Fourier Transform Infrared Spectroscopy Demonstrates The Reactivity Of The Protonated Carboxyl Group Of The Acid Salt Of Calcium Bilirubinate.

Abstract

R. D. Solowayl, J -G Wu2, D -F Xu3, Y -F Zhang4, D. K. Martinil, N -K Hongl,R. S. CrowtherlGastroenterology Division, University of Texas Medical Branch, Galveston, TX,USAI, Department of Chemistry, Peking University2, and Sinica Academica3,Beijing, China, and Jilin University, Changchun, China4.FOURIER TRANSFORM INFRARED SPECTROSCOPY DEMONSTRATES THE REACTIVITY OF THEPROTONATED CARBOXYL GROUP OF THE ACID SALT OF CALCIUM BILIRUBINATE.Calcium bilirubinate is a major salt in pigment gallstones.Bilirubin IX(H2BR) is a tetrapyrrole with 1 propionic acid side chain on both the B and Crings. A striking feature is the strong intramolecular hydrogen bonding ofboth carboxyl groups as determined by x -ray diffraction. This greatly reducesaqueous solubility. Much less is known about the structure of the salts ofcalcium bilirubinate since single crystals have not been formed. One or bothcarboxyl groups of bilirubin may coordinate with calcium in stone, forming theacid or neutral salt.The aim of this study was to compare the reactivity of the carboxylgroup in the acid salt of calcium bilirubinate [Ca(HBR)2] with the reactivityof these groups in H2BR to determine if the configuration of the carboxylgroups of these two compounds differed. The neutral salt of calcium bilirubi-nate (CaBR) was used as a control. Using a Nicolet 7199B FT -IR spectrometer(2 cm-1 resolution), peak location was assigned by computer. KBr discs, pre-pared after grinding standard and KBr together for 10 min, were compared withthe same standards examined by photoacoustic spectroscopy (PAS), utilizing aType JDV -1 photoacoustic cell from Jilin University. Protonated bilirubin waspurchased and the acid and neutral salts of calcium bilirubinate were synthe-sized. All peaks were examined for significant peak shifts of greater than 3wavenumbers (cm -l). If none of the 3 compounds demonstrated a peak shift of3cm-1, the peak was not considered. Groups contributing to each peak wereidentified from our normal coordinate analysis of protonated bilirubin. Peakshifts were calculated as: WavenumberpAS minus WavenumberKBr