The Underestimated Role of Counter Ions in Electrostatic Self‐Assembly:[1+1] Cavitand‐Calix[4]arene Capsules Based on Azinium–Sulfonate Interactions

Abstract

The apparent Ka values of the formation of [1+1] cavitand‐calix[4]arene capsules, based on azinium–sulfonate electrostatic interactions, obtained by direct titration experiments are concentration dependent due to the influence of the interaction of the charged capsule components with their initial counterions. 1H NMR and UV dilution experiments, in which this ion‐pairing is excluded, gave Ka‐values larger than 2·106 M–1 in methanol and methanol/water (1:1) for the capsule formation. The capsule encapsulates small guests like methanol, ethanol, etc., as proven by ESI‐MS. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Anion receptors based on organic frameworks: highlights from 2005 and 2006

“…This work has been followed by work on [2 + 3] systems 218 and capsule formation with tetracationic and tetraanionic cavitands. 219 Sessler and co-workers have reported the anion-induced synthesis of a new class of 2,6-diamidopyridine-bipyrrole macrocyclic receptor. 220 seems that in this case the kinetics and thermodynamics of this reaction are in fine balance and depend on subtle changes in reaction conditions.…”

Effect of buffer on heparin binding and sensing in competitive aqueous media

“…A rare example of a buffer effect in supramolecular chemistry was reported by Sirish and Schneider in 2000, who demonstrated that for the interaction of cationic porphyrins with a variety of small phosphate anions, the presence of phosphate buffer at increasing (and high) concentrations 0.1 M and 0.3 M had a significant impact on binding affinity and mode, with lipophilic interactions gaining in importance as the concentration of phosphate buffer increased (22). However, this was primarily due to the impact of ionic strength, and as noted above, such effects on molecular recognition are relatively wellknown, particularly in the field of electrostatic binding (2,(4)(5)(6)(7)(8). In very rare cases, authors have commented on specific buffer effects, such as Rebek and co-workers, who noted in passing in 2004 that for reasons which were 'not readily understood' their hydrophobic hosts showed different binding strengths and selectivities towards adamantane guests in pure water, tris buffer and phosphate buffer (23).…”

“…Ong and Kaifer explored the impact of the presence of ions on cucurbituril binding to viologens and found decreasing binding strengths as the ionic strength increased, with a dependence also on the charge of the competing cation (4). The importance of this was emphasised by Verboom and co-workers who studied the impact of counter-ions on electrostatic self-assembly (5). This type of effect has recently been revisited by García-Río and co-workers in the context of binding constant determination (6).…”

Effect of buffer at nanoscale molecular recognition interfaces – electrostatic binding of biological polyanions

“…Electrolyte can affect binding through charge-screening 2 or Hofmeister effects. 3 The impact of ions on host-guest binding, 4 selfassembly 5 and multivalent recognition 6 have been reported, and discussed with regard to binding constant determination. 7 However, the impact of buffer is less often explored.…”