Showing papers on "Steric effects published in 1995"

Unusually Mild and Selective Hydrocarbon C-H Bond Activation with Positively Charged Iridium(III) Complexes

Abstract: Certain transition metal complexes can react to break normally inert carbon-hydrogen (C-H) bonds, but these metal-based processes typically require photochemistry or elevated temperatures. In addition, most are unselective toward complicated functionalized substrates, which has limited their synthetic usefulness. The cationic iridium complex Cp*(P(CH3)3)Ir(CH3)(ClCH2Cl)+BArf− [Cp* = η5-C5(CH3)5, BArf = B(3,5-C6H3(CF3)2)4] can thermally activate methane and terminal alkanes at unprecedentedly mild temperatures (10°C). This complex will also induce C-H activation reactions in various functionalized substrates at ambient temperatures. High steric and electronic selectivity is observed, leading invariably to only one reaction product; the initial C-H activation reaction is typically followed by rapid metal-based rearrangements (that is, functionalization).

Texaphyrin compounds having improved functionalization

Abstract: Texaphyrin metal complexes having improved functionalization include the addition of electron-donating groups to positions 2, 7, 12, 15, 18 and/or 21 and/or the addition of electron-withdrawing groups to positions 15 and/or 18 of the macrocycle. Electron-donating groups at positions 2, 7, 12, 15, 18 and/or 21 contribute electrons to the aromatic π system of the macrocycle which stabilizes the metal complex to demetallation and the imine bonds to hydrolysis. The addition of substituents to the 12 and 21 positions of the macrocycle offer steric protection for the imine bonds against possible in vivo enzyme hydrolysis. Electron-withdrawing groups at positions 15 and/or 18 render the macrocycle more readily reduced, i.e. the redox potential is lower and the macrocycle more readily gains an electron to form a radical. Such texaphyrins having a low redox potential and imine bond stabilization are useful in a variety of applications.

Hydrophobic, non-hydrogen-bonding bases and base pairs in dna

Abstract: We report the properties of hydrophobic isosteres of pyrimidines and purines in synthetic DNA duplexes. Phenyl nucleosides 1 and 2 are nonpolar isosteres of the natural thymidine nucleoside, and indole nucleoside 3 is an analog of the complementary purine 2-aminodeoxyadenosine. The nucleosides were incorporated into synthetic oligodeoxynucleotides and were paired against each other and against the natural bases. Thermal denaturation experiments were used to measure the stabilities of the duplexes at neutral pH. It is found that the hydrophobic base analogs are nonselective in pairing with the four natural bases but selective for pairing with each other rather than with the natural bases. For example, compound 2 selectively pairs with itself rather than with A, T, G, or C; the magnitude of this selectivity is found to be 6.5-9.3 °C in Tm or 1.5-1.8 kcal/mol in free energy (25 °C). All possible hydrophobic pairing combinations of 1, 2, and 3 were examined. Results show that the pairing affinity depends on the nature of the pairs and on position in the duplex. The highest affinity pairs are found to be the 1-1 and 2-2 self-pairs and the 1-2 heteropair. The best stabilization occurs when the pairs are placed at the ends of duplexes rather than internally; the internal pairs may be destabilized by imperfect steric mimicry which leads to non-ideal duplex structure. In some cases the hydrophobic pairs are significantly stabilizing to the DNA duplex; for example, when situated at the end of a duplex, the 1-1 pair is more stabilizing than a T-A pair. When situated internally, the affinity of the 1-1 pair is the same as, or slightly better than, the analogous T-T mismatch pair, which is known to have two hydrogen bonds. The studies raise the possibility that hydrogen bonds may not always be required for the formation of stable duplex DNA-like structure. In addition, the results point out the importance of solvation and desolvation in natural base pairing, and lend new support to the idea that hydrogen bonds in DNA may be more important for specificity of pairing than for affinity. Finally, the study raises the possibility of using these or related base pairs to expand the genetic code beyond the natural A-T and G-C pairs.

Dioxirane oxidations: Taming the reactivity-selectivity principle

Abstract: w: Dioxiranes (1). a new class of powerful oxidants, have been employed to carry out a variety of synthetically useful transformations. Dioxirane reactivity appears to be earmarked by the propensity for easy electrophilic 0-atom transfer to nucleophilic substrates, as well as for 0-insertion into "unactivated" hydrocarbon C-H bonds. For a number of substrates of varying electron-donor power, ranging from a$-unsaturated carbonyls, to alkenes to sulfoxides to sulfides, the reactivity of dimethyldioxirane (la) exceeds that of peroxybenzoic acid by a factor of the order of 102; upon increasing substrate nucleophilicity, kinetic data show that the selectivity is not diminished, and actually appears to be enhanced. Despite its exceptional reactivity, high regioand stereoselectivities can be attained in oxyfunctionalizations at hydrocarbon C-H bonds using the powerful methyl-(trifluoromethy1)dioxirane (lb); this has been applied to accomplish remarkably high regioand stereoselective epoxidations and oxyfunctionalizations of target molecules. In these reactions, stringent steric and stereoelectronic requirements for 0-insertion seem to dictate selectivity; adoption of an FMO model provides a likely rationale.

Interaction between stacked aryl groups in 1,8-diarylnaphthalenes: Dominance of polar/π over charge-transfer effects

Abstract: Several 1,8-diarylnaphthalenes have been prepared, and the barrier to rotation around the arylhaphthalene bond has been measured. In these molecules steric congestion forces the aryls in a parallel stacked geometry. The barriers to rotation were used to evaluate the strength and to investigate the nature of the interaction between the arenes. The variance of the AG+ for the rotation upon arene substitution with electron donating or electron withdrawing groups indicates that polarh electrostatic effects dominate over charge-transfer effects in determining the arene/arene interaction.

Intramolecular interactions in the ground and excited state of tetrakis(N-methylpyridyl)porphyrins.

Abstract: The fluorescent properties of the cationic free base tetrakis(4-N-methylpyridy1)porphyrin (H2TMPyP(4)) in aqueous solution have been the subject of considerable discussion. Conclusions by various authors on the presence of homo-aggregation of H2TMPyP in these solutions are contradictory. The present work reports spectroscopic data for three isomers of H2TMPyP(n) (n = 2,3, or 4) at varying concentrations, solvent polarity, and temperature. 'H NMR spectra of H2TMPyP(4) show no ground-state monomers below M in water. Fluorescence spectra of the three isomers in aqueous solutions indicate the absence of aggregates both in the ground and excited state. Fluorescence lifetimes of the three isomers both in solution as well as adsorbed on solid surfaces can be explained by taking into account their dependence on the steric hindrance for rotation of the pyridinium groups of the three isomers with respect to the porphyrin macrocycle. Molecular mechanics confirms a higher degree of steric hindrance of H2TMPyP(2) as compared to the two other isomers. From the experimental results it is concluded that the first excited singlet state SI of the porphyrin mixes with a nearby CT state slightly above this SI state. In this CT state an electron is transferred from the porphyrin macrocycle to the pyridinium group. The amount of SI-CT mixing, responsible for the spectroscopic differences, is determined by the degree of coplanarity and resonance interaction of the porphyrin and the pyridinium n-systems, and by the solvent polarity which determines the energy difference between the two states.

Old yellow enzyme : aromatization of cyclic enones and the mechanism of a novel dismutation reaction

Abstract: The origin of charge transfer bands that develop on reaction of Old Yellow Enzyme with alpha,beta-unsaturated cyclic ketones such as 3-oxodecalin-4-ene (ODE, numbered according to the convention for steroids), 3-oxodecalin-4-ene-10-carboxaldehyde (ODEC), and 2-cyclohexenone is shown to be due to the aromatization of ODE and ODEC to 3-hydroxy-6,7,8,9-tetrahydronaphthalene (HTN) and of 2-cyclohexenone to phenol. The aromatization of ODEC to HTN is stereospecific and involves the trans dehydrogenation of the 1 beta, 2 alpha hydrogens. The aromatization occurs under aerobic as well as anaerobic conditions. With the exception of ODEC under aerobic conditions, the aromatization of these substrates is accompanied by a dismutation reaction in which the olefinic bond of a second molecule of each substrate is reduced to give the saturated cyclic ketone. Molecular oxygen may serve as the electron acceptor with ODEC and some other substrates under aerobic reaction conditions. The dismutation reaction involves an overall sequence of hydride transfer from one substrate molecule to the beta-carbon of a second substrate molecule along with a solvent proton uptake by the alpha-carbon. 19-Nortestosterone is aromatized to beta-estradiol; however, other 3-oxo-delta 4-steroids such as progesterone, testosterone, and androstene-3,17-dione bind tightly to the enzyme but are not aromatized. The NADPH-dependent reduction of the olefinic bond of alpha,beta-unsaturated carbonyl compounds is limited to aldehydes and ketones. alpha,beta-Unsaturated acids, esters, amides, and nitriles are not reduced. The reduction of the olefinic bond of ODE or cinnamaldehyde by NADPH occurs by an overall sequence of hydride transfer from the reduced pyridine nucleotide to the beta-carbon of the alpha,beta-unsaturated carbonyl compound and a solvent proton uptake by the alpha-carbon. The 4-pro-R hydride of NADPH is transferred in the reduction reaction. Structure-function relationships in the NADPH-dependent reduction of alpha,beta-unsaturated aldehydes or ketones indicate that increasing alkyl substitution at the beta-carbon results in marked decrease in the rate of reduction of the olefinic bond, consistent with a steric hindrance to hydride transfer at the beta-carbon.

Intramolecular Interactions in the Ground and Excited State of

Abstract: The fluorescent properties of the cationic free base tetrakis(4-N-methylpyridy1)porphyrin (H2TMPyP(4)) in aqueous solution have been the subject of considerable discussion. Conclusions by various authors on the presence of homo-aggregation of H2TMPyP in these solutions are contradictory. The present work reports spectroscopic data for three isomers of H2TMPyP(n) (n = 2,3, or 4) at varying concentrations, solvent polarity, and temperature. 'H NMR spectra of H2TMPyP(4) show no ground-state monomers below M in water. Fluorescence spectra of the three isomers in aqueous solutions indicate the absence of aggregates both in the ground and excited state. Fluorescence lifetimes of the three isomers both in solution as well as adsorbed on solid surfaces can be explained by taking into account their dependence on the steric hindrance for rotation of the pyridinium groups of the three isomers with respect to the porphyrin macrocycle. Molecular mechanics confirms a higher degree of steric hindrance of H2TMPyP(2) as compared to the two other isomers. From the experimental results it is concluded that the first excited singlet state SI of the porphyrin mixes with a nearby CT state slightly above this SI state. In this CT state an electron is transferred from the porphyrin macrocycle to the pyridinium group. The amount of SI-CT mixing, responsible for the spectroscopic differences, is determined by the degree of coplanarity and resonance interaction of the porphyrin and the pyridinium n-systems, and by the solvent polarity which determines the energy difference between the two states.

On the Ti-TADDOLate-Catalyzed Diels-Alder Addition of 3-Butenoyl-1,3-oxazolidin-2-one to Cyclopentadiene. General Features of Ti-BINOLate- and Ti-TADDOLate-Mediated Reactions

Abstract: A systematic investigation of the enantioselective Diels-Alder addition of 3-butenoyl-1,3-oxazolidin2-one to cyclopentadiene under the influence of catalytic amounts of dichloro-Ti complexes of a,a,a’,a’-tetraaryl-1,3-dioxolane-4,5-dimethanols (TADDOLs) is described. The influence of the mode of catalyst preparation, amount of catalyst, presence of molecular sieves, concentration of the reactands, temperature, solvent, and TADDOL structure on this reaction is studied. Best results (enantiomer ratio er 94:6) are obtained with the TADDOL-bearing Arl = C& and CsHj/CH3 substituents in the dioxolane 2-position (47) and with the Cz-symmetrical TADDOL with Arl = 2-naphthyl and two CH3 groups on the dioxolane 2-position (16). A surprising reversal of the absolute topicity of the reaction is observed with TADDOLs (15,28,32,38) bearing four 1-naphthyl groups: the 2(R)- instead of the 2(S)-bicyclo[2.2.1]hept-5-en-2-carboxylic acid derivatives are formed with enantioselectivities of up to 86:14. The crystal structures of several TADDOLs (16, 28, 47) and of the tetracyclohexylanalog 50 are described and compared with previously determined structures. A superposition of 29 structures reveals that the cyclic array of atoms of the TADDOLate moiety always has two axial and two equatorial aryl groups in a A-type conformation when derived from (R$)-tartrate and in a 8-type conformation when derived from (S,S)-tartrate. The binaphthols (BINOLs) show similar structural features (A in (P) or (SI and d in (MI or (R) enantiomers). A mnemonic rule is disclosed which applies to the steric course of Ti-BINOLate- and Ti-TADDOLatemediated reactions involving monodentate and bidentate electrophiles. The possible structure of the reactive complex involved in enantioselective reactions mediated by Ti-BINOLates and -TADDOLates, i.e., inter- and intramolecular [2 + 21 and [4 + 21 cycloadditions and ene reactions, is discussed.

Understanding barriers to internal rotation in substituted toluenes and their cations

Abstract: In substituted toluenes, the potential energy barrier to internal methyl rotation and the preferred methyl conformation depend on the position of the fluorine, amino, or methyl substituents and also on the electronic state, either S0, S1, or ground state cation. We present a unified picture of the electronic factors controlling these effects. In S0 and cation, ab initio electronic structure calculations of modest scale produce rotor potentials in good agreement with experiment. The methyl group provides a sensitive probe of local ring geometry. When the geometry of the ring in the vicinity of the rotor has good local C2v symmetry, the barrier is invariably small. In S0 ortho‐substituted toluenes, we use natural steric analysis to show that repulsive steric interactions between the halogen lone pair and the methyl CH bonds dominate over attractive donor–acceptor interactions to favor the pseudo‐trans conformation. When steric interactions are unimportant, the key determinant of rotor barrier height is the ...

Comparative study of the catalytic oxidation of catechols by copper(II) complexes of tripodal ligands

Abstract: Copper(II) complexes of the ligands tris(2-pyridylmethyl)amine (tpyma), tris(2-pyridylethyl)amine (tpyea), tris(3,5-dimethylpyrazol-1-ylmethyl)amine (tpzma) and tris(3,5-dimethylpyrazol-1-ylethyl)-amine (tpzea) were prepared. The complexes, [Cu(ligand)Cl]Cl or [Cu(ligand)(H2O)][BF4]2, were characterized by a combination of absorption and EPR spectroscopies and chemical analysis. The ability of the complexes to oxidize 3,5-di-tert-butylcatechol to 3,5-di-tert-butyl-o-benzoquinone has been studied and the results show that the rate of reaction is dependent on the nature of the heterocyclic donor, its basicity, steric considerations, the chelate ring size and the type of exogenous donor present. Large variations in the rate were observed with the most effective catalysts being those with pyridine donors which formed six-membered chelate rings; the complex [Cu(tpyea)(H2O)][BF4]2 was the most active while [Cu(tpzea)(H2O)][BF4]2 and [Cu(tpzea)Cl]Cl were inactive. Electrochemical data for the series of compounds show that there is a non-linear relationship between their ability to oxidize catechols and their reduction potentials. The most effective catalysts were those complexes which exhibited reduction potentials close to 0.00 V, while those that deviated from that potential by 200–300 mV in either direction were largely inactive. Within the range of complexes which were active, a steric match between the substrate and the complex also largely defined their reactivity. Comparisons to the biological system tyrosinase are drawn.

Quantum Chemical Calculation on the Desulfurization Reactivities of Heterocyclic Sulfur Compounds

Abstract: The relative hydrodesulfurization reactivities of heterocyclic sulfur compounds collected from our experiments and the literature were correlated with their electronic parameters estimated using MOPAC-PM3. It is found that the direct hydrogenolytic S elimination (direct hydrogenolysis) of sulfur-containing compounds is definitely correlated with the electron density of their S atoms, while the hydrogenation of the thiophenic ring or neighboring ring prior to hydrogenolysis correlated with the bond order of the most unsaturated bond in the ring. Methyl groups at 4-and/or 6-positions of dibenzothiophene may sterically retard direct hydrogenolysis. Hence, the hydrogenation of the aromatic ring adjacent to the thiophenic ring followed by the hydrogenolysis is a major route for such species. The molecular orbital calculation clarified that the hydrogenation of refractory sulfur compounds with alkyl steric hindrance accelerates their hydrogenolysis by reducing the steric hindrance through molecular puckering and by increasing the electron density on S. The calculation allows us to estimate reactivities of sulfur-containing compounds in both routes and to design the reaction scheme to ensure the most efficient hydrodesulfurization

Absolute Rate Constants for the Addition of Benzyl (PhĊH2) and Cumyl (PhĊMe2) Radicals to Alkenes in Solution

Abstract: Absolute rate constants and their temperature dependence were determined by time-resolved electron spin resonance for the addition of the radicals PhĊH2 and PhĊMe2 to a variety of alkenes in toluene solution. To vinyl monomers CH2=CXY, PhĊH2 adds at the unsubstituted C-atom with rate constants ranging from 14 M−1S−1 (ethoxyethene) to 6.7 · 103M−1S−1 (4-vinylpyridine) at 296 K, and the frequency factors are in the narrow range of log (A/M−1S−1) = 8.6 ± 0.3, whereas the activation energy varies with the substituents from ca. 51 kJ/mol to ca. 26 kJ/mol. The rate constants and the activation energies increase both with increasing exothermicity of the reaction and with increasing electron affinity of the alkenes and are mainly controlled by the reaction enthalpy, but are markedly influenced also by nucleophilic polar effects for electron-deficient substrates. For 1,2-disubstituted and trisubstituted alkenes, the rate constants are affected by additional steric substituent effects. To acrylate and styrenes, PhĊMe2 adds with rate constants similar to those of PhĊH2, and the reactivity is controlled by the same factors. A comparison with relative-rate data shows that reaction enthalpy and polar effects also dominate the copolymerization behavior of the styrene propagation radical.

Conformational behavior of six-membered rings : analysis, dynamics, and stereoelectronic effects

Abstract: 100 years of conformational analysis stereodynamics of cyclohexane and substituted cyclohexanes, substitutent A-values ab initio studies of six-membered rings, present status and future developments molecular mechancis calculations of six-membered rings stereoelectronic effects in six-membered rings conformational analysis of six-membered sulfur-containing heterocycles steric and stereoelectronic effects in 1, 3, 2-dioxaphosphorinanes.

Chiral Organometallic Reagents, XVI. Enantiomerization of α-Thio-, α-Seleno-, and α-Telluro-Substituted Alkyllithium Compounds; Kinetic and Mechanistic Studies

Abstract: The rate of enantiomerization of the racemic α-phenylselenoalkyllithium compound 6 has been determined by dynamic NMR spectroscopy in [D8]THF. The enantiomerization rate was found to be first order with respect to monomeric 6 and to show no conspicuous solvent dependence (diethyl ether; toluene + 1 eq. of THF) or change upon addition of LiClO4. The marked steric effects on the enantiomerization rate found with the α-duryl- and α-mesityl-selenoalkyllithium compounds 7c and 7d suggest that rotation about the carbanion-selenium bond may be the rate-determining step in those sterically hindered systems. Similar steric effects were detected for the enantiomerization of the corresponding α-arylthio- and α-aryltelluroalkyllithium compounds 7j and 7f, but are absent with the α-arylsilyl-substituted alkyllithium compound 7o. This finding, along with the fact that the phenyltelluro- (7e), phenylseleno- (6), and phenylthio-alkyllithium compounds (7g) have essentially the same enantiomerization barrier, lead us to propose that in these cases a reorganization within the contact ion pair is the rate limiting step for the enantiomerization.

Catalytic asymmetric hydroboration with heterotopic P-N ligands: Trends in enantioselectivity with increased steric demand

Abstract: The rhodium complexes 1 and 3 catalyse asymmetric hydroboration with differing selectivity originating in the disparate steric environments of the catalysts.

The molten helix : effects of solvation on the alpha - to 310-helical transition

Abstract: Free energy surfaces, or potentials of mean force, for the alpha- to 3(10)-helical conformational transition in polypeptides have been calculated in several solvents of different dielectric. The alpha- to 3(10)-helical transition has been suggested as potentially important in various biological processes, including protein folding, formation of voltage-gated ion channels, kinetics of substrate binding in proteins, and signal transduction mechanisms. This study investigates the thermodynamics of the alpha- to 3(10)-helical transition of a model peptide, the capped decamer of alpha-methylalanine, in order to assess the plausibility of this transition in the mechanisms of such biological processes. The free energy surfaces indicate that in each environment studied the alpha-helical conformation is the more stable of the two for the decapeptide, The thermodynamic data suggest that the alpha-helix is energetically stabilized and the 3(10)-helix is entropically favored. The inclusion of dichloromethane, acetonitrile, or water results in approximately 7 kcal/mol of relative conformational energy (favoring the alpha-helix) and 3 kcal/mol of relative conformational entropy (favoring the 3(10)-helix) in comparison to the gas phase. In polar environments, the alpha-helix is stabilized by its more favorable salute-solvent electrostatic interactions, and solute-solute steric interactions. In addition, it was concluded that in polar solvents, especially water, it is possible for the peptide to reduce some of the inherent strain of the 3(10)-helix by widening psi, the resulting weaker intrasolute hydrogen bonds being compensated for by increased-hydrogen bonding to the solvent. Lower polarity environments are associated with a marginally increased relative stability of the 3(10)-helix, which we suggest is largely due to the additional intrahelical hydrogen bond of this conformation. The data suggest that, in environments such as membranes, the interior of proteins or crystals, the complete transition from an ct-helix to a 3(10)-helix for this decapeptide would require less than 6 kcal/mol in free energy. Switching conformations for individual residues is much more facile, and shorter 3(10)-helices may actually be energetically favored, at least, in nonpolar environments. This study primarily estimates the backbone contribution to the helical transition; side chain interactions would be expected to play a significant role in stabilizing one conformer relative to the other. It is, therefore, quite feasible that the alpha- to 3(10)-helical transition could provide a possible mechanism for many biological processes. While there are many factors, such as helix length and side chain packing, that contribute to the selection of either the alpha- or the 3(10)-helical conformation or a mixture of the two, this study focuses primarily on one of these effects, that of the polarity of the environment.

Studies of the Bound Conformations of Methyl α-Lactoside and Methyl β-Allolactoside to Ricin B Chain Using Transferred NOE Experiments in the Laboratory and Rotating Frames, Assisted by Molecular Mechanics and Dynamics Calculations

Abstract: The conformation in solution of methyl β-galactopyranosyl-(14)-α-glucopyranoside (methyl α-lacto-side) and methyl β-galactopyranosyl-(16)-β-glucopyranoside (methyl β-allolactoside) has been studied through NMR spectroscopy and molecular mechanics calculations. NOE measurements both in the laboratory and rotating frames, have been interpreted in terms of an ensemble average distribution of conformers. Molecular mechanics calculations have been performed to estimate the probability distribution of conformers from the steric energy maps. The experimental results indicate that methyl α-lactoside spends about 90% of its time in a broad low-energy region close to the global minimum, while methyl β-allolactoside presents much higher flexibility. The conformational changes that occur when both disaccharides are bound to the ricin B chain in aqueous solution have been studied using transferred NOE experiments at several protein/ligand ratios. The observed data indicate that the protein causes a conformational variation in the torsion angles of methyl α-lactoside changing towards smaller angle values (ф/Ψ≈–20/–20), although the recognized conformer is still within the lowest energy region. In particular, the torsional changes separate Gal HI from Glc H3 and Glc H6 protons, with a noticeable decrease in the intensities of the corresponding NOE cross-peaks, which were clearly observed for the free disaccharide. On the other hand, different conformations around the ф, Ψ, and ω glycosidic bonds of methyl β-allolactoside are recognized by the lectin. In fact, for the methyl-β-allolactoside–ricin-B complex, only the NOESY cross-peaks corresponding to the protons of the galactose residue are negative, as expected for a molecule in the slow motion regime. In contrast, the corresponding cross peaks for the glucose residue were about zero, as expected for a molecule whose motion is practically independent of the protein. However, for the methyl-α-lactoside-ricin-B complex, all the NOESY cross-peaks for both the galactose and glucose moieties were clearly negative. From the NMR experimental point of view, it is demonstrated that the comparison of longitudinal and transversal transferred NOEs allows one to clearly differentiate direct enhancements from spin diffussion effects, which are of major concern when analysing NOE spectra of macromolecules. Finally, molecular modelling of both disaccharides in the binding site strongly suggests that, for methyl α-lactoside, apart from the expected contacts between the galactose moiety and different amino acid residues, there are also van der Waals' contacts between the protein and the remote glucose moiety, as previously deduced from binding studies using modified lactoside derivatives [Solis, D., Fernandez, P., Diaz-Maurino, T., Jimenez-Barbero, J. & Martin-Lomas, M. (1993) Eur. J. Biochem. 214, 677–683]. This result is in contrast with the X-ray crystallographic analysis of the ricin-B–lactose and ricin-B–galactose-containing diantennary hexasaccharide complexes [Rutenber, E. & Robertus, J. D. (1991) Proteins 10, 260–269]. On the other hand, for methyl β-allolactoside, the experimental data and the modelling studies demonstrate that only the galactose moiety is bound by the lectin and, therefore, the conformation around the glycosidic angles can be probably described by a distribution similar to that existing in free solution.