Binding of dipeptides and tripeptides containing lysine or arginine by p-sulfonatocalixarenes in water: NMR and microcalorimetric studiesElectronic supplementary information (ESI) available: chemical shifts experienced by different protons of KK, RR, KKK or RRR in the presence of increasing amounts of 14 or 16; heat effects observed upon titration of 14, 16 or 18 by KK, RR, KKK or RRR; COSY and ROESY 2D 1H NMR spectra for 14 complexed with KK or RR. See http://www.rsc.org/suppdata/p2/b1/b109553f/

The “True” Affinities of Metal Cations to p‐Sulfonatocalix[4]arene: A Thermodynamic Study at Neutral pH Reveals a Pitfall Due to Salt Effects in Microcalorimetry

“…The denominator in Equation (13) then indeed adopts a very large value of about 16, which accounts largely for the deviation by more than one order of magnitude. Additionally, variations in binding constants with pH (the binding of the tetraanion at pH 2 is naturally weaker than that of the pentaanion at pH 7) [17,19,20,22,46,47] and the possibility of higher-order complexes at the employed high concentrations can lead to additional deviations, such that the previous set of low binding constants [25] provides a manifestation of the interference of cation binding rather than their quantification; the same conclusion holds for the previously reported thermodynamic parameters, which are obviously in similarly unsatisfactory agreement with the data in Table 3.…”

“…Thermodynamic parameters obtained from the simultaneous fitting of ITC titrations of divalent cations to SC4 in water at neutral pH (7.0) and 298.15 K. changes. [17,19,20,46,47] Presumably, the deviation is due to the presence of large amounts of counterions (ca. 80 mm, see above) and large amounts of host used in the previous study.…”

“…For example, binding constants in the ranges of 100-6700 and 200-3900 m À1 were reported for binding of lysine and arginine, respectively, by SC4 in water at pH 1-2. [16,17,20,21] In parallel, it was firmly established that di-and trivalent metal ions bind to SC4 with significant binding constants. [18] Bakirci et al showed in 2005 by fluorescence displacement titrations that alkali metal cations and ammonium-and therefore also the innocuous sodium ions ubiquitously used in buffers-indeed bind significantly to SC4, with binding constants of 80-280 m À1 between pH 2 and 8.…”

“…[22] The same authors also emphasized that the presence of alkali metal cations prevents measurement of the true binding constants with other guests, and this was suggested as a likely explanation for the observed large spread in binding constants, for example, for amino acids. [16,17,20,21,23,24] Only one year later, another conflicting study appeared, in which the same authors, who had previously not detected any heat effect in ITC measurement, [16,18] were now able to detect heat effects by using much higher (22 mm) concentrations of SC4; [25] alkali metal cations and ammonium were now reported to show detectable heat effects and affinities, but not the crucial Na + ion and Ag + . However, the binding constants of only 2-15 m À1 were sufficiently low to eventually again justify the use of buffers in determinations of binding constants, especially since no heat effect was detected for the common Na + buffer ion.…”

Molecular recognition of organic ammonium ions in solution using synthetic receptors

“…The p -sulfonatocalix[4]arenes formed 1:1 complexes more strongly with basic amino acids with K ass values for Arg and Lys of 1520 and 740 M −1 , respectively (phosphate buffer at pH 8), than with aliphatic or aromatic amino acids: Val, Leu, Phe, His, Trp, with K ass values between 16 M −1 and 63 M −1 (phosphate buffer at pD 7.3) [292,295]. …”