Mechanism of inclusion-compound formation for binding of organic dyes, ions and surfactants to α-cyclodextrin studied by kinetic methods based on competition experiments

Abstract

This kinetic and thermodynamic study is concerned with a comparison of binding of simple anions, azo-dyes and longchain surfactants to acyclodextrin. Two competition reactions have been employed. (i) In order to obtain information on the association of simple anions and surfactants with a-cyclodextrin, the competitive binding of an azo-dye with a-cyclodextrin has been used in stopped-flow experiments. (ii) In order to study the binding of the azo-dye pyridine-2-azo-p-dimethylaniline (PADA) to acyclodextrin, the competition reaction used is that between PADA and aquo-metal ions. Ni2+(aq) and Zn2+(aq) are used as competitors for PADA in stopped-flow and Joule-heating temperature-jump experiments.It is confirmed that when azo-dyes interact with a-cyclodextrin, they do so by a two-step mechanism. The first step involves the formation of an intermediate complex in a fast pre-equilibrium step. The second step, which is rate-determining, results in the formation of the final stable inclusion complex. Studies with I-(as) as a competing ion suggest that the intermediate is associated with dye binding to the inside rather than the outside of the cyclodextrin.Small anions in general have low stability constants for binding to a-cyclodextrin. In marked contrast, surfactants of general formula C,H,,+,SO; (where n > 8) interact strongly with a-cyclodextrin, and inclusion compounds are formed in which one surfactant binds two cyclodextrin molecules. Materials and MethodsThe dyes MY7 and PADA were supplied by Aldrich and Sigma, respectively. Elemental analysis was used to confirm purity. The structures are shown below.

Solution inclusion complexes of cyclodextrins with sodium perfluorooctanoate

“…Clearly, the order of binding with SPFO is P-CD > y-CD > a-CD with the last showing very little association at all. The intersection of the linear portions of the curves for the three cyclodextrins gives the stoichiometry of the main complex formed as 1: 1 in agreement with findings from other surfactant systems (3)(4)(5)(6)(7)(8). All pre-micellar SPFO solutions displayed the same behavior.…”

“…Recent work has shown that fluorocarbon surfactants bind very strongly with P-CD (2, 3) with little tendency to form inclusates with a-CD (2). However, surfactant hydrocarbon chains of diameter 4.5 A apparently are readily accommodated within the a-CD cavity because stable complexes are formed with both a-CD and P-CD (3)(4)(5)(6)(7)(8). A complicating feature in the hydrocarbon chain situa-.…”

“…A complicating feature in the hydrocarbon chain situa-. tion is that a-CD/surfactant complexation appears to favour 2: 1 complexes while 1:l complexes predominate with P-CD (4,8). A detailed, multi-faceted study of the inclusion of a perfluorinated surfactant by the cyclodextrins in solution was deemed appropriate to shed light on the importance of size parameters in the strength and position of binding.…”

Surfactant–cyclodextrin interactions by conductance measurements

“…With aCD, the conductance curves consist of three linear sections, the intersections of which indicate 2:l and 1:l stoichiometric species. The former is more distinct and is the ratio reported by Hersey, Robinson, and Kelly (6). If the first aCD encapsulates the hydrocarbon chain of the surfactant, then the second a C D probably envelopes the polar end in a CD/S/CD complex where S represents the surfactant monomer.…”

Cyclodextrins

“…For both weakly and strongly bound drugs, we have attempted to measure the kinetics of this process by various fast kinetics techniques; in no case have we been able to estimate either the "on" or "off" rate-that is, the processes are faster than could be measured. Others have noted values for these kinetics using ultra fast kinetics techniques (Cramer et al, 1967;Rohrbach and Wojcik, 1981;Hersey and Robinson, 1984;Hersey et al, 1986) and have found that in most cases, rates approach those of diffusion-controlled limits. In only one case (unpublished work) did we see some evidence of slow kinetics when we observed nuclear magnetic resonance (NMR) line broadening for a drug with a >1 × 10 5 M -1 binding constant.…”