Chelate Cooperativity and Spacer Length Effects on the Assembly Thermodynamics and Kinetics of Divalent Pseudorotaxanes

Abstract

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.

Rotaxanes and pseudorotaxanes with threads containing viologen units

“…Having in view a multitude of works dealing with rotaxanes [101][102][103] and pseudorotaxanes, 104,105 as well as works concerning these both structures, 106,107 in the present paper only selected examples are described. Among a great number of applications of rotaxanes, such as construction of molecular devices or sensors, one should point out the possibility to use these species in the design of molecular machines.…”

Multivalenz als chemisches Organisations‐ und Wirkprinzip

“…1, handelt es sich um negative Kooperativität; der bivalente Komplex liegt dann grçßtenteils in der offenen Form vor. [36] Die effektive Molarität kann wie in Abbildung 5 an einem bivalenten Pseudorotaxan [38] gezeigt -und von Hunter et al [39][40][41] …”

Investigação eletroquímica e calorimétrica da interação de novos agentes antitumorais biscatiônicos com DNA

“…30,31 É muito utilizada no estudo de processos por complexometria, tendo como fundamento a medida do fluxo de calor envolvido na reação. 32,33 Efetua-se uma titulação microcalorimétrica para medir a extensão da interação (ou ligação) de uma molécula (componente A) que é adicionada em quantidades e concentrações definidas a outra molécula (componente B), medindo-se o calor gerado. 31,34 É uma técnica em que a constante de ligação (Kb), energia livre de ligação de Gibbs (ΔG), entalpia (ΔH) e entropia (ΔS) podem ser determinadas diretamente e com alta precisão, a partir de um único experimento.…”