Thermodynamic and kinetic stabilities of complementary double helices utilizing amidinium-carboxylate salt bridges

A series of dimer strands consisting of m-terphenyl backbones bearing complementary chiral or achiral amidines and achiral carboxylic acid residues connected by various types of linkers, such as diacetylene, Pt(II)-acetylide, and p-diethynylbenzene linkages, were synthesized by a modular strategy, and their chiroptical properties on the complementary double helix formations were investigated by absorption, circular dichroism (CD), and (1)H NMR spectroscopies. The thermodynamic and kinetic stabilities of the complementary double helices assisted by amidinium-carboxylate salt bridges are highly dependent on their linkages, and the thermodynamic analyses of the dimer duplexes revealed that the association constants increased in the order: Pt(II)-acetylide linker < p-diethynylbenzene linker < diacetylene linker, which is in agreement with the reverse order of their bulkiness. The substituents on the amidine groups were also found to affect the stabilities on the duplexes and the association constants increased in the order: isopropyl < (R)-1-phenylethyl < cyclohexyl. In addition, the introduction of electron-donating and/or electron-withdrawing substituents at the phenyl groups of the p-diethynylbenzene linkers connecting the amidine and carboxylic acid units, respectively, tends to stabilize the complementary double helices, especially in polar solvents, such as DMSO, due to the attractive charge-transfer interactions between the aromatic linkers, although the salt bridge formation is hampered in DMSO. Furthermore, the kinetic analyses of the chain exchange reactions for the duplexes bearing diacetylene and p-diethynylbenzene linkages showed that these were slow processes with negative ΔS([symbol: see text]) values, meaning that the chain exchange reactions proceed via direct exchange pathways. In contrast, those for the duplexes bearing Pt(II)-acetylide linkages were fast processes supported by positive ΔS([symbol: see text]) values, suggesting that the chain exchange reactions proceed via dissociation-exchange ones. The helix-inversion kinetics investigated for the racemic dimer duplexes composed of achiral amidines based on variable-temperature (1)H NMR measurements indicated that the barriers for the helix-inversion increased in the order: Pt(II)-acetylide linker, p-diethynylbenzene linker < diacetylene linker.     

Capture and release of CO2 by polyamidine

By CO₂ bubbling into an aqueous dimethyl sulfoxide solution, the polyamidine ( Poly‐Amd ) prepared from hexamethylenediamine and triethyl orthoacetate was converted to the bicarbonate salt, Poly‐Amd H·HCO₃, which formed aggregates. Conversely, the aggregates disappeared upon Ar bubbling to release the captured CO₂, reverting to Poly‐Amd completely. Polyethyleneimine ( PEI ) also reversibly captured and released CO₂ molecules in solution by bubbling CO₂ and Ar, respectively, in an alternate manner. No appreciable difference was observed between Poly‐Amd and PEI in CO₂ capture in solution. The binary system consisting of Poly‐Amd and polyethylene glycol ( PEG ) captured CO₂ efficiently at ordinary pressure and reached a stationary state within 200 h, at which 66% of the amidine groups bound CO₂ molecules, which was released upon exposure to a N₂ flow. In contrast to the binary system with Poly‐Amd, the binary system of PEI with PEG did not capture CO₂ efficiently, and only 5.7% of the amino groups bound CO₂ molecules after 600 h. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3404–3411     

On isomorphism and inclusion property for certain £(p,λ) spaces of strong type

Summary We prove isomorphism and inclusion theorems for certain ￡^(p,λ) spaces of strong type introduced by G. Stampacchia. These results are quite analogous to those of S. Campanato and G. N. Meyers[1], [4], F. John and L. Nirenberg[3] and L. C. Piccinini[9]. Entrata in Redazione il 14 luglio 1976.     

CHLORANIL-PHOTOSENSITIZED MONOMERIZATION OF DIMETHYLTHYMINE CYCLOBUTANE DIMERS and EFFECT OF MAGNESIUM PERCHLORATE

The photosensitized monomerization of the cyclobutane dimers of 1,3-dimethylthymine by p-chloranil was investigated by means of steady-state irradiation and laser-flash photolysis. Quantum yields for the monomerization are 0.34 for the cis,syn dimer, 0.39 for the trans,syn dimer, and much less than 10(-2) for the cis,anti isomer. Formation of the chloranil anion radical associated with quenching of triplet chloranil by the dimers demonstrates that electron transfer from dimers to triplet chloranil occurs to initiate the monomerization. Kinetic analysis suggested that the syn-dimer cation radicals undergo the ring cleavage at greater than or equal to 10(9) s-1 before escaping from the solvent cage, while the reactivity of the anti-dimer cation radical is very low. The different reactivities of the syn and anti dimer cation radicals are discussed in terms of through-bond coupling between the n orbitals of N(1) and N(1') involving the cyclobutane-ring sigma orbitals. In the cases of the syn-dimers, the sensitizer-dimer ion-radical pairs undergo the rapid geminate recombination that works as a major energy dissipating channel responsible for the lower-than-unity quantum yields. It has been found that the presence of Mg(ClO4)2 at 0.1 M enhances approximately 1.5 times either the monomerization of the syn dimers or the formation of the chloranil anion radical. A laser-flash photolysis study shows that Mg2+ forms a complex with either the triplet or the anion radical of chloranil. The net salt effects are attributed to the retardation of the rapid geminate recombination by the participation of Mg2+ in the sensitizer-dimer ion-radical pairs.     

Stereoselective [3,3]-sigmatropic rearrangement promoted by the metal [1,3]-shift of binuclear Fischer carbene complexes

Reaction of chiral homobinuclear Fischer chromium carbene complexes with allyl alcohol in the presence of NaH and the following oxidative demetalation gave alpha-allyl esters in up to 97% ee via [3,3]-sigmatropic rearrangement reaction promoted by the metal 1,3-shift. On the other hand, chiral heterobinuclear tungsten carbene complexes with arene chromium complexes afforded alpha-allyl-beta-hydroxy esters as a major product in up to 92/8 dr by the same reaction sequence.     

Oligoresorcinols fold into double helices in water

We report the first double helices with a controlled helicity in water based on oligoresorcinols as a new, simplest water-soluble structural motif. The molecular strands of the oligoresorcinols self-assemble into double helices with the aid of aromatic interactions in water as characterized by 1H NMR and absorption spectroscopies together with the X-ray crystallographic study of the pentamer. The double helix formation is sensitive to the chain length, solvent composition, and temperature. Moreover, a bias in the screw sense of the double helices was achieved by covalently attaching chiral substituents to both ends of the molecular strands.     

End-capping of a pseudorotaxane via Diels-Alder reaction for the construction of C60-terminated [2]rotaxanes

[reaction: see text] The Diels-Alder reaction of various dienophiles such as C(60) with a pseudorotaxane having a sultine moiety afforded corresponding [2]rotaxanes in moderate yields. The introduction of a porphyrin moiety on the wheel component considerably enhanced the efficiency of the end-capping reaction with C(60).