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Interaction of Na+ and K+ ions with DTPA-amide dioxa-pentaaza-cyclophanes: effect of electrostatic field in macrocyclic cavity on UV absorption spectra and protonation
The effects of electrolytes on protonation were studied by UV spectrometry on the geometrical isomers of dioxapentaazacyclophanes that were synthesized by 1+1-cyclization of diethylenetriaminepentaacetic (DTPA) dianhydride with aromatic diamine involving 1,3- or 1,4-phenylenebis(methyleneoxy) group. Absorption bands at 245 and 280nm respond to pH in association with the protonation status confirmed by H-1 NMR. The logarithmic first-protonation constants, determined from the absorptivity versus pH curves, depend on coexisting alkali metal ions in a range of 7.7-8.2 in 0.01 M solutions of the chlorides. The more pronounced effect of alkali metals occurs in the dissociation of amide proton; the logarithmic deprotonation constant, log Kp-1, of the 1,3-oxymethyl isomer is -9.48 in 0.01 M NaCl and -10.35 in 0.01 M KCl; the constant of the 1,4-oxymethyl isomer, -9.79 in 0.01 M NaCl and -11.58 in 0.01 M KCl; no amide deprotonation is observable in 0.01 M LiCl. Alkali metal ions Na+ and K+ are accommodated in a macrocyclic cavity through interaction with the oxa-donor sites, and produce an intense electrostatic field within the cavity. The resulting field causes large changes in absorptivity, and also enforces amide deprotonation. Sodium ion is so effective that the deprotonation occurs at pH as low as 9.5-9.8. Potassium ion is less effective on the amide deprotonation, but the action is selective towards the isomers.
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