Effect of methylation of β-cyclodextrin on the formation of inclusion complexes with aromatic compounds. An ionspray mass spectrometry investigation

The investigation of the inclusion complexes obtained with heptakis-(2,6-di-O-methyl)-β-cyclodextrin (DM-β-Cdx) or heptakis-(2,3,6-tri-O-methyl)-β-cyclodextrin (PM-β-Cdx) and either 1-anilinonaphthalene-8-sulfonate (ANS) or2-p-toluidinylnaphthalene-6-sulfonate (TNS) was carried out by means of ionspray mass spectrometry, both in the positive and in the negative ion mode. All the data collected showed that the heptakis-(2,3,6-tri-O-methyl)-β-cyclodextrin interacted to a very little extent with TNS and not at all with ANS. On the contrary, heptakis-(2,6-di-O-methyl)-β-cyclodextrin formed complexes with both aromatic molecules showing a more effective interaction with TNS. Small variations in the number of methoxy substituents in the DM-β-Cdx molecule did not affect the complexation behaviour significantly. The spectra recorded at different orifice potentials indicated that the complex of heptakis-(2,6-di-O-methyl)-β-cyclodextrin with TNS is more stable than the one formed with ANS. These results agreed on one hand with the conformations of both the heptakis-(2,6-di-O-methyl)-β-cyclodextrin and heptakis-(2,3,6-tri-O-methyl)-β-cyclodextrin, established by X-ray diffraction studies, and, on the other hand, with the different complexation behaviour of the guest aromatic molecules due to their own geometry.