Complete Structure Assignment of Azathioprine as a Proton-Deficient Nucleic Acid Analogue Using 1H-13C Long-Range Heteronuclear Single Quantum Multiple Bond Correlation.

In this Note, the successful structural assignment of a proton-deficient nucleic acid analogue using the 1H-13C long-range heteronuclear single quantum multiple bond correlation (LR-HSQMBC) technique is described. LR-HSQMBC is a 2D NMR technique for the sensitive detection of weak C-H spin couplings. The immunosuppressant drug, azathioprine, served as the target compound. The LR-HSQMBC measurements revealed the existence of covalent bonds between the purine and imidazole rings based on observations of 5JCH and 6JCH with good sensitivity.

Comprehensive Structural and Electronic Properties of 2-Azaadamantane N-Oxyl Derivatives Correlated with Their Catalytic Ability.

Herein, a comprehensive kinetic study is performed to compare the catalytic efficiency of 2-azaadamantane N-oxyl (AZADO) derivatives with that of 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) used as radical catalysts in the aerobic oxidation of l-menthol. Furthermore, the correlation between the catalytic activity and structural/electronic parameters of AZADOs and TEMPO is elucidated. The reaction rate constants achieved with several AZADO derivatives exhibit moderate relationships with spectroscopic parameters, such as the hyperfine coupling constant of the N atom (AN) and NO stretching vibration frequency (νNO) observed in electron spin resonance and infrared spectra, respectively. The planarity C-(NO)-C angle (φ) at the N atom, determined by density functional theory (DFT) calculations, also strongly correlates with the AN and νNO. Moreover, the bond order of NO, which strongly depends on the structural and electronic properties of NO radicals, correlates with radical activity; thus, the radical activity can be predicted by DFT calculations, thereby accelerating the synthesis of new AZADO derivatives without requiring alcohol oxidation experiments.

Near perfect head-to-head selectivity on the supramolecular photocyclodimerisation of 2-anthracenecarboxylate with self-organised gemini surfactant bilayers.

Silicificated self-organised crystalline bilayers of gemini surfactants were used as reaction templates for the supramolecular regioselective [4+4] photocyclodimerisation of 2-anthracenecarboxylate in water. A near perfect head-to-head cyclodimer selectivity, up to 97% at room temperature, was achieved with accelerated rate.

X-ray observations of single bio-supramolecular photochirogenesis.

The binding and photochirogenic behaviour of 2-anthracenecarboxylate (AC) with human serum albumin (HSA) have hitherto been investigated and comprehended as time-averaged statistical events by spectroscopic examinations and product analyses. In this study, we employed a diffracted X-ray tracking (DXT) technique to visualize the single-molecular dynamics of free and AC-loaded HSA (AC:HSA = 0, 1, 5 and 10), as well as the AC-HSA complex under photoirradiation, all of which were tethered to gold nanocrystals and hence traceable in real time by DXT. This enabled us to draw a more dynamic picture of the bio-supramolecular photochirogenesis at a single-molecule resolution, detailing the softening and flexibility enhancement of HSA upon binding of ACs to its inter-subdomain IIA-IIB site and the dynamic extrusion of AC dimers produced upon photoirradiation.

1,8-Diphenyl-9,10-Bis(arylethynyl)phenanthrenes: Synthesis, Distorted Structure, and Optical Properties.

The synthesis and optical properties of 1,8-diphenyl-9,10-bis(arylethynyl)phenanthrenes, which are distorted phenanthrenes, are reported. The presence of the two phenyl groups at the 1,8-positions of phenanthrene significantly distorts the molecular geometries, as was evidenced by X-ray crystallography. The congested substitution pattern in the K region results in a distorted aromatic framework, which leads to a redshift in the emission spectrum. These observations are in stark contrast to 9,10-bis(phenylethynyl)phenanthrene with no phenyl groups at the 1,8-positions. A large Stokes shift suggested extensive structural relaxation between the phenyl and arylethynyl units in the excited state, which was supported by theoretical calculations.

Supramolecular Photochirogenesis Driven by Higher-Order Complexation: Enantiodifferentiating Photocyclodimerization of 2-Anthracenecarboxylate to Slipped Cyclodimers via a 2:2 Complex with ß-Cyclodextrin.

Chiral slipped 5,8:9',10'-cyclodimers were preferentially produced over classical 9,10:9',10'-cyclodimers upon supramolecular photocyclodimerization of 2-anthracenecarboxylate (AC) mediated by ß-cyclodextrin (ß-CD). This photochirogenic route to the slipped cyclodimers, exclusively head-to-tail (HT) and highly enantioselective, has long been overlooked in foregoing studies but is dominant in reality and is absolutely supramolecularly activated by 2:2 complexation of AC with ß-CD. The intricate structural and photophysical aspects of this higher-order complexation-triggered process have been comprehensively elucidated, while the absolute configurations of the slipped cyclodimers have been unambiguously assigned by comparing the experimental and theoretical circular dichroism spectra. In the 2:2 complex, two ACs packed in a dual ß-CD capsule are not fully overlapped with each other but are only partially stacked in a slipped anti- or syn-HT manner. Hence, they do not spontaneously cyclodimerize upon photoexcitation but instead emit long-lived excimer fluorescence at wavelengths slightly longer than the monomer fluorescence, indicating that the slipped excimer is neither extremely reactive nor completely relaxed in conformation and energy. Because of the slipped conformation of the AC pair in the soft capsule, the subsequent photocyclodimerization becomes manipulable by various internal or external factors, such as temperature, pressure, added salt, and host modification, enabling us to exclusively obtain the slipped cyclodimers with high regio- and enantioselectivities. In this supramolecularly driven photochirogenesis, the dual ß-CD capsule functions as a chiral organophotocatalyst to trigger and accelerate the nonclassical photochirogenic route to slipped cyclodimers by preorganizing the conformation of the encapsulated AC pair, formally mimicking a catalytic antibody.

Regioselective Molecularly Imprinted Reaction Field for [4 + 4] Photocyclodimerization of 2-Anthracenecarboxylic Acid.

Molecularly imprinted cavities have functioned as a regioselective reaction field for the [4 + 4] photocyclodimerization of 2-anthracenecarboxylic acid (2-AC). Molecularly imprinted polymers were prepared by precipitation polymerization of N-methacryloyl-4-aminobenzamidine as a functional monomer to form a complex with template 2-AC and ethylene glycol dimethacrylate as a crosslinking monomer. The 2-AC-imprinted cavities thus constructed preferentially bound 2-AC with an affinity greater than that toward structurally related 9-anthracenecarboxylic acid, 2-aminoanthracene, and unsubstituted anthracene. Moreover, from the four possible regioisomeric cyclodimers, they mediated the [4 + 4] photocyclodimerization of 2-AC specifically to the anti-head-to-tail (anti-HT) isomer. This indicates that the imprinted cavities accommodate two 2-AC molecules in an anti-HT manner, thereby facilitating the subsequent regioselective photocyclodimerization.

Supramolecular Photochirogenesis with a Higher-Order Complex: Highly Accelerated Exclusively Head-to-Head Photocyclodimerization of 2-Anthracenecarboxylic Acid via 2:2 Complexation with Prolinol.

An unprecedented 2:2 complex was shown to intervene in the enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylic acid (A) mediated by a hydrogen-bonding template l-prolinol (P) to accelerate the formation of chiral anti-head-to-head and achiral syn-head-to-head cyclodimers in >99% combined yield with enhanced enantioselectivities of up to 72% ee for the former. The supramolecular complexation and photochirogenic behaviors, as well as the plausible structures, of intervening Am·Pn complexes (m, n = 1 or 2) were elucidated by combined theoretical and experimental spectroscopic, photophysical, and photochemical studies. Furthermore, the photochemical chiral amplification was achieved for the first time by utilizing the preferential 2:2 complexation of A with homochiral P to give normalized product enantioselectivities higher than those of the template used. The present strategy based on the higher-order hydrogen-bonding motif, which is potentially applicable to a variety of carboxylic acids and ß-aminoalcohols, is not only conceptually new and expandable to other (photo)chirogenic and sensing systems but also may serve as a versatile tool for achieving photochemical asymmetric amplification and constructing chiral functional supramolecular architectures.

Inherently Chiral Azonia[6]helicene-Modified ß-Cyclodextrin: Synthesis, Characterization, and Chirality Sensing of Underivatized Amino Acids in Water.

The (P)- and (M)-3-azonia[6]helicenyl ß-cyclodextrins exhibit L/D selectivities of up to 12.4 and P/M preferences of up to 28.2 upon complexation with underivatized proteinogenic amino acids in aqueous solution at pH 7.3.

pH-Independent Charge Resonance Mechanism for UV Protective Functions of Shinorine and Related Mycosporine-like Amino Acids.

The UV-protective ability of mycosporine-like amino acids (MAAs) has been well documented and is believed to serve as a protecting agent for marine organisms from solar radiation. However, the effective UV absorption by MAAs has not been well correlated to MAA (neutral) structures. In this study, the origin of UV-protecting ability of MAAs was elucidated by experimental and theoretical spectroscopic investigations. The absorption maxima of mycosporine-glycine and shinorine in the UVA region were practically unaffected over a wide range of pH 4-10 and only slightly blue-shifted at pH 1-2. It was revealed that the zwitterionic nature of the amino acid residue facilitates the protonation to the chromophoric 3-aminocyclohexenone and 1-amino-3-iminocyclohexene moieties and the operation of the charge resonance in the protonated species well accounts for their allowed low-energy transitions in the UVA region. The RI-CC2/TZVP calculations on model systems in their protonated forms well reproduced the observed transition energies and oscillator strengths of MAAs, only with insignificant systematic overestimations of the both values. The slight hypsochromic shifts at pH 1-2 were explained by (partial) protonation to a carboxylate anion in the amino acid residue, as confirmed by theory. Fluorescence spectral investigations of shinorine were also performed for the first time in water to confirm the effective nonradiative deactivation. Consequently, this study unequivocally demonstrated that the 3-aminocyclohexenone as well as 1-amino-3-iminocyclohexene moieties, which are readily protonated at a wide range of pH, are responsible for the UV-protective ability of aqueous solution of MAAs.

Mammalian serum albumins as a chiral mediator library for bio-supramolecular photochirogenesis: optimizing enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylate.

A simple strategy for choosing optimal bio-supramolecular mediators from the mammalian serum albumin library is proposed for bimolecular photochirogenic reactions. Thus, the enantiodifferentiating photocyclodimerization of 2-anthracencecarboxylate (AC) was optimized in chemical and optical yields, when mediated by porcine and canine serum albumins, both of which bound two AC molecules in the first productive site to give the (P)-enantiomer of syn-head-to-tail-cyclodimer in 69% yield and 89% enantiomeric excess (ee) for the former but the (M)-enantiomer in 77% yield and 97% ee for the latter.

Ammonia-driven chirality inversion and enhancement in enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylate mediated by diguanidino-γ-cyclodextrin.

In the supramolecular photocyclodimerization of 2-anthracenecarboxylate mediated by 6(A),6(D)-diguanidino-γ-cyclodextrin (CD), the chiral sense and enantiomeric excess of the photoproduct were dynamic functions of temperature and cosolvent to afford the (M)-anti head-to-head cyclodimer in 64% ee in aqueous methanol at -70 °C but the antipodal (P)-isomer in 86% ee in aqueous ammonia at -85 °C, while the corresponding diamino-γ-CD host did not show such unusual photochirogenic behaviors. The ee landscape was very steep against the temperature and sign-inverted against the ammonia content to reveal the opposite temperature dependence at low and high ammonia contents, for which an altered solvent structure and/or guanidinium-carboxylate interaction mode would be responsible.

Supramolecular photocyclodimerization of 2-hydroxyanthracene with a chiral hydrogen-bonding template, cyclodextrin and serum albumin.

2-Hydroxyanthracene (HA) in its neutral form smoothly photocyclodimerized to four stereoisomeric [4 + 4]-cyclodimers, which were isolated and characterized for the first time, whereas the anionic form of HA turned out to be photochemically inert. Enantiodifferentiating photocyclodimerization of HA in the presence of a chiral hydrogen-bonding template (TKS159), γ-cyclodextrin (γ-CDx) and bovine serum albumin (BSA) was examined to afford chiral syn-head-to-tail and anti-head-to-head cyclodimers in modest enantiomeric excesses with TKS159 and γ-CDx, but practically no photocyclodimerization proceeded in the presence of BSA probably due to the ionization of HA in the binding sites.

Manipulating γ-cyclodextrin-mediated photocyclodimerization of anthracenecarboxylate by wavelength, temperature, solvent and host.

Wavelength effects on the enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylate (AC) mediated by native and modified γ-cyclodextrins (CDs) were examined in different solvents at varying temperatures to manipulate the photochirogenic outcomes beyond the thermodynamically determined re/si-enantiotopic face selectivity upon 2 : 1 complexation of AC with CD in the ground state. Indeed, the stereochemical outcomes, i.e. syn/anti, head-to-tail/head-to-head (HT/HH) and in particular enantiomer ratios, were critical functions of the irradiation wavelength, irrespective of the CD host employed. Furthermore, the wavelength effects observed strongly depended on the host structure, reaction temperature and solvent employed, for which altered stacking geometry of the complexed AC pair is thought to be responsible. By optimizing the irradiation wavelength, chiral host, temperature and solvent, an enantiomeric excess of up to 54 and -37% were achieved for chiral syn-HT and anti-HH dimers.

Absolute configuration determination of the anti-head-to-head photocyclodimer of anthracene-2-carboxylic acid through cocrystallization with L-prolinol.

The absolute configuration has been established of the enantiopure anti-head-to-head cyclodimer of anthracene-2-carboxylic acid (AC) cocrystallized with L-propinol and dichloromethane [systematic name: (S)-2-(hydroxymethyl)pyrrolidin-1-ium (5R,6S,11R,12S)-8-carboxy-5,6,11,12-tetrahydro-5,12:6,11-bis([1,2]benzeno)dibenzo[a,e][8]annulene-2-carboxylate dichloromethane monosolvate], C5H12NO(+)·C30H19O4(-)·CH2Cl2. In the crystal structure, the AC dimer interacts with L-prolinol through a nine-membered hydrogen-bonded ring [R2(2)(9)], while the dichloromethane molecule is incorporated to fill the void space. The absolute configuration determined in this study verifies a recent assignment made by comparing theoretical versus experimental circular dichroism spectra.

Diffracted X-ray tracking for monitoring intramolecular motion in individual protein molecules using broad band X-ray.

Diffracted X-ray tracking (DXT) enables the tilting and twisting motions of single protein molecules to be monitored with micro- to milliradian resolution using a highly brilliant X-ray source with a wide energy bandwidth. We have developed a technique to monitor single molecules using gold nanocrystals attached to individual protein molecules using the BL28B2 beamline at SPring-8. In this paper we present the installation of a single toroidal X-ray mirror at BL28B2 to focus X-rays in an energy range of 10-20 keV (ΔE/E = 82% for an X-ray with a wide energy bandwidth). With this beamline we tracked diffraction spots from gold nanocrystals over a wide angle range than that using quasi-monochromatic X-rays. Application of the wide angle DXT technique to biological systems enabled us to observe the on-site motions of single protein molecules that have been functionalized in vivo. We further extend the capability of DXT by observing the fractional tilting and twisting motions of inner proteins under various conditions. As a proof of this methodology and to determine instrumental performance the intramolecular motions of a human serum albumin complex with 2-anthracenecarboxylic acid was investigated using the BL28B2 beamline. The random tilting and twisting intramolecular motions are shown to be directly linked to the movement of individual protein molecules in the buffer solution.

Supramolecular photochirogenesis with functional amyloid superstructures.

Chiral variants of amyloid fibrils prepared by agitating acidified solutions of bovine insulin at 45 °C not only induced quasi-mirror-imaged circular dichroism spectra upon complexation with 2-anthracenecarboxylate but also gave anti-head-to-head-cyclodimers of the opposite absolute configurations upon photoirradiation.

Photochirogenesis with mutant human serum albumins: enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylate.

Mutant human serum albumins accelerated the photocyclodimerization of 2-anthracenecarboxylate to afford chiral cyclodimers in 75-85% enantiomeric excesses, revealing that the mutations to impair non-productive sites 1 and/or 2 enhanced the substrate binding to site 3 without seriously damaging its inherently high photochirogenic ability.

Cross- versus homo-photocyclodimerization of anthracene and 2-anthracenecarboxylic acid mediated by a chiral hydrogen-bonding template. Factors controlling the cross-/homo-selectivity and enantioselectivity.

Competitive cross-/homo-photocyclodimerization of anthracene (AN) and 2-anthracenecarboxylic acid (AC) mediated by a chiral hydrogen-bonding template (TKS) was investigated under various conditions. The cross-photocyclodimerization was favored by a factor of 4-5 at all temperatures and wavelengths examined to afford the AC-AN cross-dimer in 80-84% yield even at AN/AC = 1 and in 98% yield at AN/AC = 10. The enantiomeric excesses (ee's) obtained were 27-47% for the homo-dimers and 21-24% for the cross-dimer. The absolute configuration of the cross-dimer was determined by comparing the experimental and theoretical circular dichroism spectra and further correlated with the re/si enantiotopic-face selectivity upon AC-TKS complexation in the ground state. Detailed analyses of the complexation behavior and the fluorescence lifetime and cyclodimerization rate of excited re/si complexes revealed that the product's ee is critically controlled not only by the relative abundance of the re/si complexes in the ground and excited states but also by their relative photocyclodimerization rate. Crucially, the ground-state thermodynamics and the excited-state kinetics are not synergistic but offsetting in enantiotopic-face selectivity, and the latter overwhelms the former to give the homo- and cross-dimers in modest ee's. Finally, some practical strategies for enhancing the enantioselectivity in chiral template-mediated photochirogenesis have been proposed.

Phase-controlled supramolecular photochirogenesis in cyclodextrin nanosponges.

Pyromellitate-bridged cyclodextrin nanosponges (CDNSs) evolved from sol into gel state upon gradual increase of the concentration from 0.2 to 2000 mg mL(-1) in water. The enantiodifferentiating geometrical photoisomerizations of (Z)-cyclooctene and (Z,Z)-1,3-cyclooctadiene sensitized by CDNS at various concentrations were critically affected by the phase transition of CDNS to afford the corresponding (E)- and (E,Z)-isomers in the highest enantiomeric excesses in the gel state.