Binding of calcium ions to putative antibiotics.

Abstract

Calcium-dependent antibiotics represent a family of antibiotics with so far only one clinically approved member, daptomycin. Daptomycin binds to anionic bacterial membrane in a calcium-dependent fashion and induces lipid clustering. The synthesis and design of related substances is a promising source of antibiotics at a time where antibiotic resistance looms larger than ever. By developing a series of substances that bind to anionic membranes in a manner that depends on the presence of calcium ions, we hope to determine the minimum requirements for antibiotic activity. Calcium dependent antibiotics must bind calcium to function, and therefore their affinity for calcium is essential to understanding their efficacy. We determined the calcium affinity of novel compounds by pH titration, in conjunction with UV-vis spectroscopy, in the presence and absence of calcium ions. The apparent shift in the measured pKa can be used to determine the binding constant of calcium. We then compared the calcium binding constants thus obtained with those measured using a standard Ca2+-selective electrode. The next step will be to measure Ca2+-binding constants of lipophilic analogues when associated with a lipid bilayer. Conclusions regarding the suitability of a selection of compounds are discussed.