Complexation of benzoic, 4-methylbenzoic and R- and S-2-phenylpropanoic acids and their conjugate bases by 3A-amino-3A-deoxy-(2AS,3AS)-b-cyclodextrin in aqueous solution

A potentiometric titration study of the complexation of benzoic, 4-methylbenzoic, and (R)- and (S)-2-phenylpropanoic acids and their conjugate bases by 3A-amino-3A-deoxy-(2AS,3AS)-β- cyclodextrin, βCD3NH2, in which the amino group may be protonated to produce a singly charged species, βCD3NH3+, is reported. In aqueous solution at 298.2K and I = 0.10 mol dm-3 ( KCl ), the complexation constants for the complexes indicated have the values (in dm3 mol-1) shown in parentheses: benzoic acid.βCD3NH3+ (KHA = 110�10); benzoate.βCD3NH3+ (KA = 19�2); 4-methylbenzoic acid.βCD3NH3+ (KHA = 210�10); 4-methylbenzoate.βCD3NH3+ (KA = 21�3); (R)- and (S)-2-phenylpropanoic acid.βCD3NH3+ (KRHA = 64�8, KSHA = 57�5); (R)- and (S)-2-phenylpropanoate.βCD3NH3+ (KRA = 51�6, KSA = 32�6); and (R)- and (S)-2-phenylpropanoate.βCD3NH2 (KRA′ = 13�7; KSA′ is too small to quantify reliably). These complexation constants are substantially less than those for the host-guest complexes formed by the isomeric 6A-amino-6A-deoxy-β-cyclodextrin and also for those formed by β-cyclodextrin. The origins of these differences are discussed.