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Comparison of binding of tetraphenylborate and tetraphenylphosphonium ions to cyclodextrins studied by capillary electrophoresis
Author(s) -
Nhujak Thumnoon,
Goodall David M.
Publication year - 2001
Publication title -
electrophoresis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.666
H-Index - 158
eISSN - 1522-2683
pISSN - 0173-0835
DOI - 10.1002/1522-2683(200101)22:1<117::aid-elps117>3.0.co;2-t
Subject(s) - tetraphenylborate , chemistry , ion , crystallography , capillary electrophoresis , analytical chemistry (journal) , stereochemistry , chromatography , organic chemistry
Binding constants for tetraphenylborate and tetraphenylphosphonium ions (Ph 4 B − and Ph 4 P + ) to cyclodextrins (CDs) to give 1:1 host‐guest complexes have been measured using capillary electrophoresis. Mobilities of the ions as a function of γ‐CD concentration give binding constants, K , of 1.08 × 10 5 M −1 for Ph 4 B − and 0.6 × 10 1 M −1 for Ph 4 P + . This dramatic difference of four orders of magnitude in binding constants is not seen with β‐CD ( K = 7.7 × 10 1 M −1 for Ph 4 B − and 3.7 × 10 1 M −1 for Ph 4 P + ) or dimethyl (DM)‐β‐CD ( K = 46 × 10 1 M −1 for Ph 4 B −1 and 7.7 × 10 1 M −1 for Ph 4 P + ). The crystal and hydrodynamic radii of the ions, the latter calculated from their absolute mobilities, indicate that Ph 4 B − is smaller than the γ‐CD cavity, whereas Ph 4 P + is approximately the γ‐CD cavity size. Results suggest that Ph 4 B − fits exactly into a γ‐CD cavity, with hydrophobic contacts involving several of the phenyl rings, whereas Ph 4 P + is too large to enable these multiple contacts to be made. When only a single phenyl ring can fit into the CD cavity, binding strengths are in the order DM‐β‐CD > β‐CD > γ‐CD and Ph 4 B − > Ph 4 P + . Measurement of electrophoretic mobilities of the complexes shows that their hydrodynamic radii are in the order γ‐CD < β‐CD ˜ DM‐β‐CD for Ph 4 B − and γ‐CD > β‐CD ˜ DM‐β‐CD for Ph 4 P + .