6 research outputs found
Systems chemistry: logic gates based on the stimuli-responsive gel-sol transition of a crown ether-functionalized bis(urea) gelator
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.A quite simple, achiral benzo-21-crown-7-substituted bis(urea) low-molecular weight gelator hierarchically assembles into helical fibrils, which further develop into bundles and finally form a stable gel in acetonitrile. The gel–sol transition can be controlled by three different molecular recognition events: K+ binding to the crown ethers, pseudorotaxane formation with secondary ammonium ions and Cl− binding to the urea units. Addition of a cryptand that scavenges the K+ ions and Ag+ addition to remove the chloride and bases/acids, which mediate pseudorotaxane formation, can reverse this process. With the gelator, and these chemical stimuli, a number of different systems can be designed that behave as logic gates. Depending on the choice of components, OR, AND, XOR, NOT, NOR, XNOR and INHIBIT gates have been realized. Thus, the gel–sol transition as a property of the system as a whole is influenced in a complex manner. For some cases, the type of logic gate is defined by input signal concentration so that an even more complex reaction of the gel towards the two input signals is achieved.DFG, SFB 765, Multivalenz als chemisches Organisations- und Wirkprinzip: Neue Architekturen, Funktionen und Anwendunge
Gating the photochromism of an azobenzene by strong host–guest interactions in a divalent pseudo[2]rotaxane
The ability of an E-configured azobenzene guest to undergo photoisomerisation is controlled by the presence of a complementary host. Addition of base/acid allowed for a weakening/strengthening of the interactions in the divalent pseudo[2]rotaxane complex and hence could switch on/off photochromic activity.Peer Reviewe
Thermodynamic Analysis of Allosteric and Chelate Cooperativity in Di- and Trivalent Ammonium/Crown-Ether Pseudorotaxanes
A detailed thermodynamic
analysis of the axle-wheel binding in
di- and trivalent secondary ammonium/[24]Âcrown-8 pseudorotaxanes is
presented. Isothermal titration calorimetry (ITC) data and double
mutant cycle analyses reveal an interesting interplay of positive
as well as negative allosteric and positive chelate cooperativity
thus providing profound insight into the effects governing multivalent
binding in these pseudorotaxanes
Chelate Cooperativity and Spacer Length Effects on the Assembly Thermodynamics and Kinetics of Divalent Pseudorotaxanes
Homo- and heterodivalent crown-ammonium pseudorotaxanes
with different
spacers connecting the two axle ammonium binding sites have been synthesized
and characterized by NMR spectroscopy and ESI mass spectrometry. The
homodivalent pseudorotaxanes are investigated with respect to the
thermodynamics of divalent binding and to chelate cooperativity. The
shortest spacer exhibits a chelate cooperativity much stronger than
that of the longer spacers.
On the basis of crystal structure, this can be explained by a noninnocent
spacer, which contributes to the
binding strength in addition to the two binding sites. Already very
subtle changes in the spacer length, i.e., the introduction of an
additional methylene group, cause substantial changes in the magnitude
of cooperative binding as expressed in the large differences in effective
molarity. With a similar series of heterodivalent pseudorotaxanes,
the spacer effects on the barrier for the intramolecular threading
step has been examined with the result that the shortest spacer causes
a strained transition structure and thus the second binding event
occurs slower than that of the longer spacers. The activation enthalpies
and entropies show clear trends. While the longer spacers reduce the
enthalpic strain that is present in the transition state for the shortest
member of the series, the longer spacers become entropically slightly
more unfavorable because of conformational fixation of the spacer
chain during the second binding event. These results clearly show
the noninnocent spacers to complicate the analysis of multivalent
binding. An approximate description which considers the binding sites
to be connected just by a flexible chain turns out to be more a rough
approximation than a good model. The second conclusion from the results
presented here is that multivalency is expressed in both the thermodynamics
and the kinetics in different ways. A spacer optimized for strong
binding is suboptimal for fast pseudorotaxane formation