One of the most important carbohydrate derivatives, sodium D-gluconate (Gluc–) has numerous industrial applications due to its strong ability to sequester M(II) and M(III) metal ions especially in alkaline solutions. The diastereomeric L-gulonate (Gul–) ion could have similar complexation properties based on major structural similarities; however, different configuration may modulate this picture regarding the stability and the structure of the species formed. Thus, the complexing ability of gulonate in the presence of Ca2+ ions and in strongly basic medium is worthy for a comprehensive study. Potentiometric titrations revealed that one of the OH groups of the anion underwent deprotonation to a small extent. Much higher potentiometric effect could be observed in the presence of CaCl2. Fitted data indicated that, similarly to gluconate, the predominant species under these conditions was the neutral Ca3Gul2H–4 of very high stability. The strong complexing ability of gulonate was also verified by freezing point depression measurements. Polarimetric experiments conducted at constant ligand to metal ratio and increasing pH showed remarkable decrease in optical rotation indicating complex formation, too. Moreover, this method was able to identify that the pH effect was caused by the deprotonation of the ligand on metal ion coordination. Significant changes were found in the 1H NMR spectra of Gul– on addition of CaCl2. The peak of H2 broadened significantly, while the H4 signal shifted upfield suggesting that the main coordination sites were the C2-OH and C4-OH groups (beside COO–). This structural information was further corroborated with temperature-dependent measurements. However, the main binding sites for Gluc– were the C2-OH and the C3-OH groups revealing that stereochemical difference is an important factor in the structure of the species formed.

Multinuclear complex formation between Ca(II) and L-gulonate in strongly alkaline medium

253rd National Meeting (2017)

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