POSITIVE OR ADVERSE-EFFECTS OF METHYLATION ON THE INCLUSION BEHAVIOR OF CYCLODEXTRINS - A COMPARATIVE NMR-STUDY USING PHEROMONE CONSTITUENTS OF THE OLIVE FRUIT-FLY

The complexation of alpha-, beta-, gamma-, tri-O-methyl-alpha-, tri-O- methyl-beta-, and tri-O-methyl-gamma-cyclodextrin (CD) with several co nstituents of the sex pheromone of the olive fruit fly, 1,7-dioxaspiro [5.5]undecane (1), nonanal (2), and ethyl dodecanoate (3), was studied in aqueous solution by NMR spectroscopy. Inclusion was clearly eviden t from the one-dimensional H-1 and C-13 spectra, whereas the proposed mutual disposition of the host and guest molecules was derived from tw o-dimensional POESY spectra and the respective stoichiometries. The st ructures of the complexes in conjunction with their measured associati on constants allowed for comparison of the behavior of the cyclodextri ns. The complexing ability was thus significantly enhanced by methylat ion in the case of alpha-CD, moderately reduced for beta-CD, and large ly diminished for gamma-CD. The retention of the nearly symmetrical CD shape combined with the enlargement of the hydrophobic cavity by meth ylation is considered to be responsible for the properties of tri-O-me thyl-alpha-CD. The increased cavity deformation upon enlargement of th e cavity diameter, however, further supported by the skeletal distorti ons detected for the larger methylated macrocycles, overrules the bene ficial effects of the cavity increase, resulting in poorer performance for tri-O-methyl-beta-CD and an even poorer performance for tri-O-met hyl-gamma-CD. Moreover, the methylated cyclodextrins share common feat ures in their inclusion behavior: ah undergo conformational changes an d all bind exclusively from their secondary side as opposed to normal cyclodextrins, which may use their primary side, too, depending on the guest.