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Building on Cram's legacy: stimulated gating in hemicarcerands.

Liu F, Helgeson RC, Houk KN - Acc. Chem. Res. (2014)

Bottom Line: We found that the side portals of this hemicarceplex have multiple thermally accessible conformations.Gates are built onto host molecules so that the opening or closing of such gates is stimulated by reducing or oxidizing conditions, or by ultraviolet irradiation.The experimental and computational investigations of gated hemicarcerands and several potential applications of gated hemicarceplexes are described in this Account.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095, United States.

ABSTRACT
CONSPECTUS: Donald Cram's pioneering Nobel Prize-winning work on host-guest molecules led eventually to his creation of the field of container molecules. Cram defined two types of container molecules: carcerands and hemicarcerands. Host-guest complexes of carcerands, called carceplexes, are formed during their synthesis; once a carceplex is formed, the trapped guest cannot exit without breaking covalent bonds. Cram defined a quantity called constrictive binding, arising from the mechanical force that prevents guest escape. The constrictive binding in carceplexes is high. In contrast, hemicarcerands have low constrictive binding and are able to release the incarcerated guests at elevated temperatures without breaking covalent bonds. We have designed molecules that can switch from carcerand to hemicarcerand through a change in structure that we call gating. The original discovery of gating in container molecules involved our computational studies of a Cram hemicarceplex that was observed to release a guest upon heating. We found that the side portals of this hemicarceplex have multiple thermally accessible conformations. An eight-membered ring that is part of a portal changes from a "chair" to a "boat" structure, leading to the enlargement of the side portal and the release of the guest. This type of gating is analogous to phenomena often observed with peptide loops in enzymes. We refer to this phenomenon as thermally controlled gating. We have also designed and synthesized redox and photochemically controlled gated hemicarceplexes. Gates are built onto host molecules so that the opening or closing of such gates is stimulated by reducing or oxidizing conditions, or by ultraviolet irradiation. In both cases, the appropriate stimuli can produce a carceplex (closed gates) or hemicarceplex (open gates). A hemicarceplex with closed gates behaves like a carceplex, due to its very high constrictive binding energy. When the gates are opened, constrictive binding is dramatically lowered, and guest entrance and exit become facile. This stimulated switching between open and closed states controls access of the guest to the binding site. The experimental and computational investigations of gated hemicarcerands and several potential applications of gated hemicarceplexes are described in this Account.

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Energy profilesfor the escape of acetonitrile from 1b through gating.Energies in kcal/mol.
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fig5: Energy profilesfor the escape of acetonitrile from 1b through gating.Energies in kcal/mol.

Mentions: However, the conformationalchange of the eight-membered ring fromCH2-in to CH2-out dramatically lowers the barrierof equatorial-escape, as shown in Figure 5.Ground state hemicarceplex (Figure 5, left)has to overcome the barriers for sequential conformational flips oftwo −OCH2O- moieties (22 and 26 kcal/mol, respectively)to achieve an intermediate (Figure 5, middle)with its side portal wide open. The escape of an acetonitrile throughthe open portal requires only 22 kcal/mol, less than the barrier forgate opening (Figure 5, right). The overallbarrier for decomplexation through gating is 26 kcal/mol, which correspondsto a half-life of 15 days at ambient temperature, but only 1 min at110 °C. This result agrees well with the experimental observation.Molecular dynamics simulations also showed that the escape of thefirst acetonitrile is exergonic, while the escape of a second acetonitrileis endergonic. This explains why the second acetonitrile does notexit the cavity. Our interpretation of gating as the rate-limitingstep in decomplexation was supported by the lack of a steric isotopeeffect in a similar hemicarceplex. Liu and Warmuth reported no isotopeeffect for the escape of deuterated p-xylene or naphthalenefrom a gated hemicarceplex.15


Building on Cram's legacy: stimulated gating in hemicarcerands.

Liu F, Helgeson RC, Houk KN - Acc. Chem. Res. (2014)

Energy profilesfor the escape of acetonitrile from 1b through gating.Energies in kcal/mol.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4215849&req=5

fig5: Energy profilesfor the escape of acetonitrile from 1b through gating.Energies in kcal/mol.
Mentions: However, the conformationalchange of the eight-membered ring fromCH2-in to CH2-out dramatically lowers the barrierof equatorial-escape, as shown in Figure 5.Ground state hemicarceplex (Figure 5, left)has to overcome the barriers for sequential conformational flips oftwo −OCH2O- moieties (22 and 26 kcal/mol, respectively)to achieve an intermediate (Figure 5, middle)with its side portal wide open. The escape of an acetonitrile throughthe open portal requires only 22 kcal/mol, less than the barrier forgate opening (Figure 5, right). The overallbarrier for decomplexation through gating is 26 kcal/mol, which correspondsto a half-life of 15 days at ambient temperature, but only 1 min at110 °C. This result agrees well with the experimental observation.Molecular dynamics simulations also showed that the escape of thefirst acetonitrile is exergonic, while the escape of a second acetonitrileis endergonic. This explains why the second acetonitrile does notexit the cavity. Our interpretation of gating as the rate-limitingstep in decomplexation was supported by the lack of a steric isotopeeffect in a similar hemicarceplex. Liu and Warmuth reported no isotopeeffect for the escape of deuterated p-xylene or naphthalenefrom a gated hemicarceplex.15

Bottom Line: We found that the side portals of this hemicarceplex have multiple thermally accessible conformations.Gates are built onto host molecules so that the opening or closing of such gates is stimulated by reducing or oxidizing conditions, or by ultraviolet irradiation.The experimental and computational investigations of gated hemicarcerands and several potential applications of gated hemicarceplexes are described in this Account.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095, United States.

ABSTRACT
CONSPECTUS: Donald Cram's pioneering Nobel Prize-winning work on host-guest molecules led eventually to his creation of the field of container molecules. Cram defined two types of container molecules: carcerands and hemicarcerands. Host-guest complexes of carcerands, called carceplexes, are formed during their synthesis; once a carceplex is formed, the trapped guest cannot exit without breaking covalent bonds. Cram defined a quantity called constrictive binding, arising from the mechanical force that prevents guest escape. The constrictive binding in carceplexes is high. In contrast, hemicarcerands have low constrictive binding and are able to release the incarcerated guests at elevated temperatures without breaking covalent bonds. We have designed molecules that can switch from carcerand to hemicarcerand through a change in structure that we call gating. The original discovery of gating in container molecules involved our computational studies of a Cram hemicarceplex that was observed to release a guest upon heating. We found that the side portals of this hemicarceplex have multiple thermally accessible conformations. An eight-membered ring that is part of a portal changes from a "chair" to a "boat" structure, leading to the enlargement of the side portal and the release of the guest. This type of gating is analogous to phenomena often observed with peptide loops in enzymes. We refer to this phenomenon as thermally controlled gating. We have also designed and synthesized redox and photochemically controlled gated hemicarceplexes. Gates are built onto host molecules so that the opening or closing of such gates is stimulated by reducing or oxidizing conditions, or by ultraviolet irradiation. In both cases, the appropriate stimuli can produce a carceplex (closed gates) or hemicarceplex (open gates). A hemicarceplex with closed gates behaves like a carceplex, due to its very high constrictive binding energy. When the gates are opened, constrictive binding is dramatically lowered, and guest entrance and exit become facile. This stimulated switching between open and closed states controls access of the guest to the binding site. The experimental and computational investigations of gated hemicarcerands and several potential applications of gated hemicarceplexes are described in this Account.

Show MeSH
Related in: MedlinePlus