High-pressure studies as a novel approach in determining inclusion mechanisms: Thermodynamics and kinetics of the host-guest interactions for alpha-cyclodextrin complexes

The first volume profiles for complex formation of alpha-cyclodextrins (alp ha-CD) with diphenyl azo dyes (S) are presented as a new approach in unders tanding inclusion phenomena. The following dyes were selected: sodium 4-(4- diethylaminophenylazo)benzenesulfonate (1), sodium 4-(3-carboxy-4-hydroxy-5 -methylphenylazo)benzenesulfonate (2), sodium 4-(4-hydroxy-3,5-dimethylphen ylazo)benzenesulfonate (3), and sodium 2-hydroxy-3-methyl-5-(4-sulfamoylphe nylazo)benzoate. The behavior of the dyes alone were first studied in aqueo us solutions to rule out any competition reaction. Under the experimental c onditions used for the stopped-flow kinetic studies, it has been proved tha t only monomeric species are present (no aggregation of the dye is formed b y pi-pi stacking interactions). NMR experiments and kinetic evidences have shown that only directional binding of the dye via the sulfonate/sulfonamid e group through the wide rim of the alpha-cyclodextrin was possible. The 1: 1 complex was the only stoichiometric species formed. The inclusion reactio ns for the four selected dyes were characterized by a two-step kinetics des cribed by a first fast step that yields the intermediate, S.alpha-CD*, foll owed by a slower rearrangement to form the final complex, S.alpha-CD. 2D NM R experiments served for a molecular dynamics calculation leading to a stru ctural representation of the intermediate and final complexes. An interpret ation of the volume profiles obtained from high-pressure stopped-flow kinet ic experiments have not only confirmed the so far proposed mechanisms based on "classical" kinetic investigations but offered a new focus on the inclu sion process. The inclusion mechanism can be summarized now as follows: the complexation begins with an encounter of the dye and alpha-cyclodextrin ma inly due to hydrophobic interactions followed by a partial desolvation of t he entering head of the dye. The latter interacts with the two "activated" inner water molecules of the free host and their complete release is delaye d by the primary hydroxy group barrier of the alpha-CD. At this first trans ition state, a squeezed arrangement develops inside the cavity inducing a n egative activation volume (Delta V-1,(f)double dagger approximate to -8 to -24 cm(3) mol(-1)). The subsequent intermediate is characterized by a total release of the two inner water molecules and interactions of the dye head with the primary hydroxy groups of the host in a trapped-like structure (De lta V(1)degrees approximate to -11 to -4 cm(3) mol(-1)). The latter interac tions and concurrent tail interactions with the secondary hydroxy groups of the host lend at different extents to a strained conformation of the host in the second transition state (Delta V-2,(f)double dagger approximate to - 2 to -16 cm(3) mol(-1)). In the final complex, the head of the dye is total ly rehydrated as it protrudes from the primary end of the host cavity which can now adopt a released conformation (Delta V(2)degrees approximate to +3 to +6 cm(3) mol(-1) vs +17 cm(3) mol(-1) for 1).