Determination of dissociation constants of (4RS)-[4-carboxy-5,8,11-tris(carboxymethyl)-1-phenyl-2-oxa-5,8,11-triazatridecan-13-oic acid] in water and biological fluids by means of nuclear magnetic resonance spectroscopy and the DISCO software package.

Abstract The p K A values of (4RS)-[4-carboxy-5,8,11-tris(carboxymethyl)-1-phenyl-2-oxa-5,8,11-triazatridecan-13-oic acid] (BOPTA), a polyprotic molecule whose gadolinium complex is an important magnetic resonance imaging contrast agent for clinical use, have been determined in water, in physiologic solution (PS), in serum (S), and in cerebrospinal fluid (CSF), by means of 13 C nuclear magnetic resonance spectroscopy data processed by a dedicated software package called DISCO. The aim of this study was to supply the BOPTA p K A values in media very similar to the in vivo environment and, consequently, to get a picture of the in vivo behavior of its Gd complex, whose thermodynamic stability is directly linked to the p K A values. The p K A values appeared to be almost equal both in D 2 O and in PS, while p K 1 and p K 5 values in CSF differ a little. In S, only p K 2 and p K 3 were calculated due to the narrow pH range used for data collection. However, these p K A values were found equal to those in the other media. These results represent the first direct spectroscopic evidence of a substantial invariability of BOPTA behavior in different media and they justify the extrapolation to biological fluids of the data obtained in water. The values also confirmed the high-quality performance of DISCO in calculating p K A values of polyprotic molecules in complex media.