SPECTROFLUOROMETRIC STUDY OF THE EFFECTS OF CYCLODEXTRINS ON THE ACID-BASE EQUILIBRIA OF HARMINE AND HARMANE

beta-Carboline alkaloids are important compounds because they exhibit a variety of pharmacological actions, Their acid-base behaviour can be studied by spectrofluorimetry since these molecules present a remarka ble native luminescence, Acid-base equilibria depend on the environmen t of the molecules and inclusion into cyclodextrin (CD) cavities shift s the acid-base equilibria and alters the apparent pK(a) values, The i nfluence of CDs on the acid-base equilibria of the model beta-carbolin es harmine and harmane is described, beta-CD and gamma-CD and the modi fied beta-CDs hydroxypropyl-beta-CD (HP beta-CD), 2,6-di-O-methyl-beta -CD (DM beta-CD) and 2,3,6-tri-O-methyl-beta-CD (TM beta-CD) mere used to form the corresponding complexes with harmine and harmane in the p H range 7.8-8.0. In these buffered solutions the complexes with the di fferent CDs exhibit an emission band with resolved peaks at 360 and 38 0 nm corresponding to the neutral form of harmane and with a remarkabl e enhancement in the emission intensity compared with aqueous solution , In the case of the complexes with beta-CD and gamma-CD, both the cat ionic and the neutral emission bands appear. However, for gamma-CD the cationic band is more intense than the neutral band, the inverse bein g true for beta-CD. In homogeneous aqueous solution at this pH value t he cationic band is the only one observed and therefore the presence o f the neutral band indicates the formation of inclusion complexes, In the harmane-HP beta-CD complexes, the emission bands ascribed to the a nionic form are observed after addition of NaOH. This emission is only observed in homogeneous aqueous solution in strongly alkaline media o utside the normal pH range.