9 research outputs found
Explaining the Highly Enantiomeric Photocyclodimerization of 2‑Anthracenecarboxylate Bound to Human Serum Albumin Using Time-Resolved Anisotropy Studies
The mechanism for the high enantiomeric excess (ee) (80–90%)
observed in the photocyclodimerization of 2-anthracenecarboxylate
(AC) in the chiral binding sites of human serum albumin (HSA) was
studied using fluorescence anisotropy. A long rotational correlation
time of 36 ns was observed for the excited states of the ACs bound
to the HSA site responsible for the high ee, suggesting that the ACs
have restricted rotational mobility in this site. The ACs in this
site have the same prochiral face protected by the protein, and this
protection is responsible for the high ee observed. These insights
provide a strategy for the rational design of supramolecular photochirogenic
systems
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 <i>N</i>-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 <i>anti</i>-head-to-tail (<i>anti</i>-HT) isomer. This indicates that
the imprinted cavities accommodate two 2-AC molecules in an <i>anti</i>-HT manner, thereby facilitating the subsequent regioselective
photocyclodimerization
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
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 <i>re/si</i> 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 <i>re/si</i> complexes revealed that the product’s ee is
critically controlled not only by the relative abundance of the <i>re/si</i> 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
Enantiodifferentiating Photocyclodimerization of 2‑Anthracenecarboxylic Acid via Competitive Binary/Ternary Hydrogen-Bonded Complexes with 4‑Benzamidoprolinol
Circular dichroism (CD) spectral examinations at various host/guest ratios revealed that 2-anthracenecarboxylic acid (AC) forms not only 1:1 but also novel 2:1 hydrogen-bonded/Ï€-stacked complexes with a chiral 4-benzamidoprolinol template (TKS159). The 2:1 complexation is a minor process but causes significant CD spectral changes as a consequence of the exciton coupling interaction of two AC chromophores and greatly accelerates the head-to-head photocyclodimerization to significantly affect the stereochemical outcomes
Ammonia-Driven Chirality Inversion and Enhancement in Enantiodifferentiating Photocyclodimerization of 2‑Anthracenecarboxylate Mediated by Diguanidino-γ-cyclodextrin
In the supraÂmolecular photoÂcycloÂdimerizÂation
of 2-anthraceneÂcarboxylate mediated by 6<sup>A</sup>,6<sup>D</sup>-diÂguanidino-γ-cycloÂdextrin (CD), the chiral sense
and enantioÂmeric excess of the photoÂproduct were dynamic
functions of temperature and cosolvent to afford the (<i>M</i>)-<i>anti</i> head-to-head cycloÂdimer in 64% ee in
aqueous methanol at −70 °C but the antiÂpodal (<i>P</i>)-isomer in 86% ee in aqueous ammonia at −85 °C,
while the corresponding diamino-γ-CD host did not show such
unusual photoÂchiroÂgenic behaviors. The ee landÂscape
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 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 (<b>A</b>) mediated by a hydrogen-bonding template l-prolinol (<b>P</b>) to accelerate the formation of chiral <i>anti-head-to-head</i> and achiral <i>syn-head-to-head</i> 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 <b>A</b><sub><i>m</i></sub>·<b>P</b><sub><i>n</i></sub> complexes (<i>m</i>, <i>n</i> = 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 <b>A</b> with homochiral <b>P</b> 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 β-aminoalcohols, is not
only conceptually new and expandable to other (photo)Âchirogenic and
sensing systems but also may serve as a versatile tool for achieving
photochemical asymmetric amplification and constructing chiral functional
supramolecular architectures
Supramolecular Photochirogenesis Driven by Higher-Order Complexation: Enantiodifferentiating Photocyclodimerization of 2‑Anthracenecarboxylate to Slipped Cyclodimers via a 2:2 Complex with β‑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 β-cyclodextrin (β-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 β-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 β-CD capsule are not fully
overlapped with each other but are only partially stacked in a slipped <i>anti</i>- or <i>syn</i>-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 β-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
Inherently Chiral Azonia[6]helicene-Modified β‑Cyclodextrin: Synthesis, Characterization, and Chirality Sensing of Underivatized Amino Acids in Water
The (<i>P</i>)- and (<i>M</i>)-3-azoniaÂ[6]Âhelicenyl
β-cyclodextrins exhibit l/d selectivities
of up to 12.4 and <i>P</i>/<i>M</i> preferences
of up to 28.2 upon complexation with underivatized proteinogenic amino
acids in aqueous solution at pH 7.3