Spectroscopic study on the inclusion complexes of β-cyclodextrin with selected metabolites of catecholamines

• Vanillylmandelic and homovanillic acid have importance in medical diagnosis.
• β-cyclodextrin forms inclusion complexes with these compounds as well with vanillin.
• Stability constant of complexes were calculated and geometry was established from NMR.
• Water solution of TSP as an external reference ensures more reliable values of Kassoc.
• β-cyclodextrin is able for effective entrapping of homovanillic acid and vanillin.

Inclusion complexes formed between β-cyclodextrin (β-CD) and metabolites of catecholamines, i.e. vanillylmandelic acid (VMA), homovanillic acid (HVA) as well as vanillin (VA) were studied using NMR spectroscopy. Due to the importance of these compounds for the diagnosis tumours of the sympathoadrenal system, hydrogels containing β-CD moieties for enhancing entrapping metabolites of catecholamine from aqueous solutions are located in the area of our interest. Stoichiometry and association constants of the complexes of β-CD with VMA, HVA and VA respectively were determined by using continuous variation and 1H NMR titration methods. Significant discrepancies were pointed out depending on used referencing method. In this study water solution of 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt as an external reference was used to avoid errors in the determination of association constants. β-CD formed the most stable complexes with VA and HVA molecules whilst smallest value of association constant was determined for the VMA/β-CD complex. Two-dimensional rotating-frame Overhauser effect spectroscopy (2D ROESY) allowed to establish definite information on the molecular structures of the complexes formed. Geometry of the latter was proposed basing on contour plots of the 2D ROESY spectra, which also indicated two possibilities of complexed molecule arrangement into β-cyclodextrin interior. The values of determined association constants are in good agreement with postulated geometry of the complexes.Value of association constant determined for inclusion complexes of β-cyclodextrin with homovanillic acid an vanillin indicates the strongest binding of molecules among investigated complexes, so it was finally concluded that β-CD moiety introduced into hydrogel network could be effective for homovanillic acid and vanillin entrapping.

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