbstractNew trinuclear mixed‐valent CoIII‐CoII‐CoIII complexes with salen‐type ligands have been synthesized and characterized both in the solid state and in solution. These complexescomprise a central CoCl2 unit connecting two [RCo(4,4′,7,7′tmsalen)] fragments with chlorides acting as bridging ligands between the metals, while the central CoII ion completes its octahedral coordination through the salen oxygen donors. Complex 1 (R = CH2Cl), which has cis Cl ligands at the central CoII ion, has approximate C2 symmetry, whereas the centrosymmetric complex 2 (R = CF3CH2) contains trans Cl ligands (C2h symmetry). Complexes 1 and 2 are paramagnetic with magnetic moments of 4.7 and 4.4 μB, respectively, which indicate the presence of a high‐spin CoII center in the molecule. The 1H NMR spectra, which spread over a range of nearly 190 ppm, are consistent with the presence of both cis and trans isomers in solution (with cis being the prevailing species), and indicate that exchange is slow on the NMR time‐scale. Isotropically shifted 1H NMR signals have been fully assigned by comparison with those of appropriately substituted complexes and by T1 measurements and 2D exchange experiments (EXSY). The K constant for the trans [rlarr2] cis equilibrium, as evaluated from the 1H NMR signal intensity ratio at various temperatures, has allowed us to estimate, at least qualitatively, the thermodynamic parameters ΔH° and ΔS°.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Solvent influence on the rotational isomerism in terephthalaldehyde

Experimental and theoretical studies were carried out in order to investigate the rotational isomerism of terephthalaldehyde. The dipole moment measurements and infrared spectroscopy in Ar matrix and using various solvents were performed experimentally. In order to supplement the experimental study, both static and dynamical theoretical calculations were performed. IR spectra and potential energy distribution (PED) were calculated for both cis and trans isomers of terephthalaldehyde in gas phase using B3LYP/6-31G(d,p) level of theory. Further calculations consisted of conformational analysis were performed in order to estimate the rotational barrier and relative stabilities of isomers. The DFT theory with B3LYP functional and four double-zeta and triple-zeta basis sets served as framework for this part of calculations. Semiempirical AM1 and PM3 methods were also used for gas-phase modeling. Molecular dynamics using MM3 force field was applied to study the preferences of solvent molecules’ orientation around the studied molecule. Additionally, the effect of solvent polarity on the Gibbs energy of the trans ⇌ cis equilibrium was analyzed in terms of the continuum dielectric medium models.

Solvent influence on conformational equilibrium in 3-nitrobenzaldehyde

Semi-empirical (PM3, AM1) and ab initio (B3LYP/6-31G(d,p)) calculations of the structure and energies of isolated 3-nitrobenzaldehyde (3-NBA) reveal that the planar trans and cis isomers are in equilibrium, with the trans being more stable by about 2.2 kJ/mol. The experimental dipole moments of 3-NBA in solvents of dielectric permittivity ε ranging from 1.9 to 25 have been determined. The increase in the mean dipole moment with increasing ε can be well explained in terms of the trans⇔ cis equilibrium and dipolar isomer–solvent interaction. The heterogeneous dielectric medium model with two concentric dielectric layers proved to be the best description for this interaction. On the other hand, the intrinsic experimental Gibbs energy for the trans⇔ cis conversion obtained from this model was higher than the quantum-chemical values. It is suggested that the quadrupolar interaction acts oppositely to the dipolar one stabilizing the more quadrupolar trans isomer.

NMR studies of hydrogen bonding interactions with secondary amide and urea groups

AbstractThe cis/trans ratios for six model secondary amides were determined by 1H NMR in a range of solvent systems. The trans to cis equilibrium in chloroform is only slightly affected by addition of the hydrogen bond donor, trifluorethanol, but the cis rotamer is stabilized by an average of 0.7 kcal mol−1 when acetic acid is used as an intermolecular donor–acceptor template. Conversely, amide interaction with anionic hydrogen bond acceptors decreases the percentage of cis rotamer. 15N NMR spectroscopy was used to determine the effect of hydrogen bonding on the trans amide structure. The direction and the magnitude of 15N complex‐induced‐shifts indicate that both hydrogen bond donors and acceptors raise the secondary amide rotational barrier by increasing the CN bond order. The relationship of these results to protein structure is discussed. Copyright © 2001 John Wiley & Sons, Ltd.

The Effect of Protein Conformation Change from αII to αI on the Bacteriorhodopsin Photocycle

The bacteriorhodopsin (bR) photocycle was followed by use of time-resolved Fourier-transform infrared (FTIR) spectroscopy as a function of temperature (15–85°C) as the α II → α I conformational transition occurs. The photocycle rate increases with increasing temperature, but its efficiency is found to be drastically reduced as the transition takes place. A large shift is observed in the all- trans ↔ 13- cis equilibrium due to the increased stability of the 13- cis isomer in α I form. This, together with the increase in the rate of dark adaptation as the temperature increases, leads to a large increase in the 13- cis isomer concentration in bR in the α I form. The fact that 13- cis retinal has a much-reduced absorption cross-section and its inability to pump protons leads to an observed large reduction in the concentration of the observed photocycle intermediates, as well as the proton gradient at a given light intensity. These results suggest that nature might have selected the α II rather than the α I form as the helical conformation in bR to stabilize the all- trans retinal isomer that is a better light absorber and is capable of pumping protons.

Analytical energy gradient for the reference interaction site model multiconfigurational self-consistent-field method: Application to 1,2-difluoroethylene in aqueous solution

We have reformulated the reference interaction site model self-consistent-field (RISM-SCF) method to calculate the molecular properties of solute in solutions. The key feature is to introduce the Helmholtz free energy given as a sum of the solute electronic energy estimated by ab initio molecular orbital (MO) methods and the hypernetted-chain (HNC) excess chemical potential of solvation for the extended reference interaction site model (XRISM) method. The variational conditions for the multiconfigurational self-consistent-field (MCSCF) wave functions were derived. It was also shown that, in the case of the Hartree–Fock (HF) method, the present expression gives the same Fock matrix defined previously. Moreover the expression for first derivatives of the free energy with respect to the solute nuclear coordinates were obtained. Sample calculations for the chemical equilibrium of cis- and trans-isomers of 1,2-difluoroethylene are presented. With the use of analytical energy gradients for the MCSCF and HF methods, the solute molecular geometries were determined in an aqueous solution.

Reversible formation of Pt2(.mu.-H)2H(PEt3)4+ and Pt2(.mu.-H)H2(PEt3)4+ from the cis and trans isomers of dihydrobis(triethylphosphine)platinum(II)

Irradiation of Pt(C/sub 2/O/sub 4/)L/sub 2/, where L = P(C/sub 2/H/sub 5/)/sub 3/, in Ch/sub 3/Cn solvent under a hydrogen atmosphere produces (Pt/sub 2/(..mu..-H)/sub 2/HL/sub 4/)(O/sub 2/CH), 1(O/sub 2/CH), and CO/sub 2/. Dimer 1 can also be prepared by the condensation reaction between PtH/sub 2/L/sub 2/ (a 90:10 trans:cis equilibrium mixture) and PtH(S)L/sub 2//sup +/, where S = a solvent such as acetone. When PtD/sub 2/L/sub 2/ is used in the reaction, deuterium is only incorporated into the bridging positions. Analyses of the spin and isotopic distribution patterns in the /sup 31/P, /sup 1/H, and /sup 195/Pt NMR spectra establish a solution geometry for 1 based on a cis-PtH/sub 2/L/sub 2/ molecule bound to the PtHL/sub 2//sup +/ fragment. The solution chemistry of 1 depends dramatically on the counterion. Basic anions X = I/sup -/, OCH/sub 3//sup -/, and O/sub 2/CH/sup -/ generate small equilibrium concentrations of PtH/sub 2/L/sub 2/, PtHXL/sub 2/, and (PtH(S)L/sub 2/)X. For example, 1(O/sub 2/CH) reacts completely in the presence of C/sub 2/H/sub 4/ to yield Pt(C/sub 2/H/sub 4/)L/sub 2/ whereas 1(B(C/sub 6/H/sub 5/)/sub 4/) exhibits no such reactivity. Complex 1 efficiently catalyzes the decomposition of formic acid at 25/sup 0/C; however, formate ion must bemore » added in excess to promote the reaction. Monomers appear to be the active catalysts. The species 1(OH) formed by the addition of H/sub 2/O to PtH/sub 2/L/sub 2/ also catalyzes the hydration of acetonitrile to acetamide. A symmetrical isomer, Pt/sub 2/(..mu..-H)H/sub 2/L/sub 4//sup +/ (2), of dimer 1 can be generated by the UV photolysis of Pt(C/sub 2/O/sub 4/)L/sub 2/ in methanol. It appears that the initially formed trans-PtH(OCH/sub 3/)L/sub 2/ complex undergoes ..beta..-hydride abstraction to stereoselectively yield trans-PtH/sub 2/L/sub 2/. This species is trapped by trans-PtH(OCH/sub 3/)L/sub 2/ or (trans-PtH(S)L/sub 2/)(CH/sub 3/O) to form 2 before isomerization to cis-PtH/sub 2/L/sub 2/ (and thereby 1) takes place. Dimer 2 is thermodynamically unstable with respect to 1, and bases such as pyridine or I/sup -/ catalyze the isomerization.« less

Properties and reactions of 1,3‐oxathiolans — V. Conformational energy of an axial 2‐t‐butyl group. An attempt to determine epimer equilibria by microcalorimetry

AbstractChemical equilibration of epimeric 2‐t‐butyl‐5‐methyl‐1, 3‐oxathiolans led to ‐ΔHo=4.60±0.05 kJ mol−1 and ‐ΔSo=2.9±0.16 J mol−1K−1 for the trans cis equilibrium. From them and the values of 1H vicinal coupling constants the conformational energy of a ψ‐axial 2‐t‐butyl group was estimated at 8.6±1.3 kJ mol−1. A microcalorimetric determination of the enthalpy difference for the epimer equilibrium resulted in a less accurate value of 4.5±1.0 kJ mol−1 and pointed out some disadvantages connected with the latter method.

Infrared spectra and conformational equilibrium of cis- and trans-2-chloromethylcyclohexanes

Infrared spectra and conformational equilibrium of cis- and trans-2-chloromethylcyclohexanes

On the conformation of dipeptides in aqueous solutions

The IR spectra of Gly-Leu, Leu-Gly, Gly-Phe and Phe-Gly in KBr, water and deuterium oxide, and also IR spectra of lyophilysed dipeptides in nujol were investigated. It was found that in the aqueous solution there exists an equilibrium of cis- and trans-isomers. On this ground the apparent incoherence between ORD and NMR measurements for the dipeptides is clarified.

On the conformation of dipeptides in aqueous solutions

The IR spectra of Gly-Leu, Leu-Gly, Gly-Phe and Phe-Gly in KBr, water and deuterium oxide, and also IR spectra of lyophilysed dipeptides in nujol were investigated. It was found that in the aqueous solution there exists an equilibrium of cis- and trans-isomers. On this ground the apparent incoherence between ORD and NMR measurements for the dipeptides is clarified.