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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.

Water-soluble octaacid cavitands (OAs) form dimeric capsules suitable for guest incorporation. Our studies reveal that the mechanism of pyrene (Py) binding involves the rapid (<1 ms) formation of the Py·OA complex followed by slower binding with the second OA. The dissociation of the capsular OA·Py·OA complex occurs with a lifetime of 2.7 s, which is 5 orders of magnitude slower than the microsecond opening/closing ("breathing") previously observed to provide access of small molecules to the encapsulated guest. These different dynamics of the capsules have a potential impact on how the chemistry of included guests could be altered.

Photocyclodimerization of 2-anthracenecarboxylate mediated by molecular chaperone protein was performed for the first time to afford chiral syn-head-to-tail and anti-head-to-head dimers (2 and 3) in 10% and 16% enantiomeric excess, respectively, with enhanced yields of sterically and electrostatically less-favored head-to-head dimers (3 and 4).

The photochirogenesis of 2-anthracenecarboxylic acid (AC) complexed to a hydrogen-bonding template (TKS159) was investigated to obtain mechanistic information on how chirogenesis is achieved for the dimerization of AC. Complexation of AC to TKS159 leads to the shielding of one of the two surfaces of the prochiral AC molecule. The two diastereomeric AC-TKS complexes, i.e., re-AC-TKS and si-AC-TKS, were characterized by changes in the UV-vis, fluorescence, and circular dichroism spectra and excited-state lifetimes. The ee is not simply determined by the diastereomeric ratio of the re- and si-AC-TKS complexes but also depends on the relative lifetimes of the diastereomeric complexes. The relative population of the re and si complexes was calculated from the enantiomeric excess (ee) for the products, taking into account the relative lifetimes of the two complexes. These studies established a protocol that can be used to reveal the mechanism for photochirogenesis by investigating the ground state and the excited state behavior of supramolecular systems.

The mechanism for the chirogenesis in the photocyclodimerization of 2-anthracenecarboxylate (AC) bound to human serum albumin (HSA) was investigated using time-resolved fluorescence measurements in the presence of HSA inhibitors and/or an AC singlet excited state quencher. The photophysical studies were correlated with product studies to explain the high enantiomeric excess (ee) observed for the chiral photoproducts. AC binds to HSA in five different binding sites with decreasing affinities. AC bound to the sites with the highest affinity (sites 1 and 2) is unreactive, and the AC can be displaced from these sites by the use of known inhibitors. Time-resolved fluorescence studies isolated a singlet excited state AC bound to a site which exhibited moderate protection from interactions with species in the aqueous phase. This site was assigned to binding site 3, where the chiral photoproducts are formed with a high ee based on the correlation of the photophysical studies with product studies in the presence of a quencher. These results show that the use of inhibitors for multiple binding site proteins is useful to uncover the properties of binding sites for which guest binding has only moderate affinity and where the photophysical characterization of these binding sites is not possible in the absence of inhibitors.

Photophysics and photochemistry of 2-anthracenecarboxylate (AC) bound to bovine serum albumin (BSA) were investigated in detail for the first time by electronic absorption, circular dichroism (CD), steady-state and time-resolved fluorescence, fluorescence quenching, and product analysis studies. Through the spectroscopic investigations, it was revealed that the four independent binding pockets of BSA, which are known to accommodate 1, 3, 2, and 3 AC molecules in the order of decreasing affinity, are distinctly different in hydrophobicity, chiral environment, and accessibility. Interestingly, AC bound to site 1 gave highly structured fluorescence with dual lifetimes of 4.8 and 2.1 ns in an intensity ratio of 3:2, which may be assigned to the existence of two positional or orientational isomers within the very hydrophobic site 1. In contrast, the lifetime of AC in site 2 was much longer (13.3 ns), and ACs in sites 3 and 4 have broader fluorescence spectra with lifetimes that were practically indistinguishable from that in bulk water (15.8 ns). Although each of sites 2-4 simultaneously binds multiple AC molecules, no CD exciton coupling or static fluorescence quenching was detected, indicating that ACs bound to each site are not in close proximity to each other. Quenching studies with nitromethane further confirmed the significant difference in accessibility among the binding sites; thus, ACs bound to sites 1 and 2 are highly protected from the attack of the quencher, affording 32 and 10 times smaller rate constants than that for free AC in water. Product studies in the presence and absence of nitromethane more clearly revealed the photochirogenic performance of each binding site. Although the addition of nitromethane did not greatly alter the product distribution, the enantiomeric excesses (ee's) of chiral cycloadducts 2 and 3 were critically manipulated by selectively retarding the photoreaction occurring at the more accessible binding sites. Thus, the highest ee of 38% was obtained for 2 in the presence of 18 mM nitromethane, while the highest ee of 58% was attained for 3 in the absence of nitromethane, both at [AC]/[BSA]=3.6.