Maki Ito’s research while affiliated with Tohoku University and other places

A γ-cyclodextrin (γ-CyD) derivative (1) having a pyrene moiety, connected through a 4,13-diaza-18-crown-6 ether moiety to γ-CyD, was synthesized and evaluated for guest binding and sensing properties. In aqueous solution, 1 existed as an association dimer in which the secondary hydroxyl sides faced each other to accommodate two pyrene moieties. Photo- induced electron transfer (PET) between the amino group and the excited pyrene moiety regulated the monomer fluorescence intensity of 1. The monomer- dimer equilibrium and the PET indicated that I may be used as a host capable of detecting guest complexation by changes in fluorescence intensity from the pyrene moiety. Deoxycholic acids were found to be good guests for detection by 1, and deoxycholic acid itself induced different fluorescence changes compared to the other deoxycholic acids. This indicated that 1 could recognize the position of the hydroxy groups on the steroidal framework. The azacrown part may participate in the guest selectivity for the deoxycholic acids by regulating the distance between the amino group and the pyrene moiety, modifying PET efficiency.

The β-cyclodextrin derivative (DN-β-CyD) having a branched arm derived from L-ornitine and two molecules of 2-naphthoic acid was synthesized for exploring the conformational change associated with guest binding. When DN- β-CyD was alone in a 10% ethylene glycol aqueous solution, one of the naphthoyl residues in the branched arm was accommodated in the DN-β-CyD cavity. This intramolecular self complex was stabilized by the other naphthoyl moiety located outside of the cavity on the basis of a hydrophobic capping effect. Upon guest binding, the naphthoyl residue residing in the cavity was expelled from the cavity, forming an excimer with the other naphthoyl residue outside of the cavity. DN-β-CyD was not able to bind the guests strongly, owing to the stability of the intramolecular complexation form. Although DN-β-CyD was proven to be a poor host in guest binding, the guest recognition ability of DN-β-CyD could be estimated by the use of guest-induced fluorescence variations. DN-β-CyD could bind both monoterpenes and cholic acid derivatives to nearly the same extent. However, variations in fluorescence intensity induced by guest binding indicated, as a fluorescent sensing system, that DN-β-CyD was able to detect cholic acid derivatives with higher sensitivity than terpenoids.

Gamma-cyclodextrins bearing a pyrenylamide moiety at primary or secondary hydroxyl side (1 and 2, respectively) showed red-shifted fluorescence as well as normal fluorescence in a solution of pH below 3. This red-shifted fluorescence was emitted from the pyrenylamide moiety protonated in the excited state. Guest binding ability of 2 was markedly affected by the protonation, while that of 1 was hardly affected.