Synthesis and Hydrogen Bonding Capabilities of Biphenyl-Based Amino Acids Designed To Nucleate beta-Sheet Structure

The syntheses of 3'-(aminoethyl)-2-biphenylpropionic acid (1) and 2-amino-3'-biphenylcarboxylic acid (2) are described. These residues were designed to nucleate beta-sheet structure in aqueous solution when incorporated into small, amphiphilic peptides in place of the backbone of the i + 1 and i + 2 residues of the beta-turn. N-Benzyl-3'-(2-(benzylamido)ethyl)-2-biphenylpropamide (3) and N-benzyl-(2-benzylamido)-3'-biphenylamide (4) were synthesized and studied as model compounds to investigate the hydrogen-bonding capabilities of residues 1 and 2, respectively. The X-ray crystal structure of 3 indicates that a 13-membered intramolecular hydrogen-bonded ring is formed, while the remaining amide proton and carbonyl are involved in intermolecular hydrogen bonding. Infrared and variable-temperature NMR experiments indicate that, in solution (CH(2)Cl(2)), 3 exists as an equilibrium mixture of the 13- and the 15-membered intramolecularly hydrogen-bonded conformers with the 15-membered ring conformer being favored. Amide 4 was shown to exist in solution (CH(2)Cl(2)) as an equilibrium mixture of the 11-membered intramolecular hydrogen-bonded ring and a nonbonded conformation. No contribution from the 9-membered hydrogen-bonded ring conformation was observed. The X-ray crystal structure of 4 indicated the absence of intramolecular hydrogen bonding in the solid state.     

Synthesis and Conformational Preferences of a Potential beta-Sheet Nucleator Based on the 9,9-Dimethylxanthene Skeleton

A 4,5-disubstituted-9,9-dimethylxanthene-based amino acid (10) has been synthesized for incorporation into peptide sequences which have a propensity to adopt beta-sheet structure. Molecular dynamics studies support the FT-IR and NMR results which demonstrate that amides based on this residue utilize the NH and the C=O from the xanthene residue to form an intramolecular hydrogen bond (13-membered ring), unlike the previously studied dibenzofuran-based amino acid residues in which the NH and the C=O of the attached amide groups participate in intramolecular hydrogen bonding (15-membered ring). Interestingly, residue 10 derivatized as a simple amide prefers to adopt a trans conformation where the aliphatic side chains are placed on opposite sides of the plane of the 9,9-dimethylxanthene ring system. This is different than the conformational preferences of the dibenzofuran-based amino acids which adopt a cis conformation that is preorganized to nucleate beta-sheet formation. It will be interesting to see how these conformational differences effect nucleation in aqueous solution.     

Anion and carboxylic acid binding to monotopic and ditopic amidopyridine macrocycles

Binding of inorganic anions, carboxylic acids, and tetraalkylammonium carboxylates by macrocyclic compounds of different size was studied by NMR in DMSO-d6. It has been shown that at least a 15-membered ring is necessary for successful recognition of fluoride. Larger macrocycles were shown to bind HSO4(-), H2PO4(-), Cl(-), and carboxylic acid salts. Effects of binding topicity are discussed. The 30-membered macrocycles 4 and 4m selectively bind substrates that are size- and shape-complementary: maximum binding is observed for dicarboxylic acids and dicarboxylates with four-carbon chains, and the binding constant for association of fumaric acid and 4 is ca. 5 orders of magnitude higher than that of maleic acid. The 30-membered macrocycle 4m showed selectivity toward alpha-ketocarboxylic acids. Secondary amino groups were not crucial for binding of fluoride to the macrocycles; however, they proved to be very important for selectivity and strength of carboxylic acid binding. The X-ray structure of the adduct of 4 and nitrobenzoic acid confirmed the guest H-bonding with both the amide and the secondary amino groups of the 30-membered macrocyclic host.     

Triple helical structure constructed by covalent bondings: effective synthesis by a pre-organized partial structure and helicity induced by aromatic-aromatic interactions

Discotic helical macrocycle constructed by aromatic tertiary amide was effectively synthesized using dichlorotriphenylphosphorane as the condensation reagent. The yield was significantly higher when meta-substituted diaminobenzene was used as the linkage for a two 1,3,5-tris(4-carboxyphenyl)benzene moiety than when para-substituted diaminobenzene was used. The yield is thus dependent on whether the pre-organized partial diamide structure suits the construction of the macrocyclic structure.     

Cyclization and rearrangement reactions of a(n) fragment ions of protonated peptides

a(n) ions are frequently formed in collision-induced dissociation (CID) of protonated peptides in tandem mass spectrometry (MS/MS) based sequencing experiments. These ions have generally been assumed to exist as immonium derivatives (-HN(+)═CHR). Using a quadrupole ion trap mass spectrometer, MS/MS experiments have been performed and the structure of a(n) ions formed from oligoglycines was probed by infrared spectroscopy. The structure and isomerization reactions of the same ions were studied using density functional theory. Overall, theory and infrared spectroscopy provide compelling evidence that a(n) ions undergo cyclization and/or rearrangement reactions, and the resulting structure(s) observed under our experimental conditions depends on the size (n). The a(2) ion (GG sequence) undergoes cyclization to form a 5-membered ring isomer. The a(3) ion (GGG sequence) undergoes cyclization initiated by nucleophilic attack of the carbonyl oxygen of the N-terminal glycine residue on the carbon center of the C-terminal immonium group forming a 7-membered ring isomer. The barrier to this reaction is comparatively low at 10.5 kcal mol(-1), and the resulting cyclic isomer (-5.4 kcal mol(-1)) is more energetically favorable than the linear form. The a(4) ion with the GGGG sequence undergoes head-to-tail cyclization via nucleophilic attack of the N-terminal amino group on the carbon center of the C-terminal immonium ion, forming an 11-membered macroring which contains a secondary amine and three trans amide bonds. Then an intermolecular proton transfer isomerizes the initially formed secondary amine moiety (-CH(2)-NH(2)(+)-CH(2)-NH-CO-) to form a new -CH(2)-NH-CH(2)-NH(2)(+)-CO- form. This structure is readily cleaved at the -CH(2)-NH(2)(+)- bond, leading to opening of the macrocycle and formation of a rearranged linear isomer with the H(2)C═NH(+)-CH(2)- moiety at the N terminus and the -CO-NH(2) amide bond at the C terminus. This rearranged linear structure is much more energetically favorable (-14.0 kcal mol(-1)) than the initially formed imine-protonated linear a(4) ion structure. Furthermore, the barriers to these cyclization and ring-opening reactions are low (8-11 kcal mol(-1)), allowing facile formation of the rearranged linear species in the mass spectrometer. This finding is not limited to 'simple' glycine-containing systems, as evidenced by the IRMPD spectrum of the a(4) ion generated from protonated AAAAA, which shows a stronger tendency toward formation of the energetically favorable (-12.3 kcal mol(-1)) rearranged linear structure with the MeHC═NH(+)-CHMe- moiety at the N terminus and the -CO-NH(2) amide bond at the C terminus. Our results indicate that one needs to consider a complex variety of cyclization and rearrangement reactions in order to decipher the structure and fragmentation pathways of peptide a(n) ions. The implications this potentially has for peptide sequencing are also discussed.     

Anion binding versus intramolecular hydrogen bonding in neutral macrocyclic amides

Although amide groups are important hydrogen-bond donors in natural and synthetic anion receptors, studies on structure-affinity relationships of amide-based macrocyclic receptors are still very limited. Therefore, we synthesized a series of macrocyclic tetraamides 5-8 derived from 1,3-benzenedicarboxylic (isophthalic) acid and aliphatic alpha,omega-diamines of different lengths. (1)H NMR titrations in DMSO solution show that the anion affinity of these receptors decreases with increasing size of the macrocycle irrespective of the anion, and this suggests a minor role of geometric complementarity. Comparison with their previously studied pyridine congeners reveals that the isophthalic acid based macrocycles are less potent, in contrast to what was found for simple model diamides. Combined theoretical and experimental structural studies were carried out to determine the reasons behind this behaviour. The results show that the unexpectedly low anion binding ability of the isophthalic acid-based receptors is due to the self-complementary nature of the isophthalic bis-amide fragments: when two such moieties are present within a sufficiently flexible macrocycle, they adopt syn-anti conformations and bind each other by two strong intramolecular hydrogen bonds that close the macrocyclic cavity. Nevertheless, anion binding is able to break these hydrogen bonds and switch a macrocycle into a convergent all-syn conformation. Despite the ill-preorganized conformation, 20-membered receptor 6 is better than either its open-chain analogue (macrocyclic effect) and/or its isomer having differently placed carbonyl groups. The crystal structures of four anion complexes of the macrocyclic receptors are reported. X-ray studies and solution NMR data confirmed the inclusive nature of the complexes and pointed to strong involvement of aromatic CH hydrogen atoms in anion binding.     

Is [N+-H...O] hydrogen bonding the most important noncovalent interaction in macrocycle-dibenzylammonium ion complexes?

We report a new host molecule in which one diethylene glycol chain (i.e., a loop possessing only three oxygen atoms) incorporated along with two phenolic aromatic rings is linked by a xylene spacer into a macroring. The design of the molecular structure of this macrocycle "amplifies" any potential [cation...pi], [N+-H...pi], and [N+C-H...pi] interactions between the dibenzylammonium (DBA+) ion and the phenolic rings of the macrocycle; as such, these species display a very strong binding affinity in CD3NO2 (Ka = 15,000 M(-1)). The macroring also coordinates to bipyridinium ions in a [2]pseudorotaxane fashion, which makes it the smallest macrocycle (i.e., a 25-membered ring) known to complex both DBA+ and bipyridinium ions in solution. To confirm unambiguously that these pseudorotaxanes exist in solution, we synthesized their corresponding interlocked molecules, namely rotaxanes and catenanes.     

Negative ion mass spectra of Cys-containing peptides. The characteristic Cys gamma backbone cleavage: a joint experimental and theoretical study

The Cys residue initiates characteristic backbone cleavages of [M-H](-) anions of Cys-containing peptides. A combination of experiment and theory suggests that these processes are initiated by molecular recognition between the C-terminal CONH(-) group (in this study all peptides have C-terminal CONH(2) groups) and the SH in the Cys side chain to form an S-H...O=C hydrogen bond. This process is exothermic by 60 kJ mol(-1) (calculations at the HF/6-31G(d)//AM1 level of theory). The structure of this reactive intermediate has the NH(-) of the amide group and the central CH of the Cys residue locked into position such that these groups effect an S(N)2 process to form an intermediate which can either (i) dissociate to give an RNH(-) species [the delta ion (process endothermic by 37 kJ mol(-1) with a barrier of 132 kJ mol(-1))], or (ii) effect deprotonation within the intermediate to eliminate RNH(2) to give the gamma backbone cleavage anion in a reaction exothermic by 40 kJ mol(-1) with a barrier of 132 kJ mol(-1). Collision-induced mass spectra of the [M-H](-) anions of five selected Cys-containing peptides all contain gamma and (gamma-H(2)S) anions. Three of these spectra also show the less favoured delta cleavage anions.     

Stable conformations of tripeptides in aqueous solution studied by UV circular dichroism spectroscopy

Determination of the precise solution structure of peptides is of utmost importance to the understanding of protein folding and peptide drugs. Herein, we have measured the UV circular dichroism (UVCD) spectra of tri-alanine dissolved in D(2)O, H(2)O, and glycerol. The results clearly show the coexistence of a polyproline II or 3(1)-helix and a somewhat disordered flat beta-strand conformation, in complete agreement with recent predictions from spectroscopic data (Eker et al. J. Am. Chem. Soc. 2002, 124, 14 330-14 341). A thermodynamic analysis revealed that enthalpic contributions of about 11 and 17 kJ/mol stabilize polyproline II in D(2)O and H(2)O, respectively, but at room temperature they are counterbalanced by entropic contributions, which clearly favor the more disordered beta-strand conformation. It is hypothesized that this delicate balance is the reason for the variety of structural propensities of amino acid residues in the absence of nonlocal interactions. The isotope effect yielding a higher occupation of polyproline II in H(2)O with respect to D(2)O strongly suggests that a hydrogen-bonding network involving the peptide and water molecules in the hydration shell plays a major role in stabilizing this conformation. The equilibrium between polyproline II and beta-strand is practically maintained in glycerol, which suggests that glycerol can substitute water as stabilizing solvent for the polyproline II conformation. We also measured the UVCD spectra of tri-valine and tri-lysine (both at acidic pD) in D(2)O and found them to adopt a flat beta-strand and left-handed turn structure, respectively, in accordance with recent analyses of vibrational spectroscopy data. Generally, the present study adds substantial evidence to the notion that the so-called random coil state of peptides is much more structured than generally assumed.     

Double-helical cyclic peptides: design, synthesis, and crystal structure of figure-eight mirror-image conformers of adamantane-constrained cystine-containing cyclic peptide cyclo (Adm-Cyst)(3)

A large number of macrocycles containing alternating repeats of cystine diOMe(-NH-CH(CO(2)Me)-CH(2)-S-)(2) and either a conformationally rigid aromatic/alicyclic moiety or a flexible polymethylene unit (X) in the cyclic backbone with ring size varying from 13- to 78-membered have been examined by spectral ((1)H NMR, FT-IR, CD) and X-ray crystallography studies for unusual conformational preferences. While (1)H NMR measurements indicated a turnlike conformation for all macrocycles, stabilized by intramolecular NH.CO hydrogen bonding, as also supported by FT-IR spectra in chloroform, convincing proof for beta-turn structures was provided by circular dichroism studies. Single-crystal X-ray studies on 39-membered cyclo (Adm-L-Cyst)(3) revealed a double-helical fold (figure-eight motif) for the macrocycle. Only a right-handed double helix was seen in the macrocycle constructed from L-cystine. The mirror-image macrocycle made up of D-cystine units exhibited a double helix with exactly the opposite screw sense, as expected. The enantiomeric figure-eights were stabilized by two intramolecular NH. CO hydrogen bonds and exhibited identical (1) H NMR and FT-IR spectra. The CD spectra of both isomers had a mirror-image relationship. The present results have clearly brought out the importance of cystine residues in inducing turn conformation that may be an important deciding factor for the adoption of topologically important structures by macrocycles containing multiple S-S linkages.     

Coupling of intramolecular hydrogen bonding to the cis-to-trans isomerization of a proline imide bond of small model peptides

A relationship between intramolecular hydrogen bonding and the cis-trans isomerization of a proline imide bond for proline-containing short peptides were studied by proton NMR and infrared spectroscopy using DMSO-d6/CDCl3 mixed solvents. The percentage of the trans form increases with increasing fraction of CDCl3 in the mixed solvents except for compounds without possibility of intramolecular hydrogen bonding. Chemical shift variations of amide protons with solvent mixing ratios were found to be useful for judging whether the amide protons take part in the intramolecular hydrogen bonding to a considerable degree or not. These results and infrared spectra were used to specify intramolecularly hydrogen bonded structures of the peptides. Formation of the 10-membered or 13-membered hydrogen bonded ring which includes the carbonyl group precedent to the prolyl residue facilitates the cis-to-trans isomerization and these hydrogen bonded rings are strong enough to restrict the proline imide bond to the trans form in CDCl3 solution. On the other hand, a 7-membered hydrogen bonded ring is not so effective in restricting the proline imide bond.     

Electronic control of amide cis-trans isomerism via the aromatic-prolyl interaction

The cis-trans isomerization of prolyl amide bonds results in large structural and functional changes in proteins and is a rate-determining step in protein folding. We describe a novel electronic strategy to control cis-trans isomerization, based on the demonstration that interactions between aromatic residues and proline are tunable by aromatic electronics. A series of peptides of sequence TXPN, X = Trp, pyridylalanine, pentafluorophenylalanine, or 4-Z-phenylalanine derivatives (Z = electron-donating, electron-withdrawing, or electron-neutral substituents), was synthesized and Ktrans/cis analyzed by NMR. Electron-rich aromatic residues stabilized cis amide bond formation, while electron-poor aromatics relatively favored trans amide bond formation. A Hammett correlation between aromatic electronics and cis-trans isomerization was observed. These results indicate that the interaction between aromatic residues and proline, which is observed to stabilize cis amide bonds and is also a general stabilizing interaction ubiquitous in proteins and protein-protein complexes, is not stabilized exclusively by a classical hydrophobic effect. To a large extent, the aromatic-prolyl interaction is driven and controllable by an electronic effect between the aromatic ring pi-electrons and the proline ring, consistent with a C-H-pi interaction as the key stabilizing force. The aromatic-prolyl interaction is electronically tunable by 0.9 kcal/mol and is enthalpic in nature. In addition, by combining aromatic ring electronics and stereoelectronic effects using 4-fluoroprolines, we demonstrate broad tuning (2.0 kcal/mol) of cis-trans isomerism in tetrapeptides. We demonstrate a simple tetrapeptide, TWflpN, that exhibits 60% cis amide bond and adopts a type VIa1 beta-turn conformation.     

Quantum chemical estimation of the macrocyclization of 1,4-dithiine

Ab initio quantum chemical methods are employed to study the spatial and electronic structure of a 1,4-dithiine molecule. Calculations are performed with electron correlation treatment by MP2 and B3LYP methods in 6-311++G(d,p) and 6-311G basis sets. It is found that the macrocyclization of 1,4-dithiine is energetically favorable. The addition of each subsequent dithiine ring decreases the relative energy of the system by ??0.5÷2.3 kcal/mol. The cycle consisting of 7 dithiine rings is the most comfortable from the standpoint of the internal strain of the macrocycle. With an increase in the number of rings up to 10, the macrocycle starts to break into 4-, 5-, 6-, or 7-membered fragments. The formation of a nanotube based on a 7-membered macrocycle is energetically favorable. The inner diameter of this nanotube is 6.9     

Formal synthesis of (-)-apicularen A

[reaction: see text] A formal synthesis of (-)-apicularen A has been completed. The synthesis features a cyanohydrin acetonide coupling as a convergent approach to the C9-C18 segment and an intramolecular Diels-Alder addition sequence to create both the 10-membered macrocycle and the aromatic ring.     

A Comparison of the Effects of Amide and Acid Groups at the C-Terminus on the Collision-Induced Dissociation of Deprotonated Peptides

The dissociative behavior of peptide amides and free acids was explored using low-energy collision-induced dissociation and high level computational theory. Both positive and negative ion modes were utilized, but the most profound differences were observed for the deprotonated species. Deprotonated peptide amides produce a characteristic c(m-2) (-) product ion (where m is the number of residues in the peptide) that is either absent or in low abundance in the analogous peptide acid spectrum. Peptide acids show an enhanced formation of c(m-3) (-); however, this is not generally as pronounced as c(m-2) (-) production from amides. The most notable occurrence of an amide-specific product ion is for laminin amide (YIGSR-NH(2)) and this case was investigated using several modified peptides. Mechanisms involving 6- and 9-membered ring formation were proposed, and their energetic properties were investigated using G3(MP2) molecular orbital theory calculations. For example, with C-terminal deprotonation of pentaglycine amide, formation of c(m-2) (-) and a 6-membered ring diketopiperazine neutral requires >31.6?kcal/mol, which is 26.1?kcal/mol less than the analogous process involving the peptide acid. The end group specific fragmentation of peptide amides in the negative ion mode may be useful for identifying such groups in proteomic applications.     

Vibrational Raman optical activity characterization of poly(l-proline) II helix in alanine oligopeptides

A vibrational Raman optical activity (ROA) study of a series of alanine peptides in aqueous solution is presented. The seven-alanine peptide Acetyl-OOAAAAAAAOO-Amide (OAO), recently shown by NMR and UVCD to adopt a predominantly poly(l-proline II) (PPII) helical conformation in aqueous solution, gave an ROA spectrum very similar to that of disordered poly(l-glutamic acid) which has long been considered to adopt the PPII conformation, both being dominated by a strong positive extended amide III ROA band at approximately 1319 cm-1 together with weak positive amide I ROA intensity at approximately 1675 cm-1. A series of alanine peptides Ala2-Ala6 studied in their cationic states in aqueous solution at low pH displayed ROA spectra which steadily evolved toward that of OAO with increasing chain length. As well as confirming that alanine peptides can support the PPII conformation in aqueous solution, our results also confirm the previous ROA band assignments for PPII structure, thereby reinforcing the foundation for ongoing ROA studies of unfolded and partially folded proteins.     

Properties of a new 4-imidazolyl derivative of a 14-membered tetraazamacrocyclic chelating agent

A new bis-N,N'-(5-methylimidazol-4-ylmethyl) derivative of a 14-membered tetraazamacrocycle, L1, has been synthesized. The protonation constants of this compound and the stability constants of its complexes with divalent first-row transition metal ions and Fe3+ were determined at 298.2 K in aqueous 0.10 mol dm(-3) KNO3. Compound L1 exhibits high overall basicity, which is mainly conferred by the imidazolyl groups. The complexes of the divalent first row-transition metal ions of L1 follow the Irving-Williams order of stability with the maximum for Cu2+ as expected, but a steep fall of constants is verified for the Mn2+, Fe2+ and Co2+, in one side, and for the Zn2+ complexes, in the other side. Additionally, L1 shows a large affinity for Fe3+, and the relative stability constants for its Cd2+ and Pb2+ complexes indicate that L1 may be useful for the complexometric determination of these two toxic metal ions in solutions containing both metal ions. These studies together with NMR, UV-vis and EPR spectroscopic data indicated the presence of mononuclear complexes, which adopt distorted pyramidal or octahedral geometries depending on the metal centre. The X-ray crystal structure of [Cu(HL1)](PF6)2(NO3).H2O showed that the coordination sphere of the copper centre can be described as a distorted square pyramid with the basal plane defined by three nitrogen donors of the macrocycle backbone and one nitrogen atom from one imidazolyl pendant arm. The apical position is occupied by the nitrogen atom of the macrocycle trans to the pyridine ring. To achieve this coordination environment, the macrocycle is folded along the axis defined by the two N atoms contiguous to the pyridine ring. The free methylimidazolyl arm points away from the metal centre leading to an intramolecular Cu[dot dot dot]N distance of 5.155(1) A.     

Estimation on the intramolecular 10-membered ring N-H...O=C hydrogen-bonding energies in glycine and alanine peptides

Computation of accurate intramolecular hydrogen-bonding energies for peptides is of great importance in understanding the conformational stabilities of peptides and developing a more accurate force field for proteins. We have proposed a method to determine the intramolecular seven-membered ring N-H...O=C hydrogen-bonding energies in glycine and alanine peptides. In this article, the method is further applied to evaluate the intramolecular 10-membered ring N-H...O=C hydrogen-bonding energies in peptides. The optimal structures of the intramolecular 10-membered ring N-H...O=C hydrogen bonds in glycine and alanine tripetide molecules are obtained at the MP2 level with 6-31G(d), 6-31G(d,p), and 6-31+G(d,p) basis sets. The intramolecular 10-membered ring N-H...O=C hydrogen-bonding energies are then evaluated based on our method at the MP2/6-311++G(3df,2p) level with basis set superposition error correction. The intramolecular 10-membered ring N-H...O=C hydrogen-bonding energies are calculated to be in the range of -6.84 to -7.66, -4.44 to -4.98, and -6.95 to -7.88 kcal/mol. The method is also applied to estimate the individual intermolecular hydrogen-bonding energies in the dimers of amino-acetaldehyde, 2-amino-acetamide, formamide, and oxalamide, each dimer having two identical intermolecular hydrogen bonds. According to our method, the individual intermolecular hydrogen-bonding energies in the four dimers are calculated to be -1.77, -1.67, -6.35, and -4.82 kcal/mol at the MP2/6-311++G(d,p) level, which are in good agreement with the values of -1.84, -1.72, -6.23, and -4.93 kcal/mol predicted by the supermolecular method.     

Quantitative determination of intermolecular interactions with fluorinated aromatic rings

The chemical double mutant cycle approach has been used to investigate substituent effects on intermolecular interactions between aromatic rings and pentafluorophenyl pi-systems. The complexes have been characterised using 1H and 19F NMR titrations, X-ray crystal structures of model compounds and molecular mechanics calculations. In the molecular zipper system used for these experiments, H-bonds and the geometries of the interacting surfaces favour the approach of the edge of the aromatic ring with the face of the pentafluorophenyl pi-system. The interactions are generally repulsive and this repulsion increases with more electron-withdrawing substituents up to a limit of +2.2 kJ mol(-1), when the complex distorts to minimise the unfavourable interaction. Strongly electron-donating groups cause a change in the geometry of the aromatic interaction and attractive stacking interactions are found (-1.6 kJ mol(-1) for NMe2). These results are generally consistent with an electrostatic model: the polarisation of the pentafluorophenyl ring leads to a partial positive charge located at the centre and this leads to repulsive interactions with the positive charges on the protons on the edge of the aromatic ring; when the aromatic ring has a high pi-electron density there is a large electrostatic driving force in favour of the stacked geometry which places this pi-electron density over the centre of the positive charge on the pentafluorophenyl group.     

Solvent-assisted catalysis mechanism on Keto–enol tautomerism of cyameluric acid

The keto–enol tautomerism of cyameluric acid, both in gas phase and in water and methanol solution, has been studied at the B3LYP/6-31++g(d,P) level of theory in this paper. The harmonic frequencies of all the structures are calculated. The results show that the transition states of the tautomerism are 4-membered ring conformations in gas phase, whereas 6-membered ring conformations in solution. In the first proton transfer, activation energy ΔE^# is 56.4 and 50.9 kJ/mol for water and methanol solution, respectively, which is much lower than that in gas phase (163.2 kJ/mol). Solvent molecules (water and methanol) produce an important catalytic effect in the tautomerism, especially for methanol-solvated system. NBO analysis shows that there is a strong interaction between cyameluric acid and solvent molecules in transition states. AIM charge analysis indicates that the keto–enol tautomerism shows a certain degree of proton transfer character. From the reaction enthalpy and reaction rate point of view, keto–enol tautomerism in water-solvated and methanol-solvated system is easier than that in gas phase. The keto–enol tautomerisms are endothermic both in gas phase and in solution, so the enol forms are less stable than the keto ones.