Dynamic molecular motions of p-methylcinnamic acid included into β-cyclodextrin derivatives: a new type of free-energy relationship in complex formation

Abstract

The deuterium quadrupolar relaxation times of (E)-[β,methyl-²H₂]-p-methylcinnamic acid (G) included into various β-cyclodextrin derivatives have been measured by line-shape analysis. The association constants for the complexation of host with (G) are largely affected by additional recognition elements substituted into the parent β-cyclodextrin. The correlation times for the molecular motion of G which have been estimated from the corresponding relaxation times are also greatly affected by these additional recognition elements. Thus, the relationship between the association constants and the correlation times of (G) may be described simply in the present case, i.e., the host molecule having the larger association constant restricts the molecular motion of (G) more significantly. One exception is 6,6′-dideoxy-6,6′-di-iodo-β-cyclodextrin (CDI₂) which has a large association constant but restricts only slightly the molecular motion of (G). The observed large association constant and the small correlation time for CDI₂ are reasonably interpreted in terms of the effect of the large polarizability of iodine. The free-energy relationship between the binding process and the molecular motion of (G) is discussed on the basis of these new observations.