Peptide recognition: encapsulation and alpha-helical folding of a nine-residue peptide within a hydrophobic dimeric capsule of a bowl-shaped host

A dimeric capsule of coordination bowl 1 encapsulated a nine-residue peptide (Trp-Ala-Glu-Ala-Ala-Ala-Glu-Ala-Trp; 2) within the large hydrophobic cavity in water, and stabilized the alpha-helical conformation of bound 2. An NMR titration experiment revealed that monomeric bowl 1 recognized two Trp residues at the both terminals of 2 through 1/2 = 1:1 to 2:1 complexation. The 1:1 and 2:1 species exist in equilibrium even in the presence of excess 1. It was found that the formation of the 2:1 complex, in which two bowls of 1 wrapped the whole of 2, became dominant by the addition of NaNO3 due to the fact that the enhanced ion strength increased the hydrophobic interaction between Trp residues and the cavity of 1. The alpha-helical conformation of 2 within the dimeric capsule of 1 was elucidated from detailed NOESY analysis.     

Assembly of a supramolecular capsule on a molecular printboard

A molecular capsule based on ionic interactions between two oppositely charged calix[4]arenes, 1 and 2, was assembled both in solution and on a surface. In solution, the formation of the equimolar assembly 1.2 was studied by (1)H NMR, ESI-MS, and isothermal titration calorimetry, giving an association constant (K(a)) of 7.5 x 10(5) M(-1). A beta-cyclodextrin self-assembled monolayer (beta-CD SAM) on gold was used as a molecular printboard to anchor the tetraguanidinium calix[4]arene (2). The binding of tetrasulfonate calix[4]arene 1 was monitored by surface plasmon resonance spectroscopy. Rinsing of the surface with a high ionic strength aqueous solution allows the removal of the tetrasulfonate calix[4]arene (1), while by rinsing with 2-propanol it is possible to achieve the complete desorption of the tetraguanidinium calix[4]arene (2) from the beta-CD SAM. The K(a) for the capsule formation on a surface is 3.5 x 10(6) M(-1), thus comparing well with the K(a) determined in solution.     

Reductive activation of dioxygen by a myoglobin reconstituted with a flavohemin

We successfully converted myoglobin, an oxygen-storage hemoprotein, into an oxygen-activating hemoprotein like cytochrome P450s by replacing the native hemin with the artificially created flavohemin. The reconstituted myoglobin, rMb(1), was chacterized by ESI-TOF-mass, UV-vis, and fluorescence spectra. The 1H NMR spectrum of cyanomet rMb(1) indicates that two hemin conformers are present in a ratio of 1:1. Upon the addition of NADH to the buffer solution of rMb(1) in the presence of SOD and catalase, the oxymyoglobin was rapidly formed. As compared with the formation of the oxygenated native myoglobin in the presence of 10-N-(acetylaminoethyl)isoalloxazine, the rate constant of the oxyheme formation in rMb(1) is 6 times larger. This is because the flavin covalently linked to the terminal heme propionate functions as an effective mediator of an electron transfer from NADH to the hemin in rMb(1). Furthermore, rMb(1) shows the deformylation activity, when 2-phenylpropionaldehyde (2-PPA) was employed as a substrate. This result indicates that the oxyheme is reductively activated to Fe(III)-peroxoanion (Fe(III)-O22-). The result in this report is the first example of the activation of dioxygen by myoglobin. This study shows the utility of the replacement of the native hemin with a chemically modified one for the functionalization of myoglobin.     

Prenylation of a nonaromatic carbon of indolylbutenone by a fungal indole prenyltransferase

FtmPT1 from Aspergillus fumigatus is a fungal indole prenyltransferase (PT) that normally catalyzes the regiospecific prenylation of brevianamide F (cyclo-L-Trp-L-Pro) at the C-2 position of the indole ring with dimethylallyl diphosphate (DMAPP). Interestingly, FtmPT1 exhibited remarkable substrate tolerance and accepted (E)-4-(1H-indol-3-yl)but-3-en-2-one (1) as a substrate to produce an unnatural novel α-prenylindolylbutenone (1a). This is the first demonstration of the prenylation of a nonaromatic carbon of the acceptor substrate by a fungal indole PT.     

Isolation of a metastable geometrical isomer of a hexacoordinated dihydrophosphate: elucidation of its enhanced reactivity in umpolung of a hydrogen atom of water

Two of five conceivable geometrical isomers of a hexacoordinated dihydrophosphate bearing two sets of a bidentate ligand were investigated. X-ray crystallographic analysis of both of isomers, 1a-TPP and 1b-TEA, revealed their octahedral geometries of C(2) and C(1) symmetry, respectively, which were consistent with the NMR spectra. The isomer 1b-TEA underwent both hydride reduction of an aldehyde and proton exchange with water at room temperature in DMSO without any additive. A one-pot reaction of both of the reactions of 1b-TEA with D(2)O and an aldehyde or a ketone under the above conditions proceeded successfully to give the deuterated alcohol. Thus, umpolung of a hydrogen atom of water with 1b-TEA was achieved under much milder conditions than those used in the reaction with another isomer, 1a-TEA. Quantitative isomerization of 1b-TEA to 1a-TEA occurred in methanol at room temperature. Calculations on the five conceivable geometrical isomers of the anionic part of the dihydrophosphate revealed their relative stability, which reasonably explained the isomerization, and the larger negative charge at the atoms located at the trans positions of the oxygen atoms. The smaller coupling constants of the P-H and P-C bonds located at the rear of an oxygen atom in the NMR spectra resulted in the smaller s character of these bonds. The differences in both hydride-donation and proton-exchange reactivities between 1a-TEA and 1b-TEA could be explained by the differences in the atomic charge of the hydrogen atom and the stability difference of the initially formed phosphorane intermediates, respectively.     

Cyclodextrin complexation of a stilbene and the self-assembly of a simple molecular device

(E)-4-tert-Butyl-4'-oxystilbene, 1(-), is thermally stable as the (E)-1(-) isomer but may be photoisomerized to the (Z)-1(-) isomer as shown by UV-vis and (1)H NMR studies in aqueous solution. When (E)-1(-) is complexed by alphaCD two inclusion isomers (includomers) form in which alphaCD assumes either of the two possible orientations about the axis of (E)-1(-) in alphaCD.(E)-1(-) for which (1)H NMR studies yield the parameters: k(1)(298 K)= 12.3 +/- 0.6 s(-1), DeltaH(1)(++)= 94.3 +/- 4.7 kJ mol(-1), DeltaS1(++)= 92.0 +/- 5.0 J K(-1) mol(-1), and k(2)(298 K)= 10.7 +/- 0.5 s(-1), DeltaH(2)(++)= 93.1 +/- 4.7 kJ mol(-1), DeltaS2(++)= 87.3 +/- 5.0 J K(-1) mol(-1) for the minor and major includomers, respectively. The betaCD.(E)-1(-) complex either forms a single includomer or its includomers interchange at the fast exchange limit of the (1)H NMR timescale. Complexation of 1(-) by N-(6(A)-deoxy- alpha-cyclodextrin-6(A)-yl)-N'-(6(A)-deoxy- beta-cyclodextrin-6(A)-yl)urea, results in the binary complexes 2.(E)-1(-) in which both CD component annuli are occupied by (E)-1(-) and which exists exclusively in darkness and 2.(Z)-1(-) in which only one CD component is occupied by (Z)-1(-) and exists exclusively in daylight at lambda > or = 300 nm. Irradiation of solutions of the binary complexes at 300 and 355 nm results in photostationary states dominated by 2.(E)-1(-) and 2.(Z)-1(-), respectively. In the presence of 4-methylbenzoate, 4(-), 2.(Z)-1(-) forms the ternary complex 2.(Z)-1(-).4(-) where 4(-) occupies the second CD annulus. Interconversion occurs between 2.(Z)-1(-).4(-) and 2.(E)-1(-)+4(-) under the same conditions as for the binary complexes alone. Similar interactions occur in the presence of 4-methylphenolate and 4-methylphenylsulfonate. The two isomers of each of these systems represent different states of a molecular device, as do the analogous binary complexes of N,N-bis(6(A)-deoxy- beta-cyclodextrin-6(A)-yl)urea, 3, [3.(E)-1(-) and 3.(Z)-1(-), where the latter also forms a ternary complex with 4(-).     

Controlled expansion of a molecular cavity in a steroid host compound.

Expansion of a molecular cavity is described by using elongation of the side chain of a bile acid host compound. Bishomocholic acid (2), which has a side chain that is longer by two methylene unit than cholic acid (1), includes many organic substances at 1:1 host:guest ratios. X-ray crystallographic studies revealed that 2 has two types of open host frameworks: a bilayer type and a crossing type. Both of them are isostructural to those of 1, indicating that they are robust against the elongation of the side chain. In the former type, the increment of the width of the host channel corresponds to that of the length of the molecular structures. Larger aromatic guest components such as 1-methylnaphthalene and 1-tetralone, are included in 2, but not in 1.     

Induced Chemical Defense of a Mushroom by a Double-Bond-Shifting Polyene Synthase

The antilarval mushroom polyenes 18-methyl-19-oxoicosaoctaenoic acid and 20-methyl-21-oxodocosanonaenoic acid appear in response to injury of the mycelium of the stereaceous mushroom BY1. We identified a polyketide synthase (PPS1) which belongs to a hitherto completely uncharacterized clade of polyketide synthases. Expression of the PPS1 gene is massively upregulated following mycelial damage. The synthesis of the above polyenes was reconstituted in the mold Aspergillus niger as a heterologous host. This demonstrates that PPS1 1) synchronously produces branched-chain polyketides of varied lengths, and 2) catalyzes the unprecedented shift of eight or nine double bonds. This study represents the first characterization of a reducing polyketide synthase from a mushroom. We also show that injury-induced de novo synthesis of polyketides is a fungal response strategy.     

Cation-pi interactions of a thiocarbonyl group and a carbonyl group with a pyridinium nucleus

Attractive interactions between a thiocarbonyl group and a pyridinium nucleus, and between a carbonyl group and a pyridinium nucleus have been proven by (1)H and (13)C NMR studies, UV-vis spectral analyses, and X-ray crystallographic analyses of nicotinic amides 1 and 3, and pyridinium salts 2 and 4. Comparison of the Deltadelta values, which are the differences in the chemical shifts with reference compounds 5 or 6, showed that the absolute Deltadelta values of 2 and 4 are much larger than those of 1 and 3. In the UV-vis spectra, the n-->pi absorption of the C=S group of 2a exhibited a significant blue shift in CHCl(3). X-ray crystallographic analysis of 1-4 clearly showed that the C=S group of 2a and the C=O group of 4 are very close to the pyridinium moiety compared to the case of 1 and 3. In addition, the X-ray crystal packing structure of 2a showed the C=S group is sandwiched between two pyridinium rings. These experimental results strongly suggested the existence of attractive (C=S)...Py(+) and (C=O)...Py(+) interactions in solution and in crystal. The optimized geometries of 1 and 2 calculated at the HF/6-311G level are in good agreement with their X-ray geometries. MP2/6-311G calculations for the model systems of pyridinium salts 2 and 4 predicted that the electrostatic and induction energies are the major source of the attractive interactions. Since the larger contribution of electrostatic and induction interactions are characteristic features of cation-pi interactions, the (C=S)...Py(+) and (C=O)...Py(+) interactions would be classified as a cation-pi interaction.     

Preparation of a lipid a derivative that contains a 27-hydroxyoctacosanoic acid moiety

[structure: see text] A general synthetic strategy for long-chain omega-1 hydroxy fatty acids has been developed, which employs as a key reaction step a cross metathesis between omega-unsaturated ester and 3-butene-2-ol. The resulting lipids were used for the preparation of lipid A derivatives of Rhizobium sin-1, which have the ability to inhibit the E. coli LPS-dependent synthesis of tumor necrosis factor by human monocytes.     

Self-assembly of a hexameric aggregate of a lipophilic calix[4]pyrrole-resorcinarene hybrid in solution: a diffusion NMR study

The lipophilic calix[4]pyrrole-resorcinarene hybrid 1b, the extended analogue of resorcin[4]arene 2b, was synthesized for the first time, and its self-assembly in solution was studied using (1)H and diffusion NMR. It was found that 1b self-assembles to hexameric aggregates in CDCl(3) solution. The interaction of trialkylamine guests with the hexameric aggregate of 1b was explored, and it appears that under the conditions used in the present study these guests interact with the external faces of the hexameric aggregate of 1b.     

Sequential protonation and methylation of a hydride-osmium complex containing a cyclopentadienyl ligand with a pendant amine group

Complex OsH{eta5-C5H4(CH2)2NMe2}(P(i)Pr3)2 (1) reacts with 1 equiv of trifluoromethanesulfonic acid (HOTf) and trifluoromethanesulfonic acid-d1 (DOTf) to produce the dihydride and hydride-deuteride complexes, [OsHE{eta5-C5H4(CH2)2NMe2}(P(i)Pr3)2]OTf (E = H (2), D (2-d1), respectively. Treatment of 2 and 2-d1 with a second equivalent of HOTf gives [OsHE{eta5-C5H4(CH2)2NHMe2}(P(i)Pr3)2][OTf]2 (E = H (3), D (3-d1) as a result of the protonation of the nitrogen atom. While the hydride and deuteride ligands of 2, 2-d1, 3, and 3-d1 do not undergo any H/D exchange process with the solvent, in acetone-d6, the NH proton of 3 and 3-d1 changes places with a deuterium atom of the solvent to yield [OsHE{eta5-C5H4(CH2)2NDMe2}(P(i)Pr3)2][OTf]2 (E = H (3-Nd1), D (3-d2)). Complex 3-Nd1 can also be obtained from the treatment of complex 2 with DOTf in dichloromethane. No exchange process between the hydride and the ND positions in 3-Nd1 or between the deuteride and NH positions in 3-d1 has been observed. Treatment of 3-Nd1 and 3-d1 with sodium methoxide results in a selective reaction of the base with the ammonium group to regenerate 2 and 2-d1, respectively. Complex 1 also reacts with methyl and methyl-d3 trifluoromethanesulfonate (CH3OTf and CD3OTf, respectively) to give [OsH{eta5-C5H4(CH2)2NMe2CE3}(P(i)Pr3)2]OTf (E = H (4), D (4-d3)) as a result of the addition of the CE3 (E = H, D) group to the nitrogen atom. Complex 4 has been characterized by an X-ray diffraction analysis. It reacts with a second molecule of CH3OTf or CD3OTf to produce [OsH{eta5-C5H4(CH2)2NMe3}{CH2CH(CH3)P(i)P2}(P(i)Pr3)[OTf]2 (5). Similarly, complex 4-d3 reacts with a second molecule of CH3OTf or CD3OTf to yield [OsH{eta5-C5H4(CH2)2NMe2CD3}{CH2CH(CH3)P(i)P2}(P(i)Pr3)[OTf]2 (5-d3). In acetonitrile, complex 5 evolves to an equilibrium mixture of the acetonitrile adducts [Os{eta5-C5H4(CH2)2NMe3}(NCCH3)(P(i)Pr3)2][OTf]2 (7) and [Os{eta5-C5H4(CH2)2NMe3}(NCCH3)2(P(i)Pr3)][OTf]2 (8). In methanol or methanol-d4, complex 4 is not stable and loses trimethylamine to give the vinylcyclopentadienyl derivatives [OsHE(eta5-C5H4CH=CH2)(P(i)Pr3)2]OTf (E = H (9), D (9-d1)) as a result of the protonation or deuteration of the metallic center and a subsequent Hofmann elimination. Protonation of 4 with HOTf gives the dihydride-trimethylammonium derivative [OsH2{eta5-C5H4(CH2)2NMe3}(P(i)Pr3)2][OTf]2 (10). Treatment of 9 with sodium methoxide produces OsH(eta5-C5H4CH=CH2)(P(i)Pr3)2 (11).     

Photoswitchable electrochemical behaviour of a [FeFe] hydrogenase model with a dithienylethene derivative

A diiron dithiolate complex 1o with a dithienylethene (DTE) phosphine ligand has been elaborately designed and fully investigated by spectroscopic and DFT computational studies. Upon irradiation with UV light, the DTE moiety in complex 1o undergoes an excellent photocyclization reaction to attain ring-closed state 1c in high yield (>95%), accompanied by a colour change from orange to deep blue. On the other hand, upon irradiation with visible light (>460 nm), ring-closed form 1c in CH(3)CN solution reverts perfectly into ring-open form 1o. Both 1o and 1c were characterised by IR, (1)H, (31)P, (19)F NMR and electrochemical spectroscopy. The electrochemical behaviours of both the open and closed forms were investigated by cyclic voltammetry. Upon photocyclization reaction, a 290 mV (from -2.29 V to -2.00 V) positive shift is induced in the potential of electrochemical catalytic proton reduction, due to the electron-withdrawing effect of the ring-closed DTE moiety. Consequently, complex 1 can reversibly photoswitch the potential of proton reduction on the [FeFe] moiety.     

Use of a chiral surfactant for enantioselective reduction of a ketone

The influence of a chiral surfactant and a polymer-supported chiral additive on reduction of ketones using sodium borohydride will be described. Initial preparations involved methylation of (S)-leucinol to give (2S)-N , N-dimethyl-2-amino-4-methyl-1-pentanol (1) (67%). The chiral surfactant (2) was synthesized by reacting (1) with bromohexadecane (71%). The functionalized styrene for the polymer-supported chiral additive (5) was synthesized by reacting (1) with 4-vinylbenzyl chloride. Polymerization was carried out with 10% of the functionalized monomer (4), 5% cross-linking agent divinylbenzene, and 85% styrene with AIBN as the initiator. The activity of the chiral surfactant and polymeric additive were examined by using them as additives in a standard reduction of 2-pentanone with sodium borohydride to yield (R)- and (S)-2-pentanol (3) (20%). The resulting alcohol was analyzed by polarimetry (ee 9.5%) and also esterified with (2S)-methylbutyric acid prior to characterization by NMR. 13C NMR indicated an enantiomeric excess of 5.2% when the chiral surfactant was used, and 7% when the polymeric additive was used.     

Electronic angular momentum polarizations of photofragments: a case study of ICN photodissociation from a perpendicular transition

A quantum treatment on ICN photodissociation from an initial perpendicular transition (Omega'=+/-1<--Omega"=0) to the asymptote CN(|Sigma+,J'M'N'1/2>)+I(2P3/2) is presented. Density matrices of both photofragments are derived and explicit expressions of the state multipoles in terms of the angular momentum coupling coefficients and the rotation-bending factors have been obtained. To perceive the physical origin of electronic angular momentum polarizations of the iodine photofragments, a correlation scheme which considers the magnetic dipolar and the electrostatic dipole-quadrupole interactions between I and CN cofragments is proposed. For ICN precursors in the vibrational ground state or in the equally populated l-type split levels, the alignment parameters of the iodine photofragments in the molecular frame can be calculated according to this long-range interaction model. For the perpendicular transition |1Pi1><--|1Sigma0+>, its alignment parameters of I(2P3/2) from the incoherent and coherent transitions to the |Omega'=1> and |Omega'=-1> components are rho(0)2(1Pi1)=0.756 and rho2(2)(1Pi1)=-0.656, respectively. For the perpendicular transition to |3Pi1>, rho(0)2(3Pi1)=-0.878 and rho2(2)(3Pi1)=0.328 are from the incoherent transition, whereas rho(0)2(3Pi1)=0.122 and rho2(2)(3Pi1)=0.328 are from the coherent transition. To analyze the photoion images of iodine photofragments, angular distributions of I+ from the 2+1 resonance-enhanced multiphoton ionization detection scheme are derived.     

Design of a five-coordinate heme protein maquette: a spectroscopic model of deoxymyoglobin

The substitution of 1-methyl-l-histidine for the histidine heme ligands in a de novo designed four-alpha-helix bundle scaffold results in conversion of a six-coordinate cytochrome maquette into a self-assembled five-coordinate mono-(1-methyl-histidine)-ligated heme as an initial maquette for the dioxygen carrier protein myoglobin. UV-vis, magnetic circular dichroism, and resonance Raman spectroscopies demonstrate the presence of five-coordinate mono-(1-methyl-histidine) ligated ferrous heme spectroscopically similar to deoxymyoglobin. Thermodynamic analysis of the ferric and ferrous heme dissociation constants indicates greater destabilization of the ferric state than the ferrous state. The ferrous heme protein reacts with carbon monoxide to form a (1-methyl-histidine)-Fe(II)(heme)-CO complex; however, reaction with dioxygen leads to autoxidation and ferric heme dissociation. These results indicate that negative protein design can be used to generate a five-coordinate heme within a maquette scaffold.     

Continuous monitoring of a heavy water plant effluent with a sulphide-selective electrode

A monitor, based on a sulphide ion-selective electrode, has been developed and applied to measure the total dissolved sulphide concentration (0.01–100 mg kg^-1) in heavy water plant effluents. The monitor is reliable and accurate, and has minimal maintenance and calibration requirements under continuous operation. The Orion 94-16A sulphide electrode gives Nernstian response to sulphide concentrations as low as ca. 0.01 mg kg^-1 in strongly alkaline solutions in the presence of anti-oxidants, but its sensitivity decreases with use. One electrode failed to give Nernstian response to sulphide concentrations below 1 mg kg^-1.     

Synthesis and photochromic reactivity of a diarylethene dimer linked by a phenyl group

A diarylethene dimer linked by a phenyl group was synthesized and the photochromic behavior was examined. Upon irradiation with ultraviolet light (λ=313 nm), a hexane solution of the diarylethene dimer (1a) turned purple blue. Upon further prolonged irradiation the color changed to blue. The purple-blue and blue colors are due to the formation of a dimer having one open- and one closed-ring forms (1b) and a dimer having two closed-ring forms (1c), respectively. Both 1b and 1c returned to 1a by irradiation with visible light (λ>500 nm). The photochromic reactivity was evaluated by measuring quantum yields of the photocyclization and photocycloreversion reactions. The photocyclization quantum yield was 0.50. The cycloreversion quantum yield from 1c to 1b (0.0026) was lower than that from 1b to 1a (0.0094).     

Dynamic molecular recognition in the generation of a new crystal-engineering motif: a unique case study of a dicarboxylic acid with a ditopic receptor favouring a polymeric over a dimeric hydrogen-bonded supramolecular complex

A new crystal-engineering motif has been developed where a ditopic receptor 1 shows a novel syn-syn hydrogen-bonded polymeric supramolecular complex (Fig. 4b) (instead of a 1:1 dimeric syn-syn or polymeric syn-anti complex) giving rise to a hydrogen-bonded stair-like polymeric ribbon structure between the binding groups of the receptor pyridine amide and the carboxyl groups of the guest substrate.