SALICYLIC ACID RECOGNITION
Lanthanide luminescent sensor detects active form of aspirin
Supramolecular systems based on terbium(III) complexes bind and recognize aromatic carboxylic acids, including salicylic acid, in water, chemists in Ireland and Northern Ireland have shown.
The chemists observed that terbium(III) complexes of a tetraazacyclodecane possessing two labile water molecules bound to the lanthanide atom give rise to large enhancements in lanthanide luminescence when the lanthanide atom chelates to aromatic carboxylic anions.
The work was done by organic chemistry lecturer Thorfinnur Gunnlaugsson and graduate students Andrew J. Harte and Joseph P. Leonard at Trinity College Dublin, and X-ray crystallographer Mark Nieuwenhuyzen at Queen's University of Belfast [Chem. Commun., 2002, 2134].
"Our terbium(III)-based chemosensor selectively recognizes salicylic acid under mimicked physiological conditions," Gunnlaugsson says. "The acid is the active painkiller that is formed in the stomach by the hydrolysis of its acetate ester--the prodrug aspirin. The chemosensor does not bind to the ester and therefore discriminates between the active and administered forms of the drug."
DISPLACEABLE Two water molecules occupy vacant coordination sites of terbium(III) complex. These supramolecular systems are kinetically and thermodynamically stable.
The complex's sensing action results from the enhanced sensitization of the ion's excited state by the aromatic acid when it displaces the labile metal-bound water molecules.
"Prior to recognition, the chemosensors are 'photophysically' silent since no light is emitted until they bind with the acid," Gunnlaugsson explains. "Our systems emit signals well after fluorescence of the surrounding biological environment has occurred. There is therefore no pollution from fluorescent moieties in the human body such as proteins."
The group is now trying to improve the sensor's sensitivity and develop the systems for other drugs.