Fluorescence properties of some isoquinoline alkaloids

Abstract

The fluorescence properties of 28 isoquinoline alkaloids have been investigated. In most of them the chromophore responsible for fluorescence was the benzene ring with electron-donor substituents containing oxygen. The long-wave excitation peak practically coincides with the long-wave absorption peak of these substances, covering the region from 284 to 293 nm, the maximum emission being in the range 320-332 nm. With alkaloids having a number of conjugated rings, both excitation and emission were observed at higher wavelengths. Only protoberberine alkaloids behaved as hydrophobic probes, i.e. transfer of these compounds into a less polar medium produced a marked hypsochromic shift and a higher intensity of emission. The effect of polarity on the behaviour of tetrahydroprotoberberines, protopines, pavinanes, aporphines and benzophenanthridines was not so pronounced. Changes of pH manifested themselves most markedly in compounds with dissociable hydroxyl groups; the majority of phenolates did not fluoresce.The phenol group pK values of these compounds in the excited state were lower than in the ground state (which ranged between 8.4 and 10.4). The relations between the apparent pK, determined from fluorescence data, and the pK's of these compounds in the ground and the excited states are discussed.