Complexation of Charged Porphyrins by Charged and Metal‐Chelated EDTA‐Tethered β‐Cyclodextrin Dimers: A Thermodynamic Study on the Influence of Tether Charge and Flexibility on Binding Affinity

Abstract

AbstractComplex formation between charged porphyrin guests and charged or neutral β‐cyclodextrin (CD) host dimers linked by ethylenediamine‐N,N,N,N‐tetraacetate (EDTA) tethers was assessed by microcalorimetry. Two EDTA‐tethered CD dimers (1 and 2) were employed to obtain a series of structurally similar CD dimers with different tether charges and flexibilities through either (partial) protonation or metal chelation (with CaII or EuIII) of the EDTA tether. Both the tether charge and the flexibility of the CD dimers strongly influenced the binding properties, giving rise to binding constants differing by factors of up to 22. The effect of tether charge was elucidated by the complexation of tetraanionic meso‐tetrakis(4‐sulfonatophenyl)porphyrin (TSPP) by the different forms of CD dimer 1. Tether charge‐dependent complex (de)solvation gave rise to large systematic and counteracting changes in binding enthalpy and entropy, resulting in a binding affinity for the positively charged europium complex of the CD dimer five times larger than that of the negatively charged free li‐gand CD dimer. No marked differences in binding affinities were observed for the complexation of TSPP by CD dimer 2. Both restricted tether flexibility by metal chelation and repulsive electrostatic interactions resulted in diminished binding affinity of CD dimer 1 for the relatively large tetracationic p‐tert‐butylbenzyl‐functionalized p‐pyridylporphyrin (TBPyP). The europium complex of CD dimer 1 binds TBPyP less strongly than the corresponding negatively charged uncomplexed forms of the CD dimer, by a factor of up to 22. The tether of CD dimer 2 is too short to allow strong binding of TBPyP. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)