Exploring the nature of the clopidogrel-bromocresol green interaction via spectrophotometric measurements and quantum chemical calculations.

Clopidogrel is an oral, thienopyridine class antiplatelet agent used to inhibit blood clots in coronary arteries, peripheral vascular and cerebrovascular diseases. A spectrophotometric method was developed for clopidogrel bisulfate (CLOP·H2SO4) determination using bromocresol green (BCG) as an ion-pairing agent. To explore the binding nature of CLOP·H2SO4 with BCG at a molecular level, quantum chemical calculations have been performed. DFT based full geometry optimization has been carried out for BCG and clopidogrel in basic (CLOP) and protonated (CLOP+) forms as well as for BCG ion-pairs with CLOP and CLOP·H2SO4. The DFT calculations referred to the stability of the BCG-CLOP+ ion-pair and its spontaneous formation reaction from BCG and CLOP·H2SO4 compared to the BCG-CLOP-ion-pair. Furthermore, the UV-visible spectra and their corresponding excited states and electronic transitions for BCG, BCG-CLOP+ ion-pair, and BCG-CLOP ion-pair have been investigated. These spectra provided a molecular level understanding of the nature of the different intra-molecular and intermolecular electronic transitions in the BCG ion-pairs with CLOP+. Moreover, the quantitative analysis based on extracting a yellow-formed ion-pair into chloroform from aqueous medium was carried out. The ion-pair exhibits an absorption maximum at 413 nm. The optimum conditions of the reactions were studied experimentally and optimized. The calibration graph shows that CLOP·H2SO4 can be determined up to 100.0 µg mL-1 with detection limit (LOD) of 0.57 µg mL-1 and quantification limit (LOQ) of 1.86 µg mL-1. The low relative standard deviation values, 0.16-1.16, indicate good precision. The results were compared to other published data and were treated statistically using F and t-tests.