INTERACTIONS OF LINEAR DICATIONIC MOLECULES WITH LIPID-A - STRUCTURALREQUISITES FOR OPTIMAL BINDING-AFFINITY

The structural determinants of the binding affinity of linear dication ic molecules toward lipid A have been examined with respect to the dis tance between the terminal cationic functions, the basicity, and the t ype of cationic moieties using a series of spermidine derivatives and pentamidine analogs by fluorescence spectroscopic methods, The presenc e of two terminal cationic groups corresponds to enhanced affinity, A distinct sigmoidal relationship between the intercationic distance and affinity was observed with a sharp increase at 11 Angstrom, levelling off at about 13 Angstrom. The basicity (pK) and nature of the cationi c functions are poor correlates of binding potency, since molecules be aring primary amino, imidazolino, or guanido termini are equipotent, T he interaction of pentamidine, a bisamidine drug, with lipid A, charac terized in considerable detail employing the putative intermolecular e xcimerization of the drug, suggests a stoichiometry of 1:1 in the resu ltant complex, The binding is driven almost exclusively by electrostat ic forces, and is dependent on the ionization states of both lipid A a nd the drug, Under conditions when lipid A is highly disaggregated, pe ntamidine binds specifically to bis-phosphoryl- but not to monophospho ryl-lipid A indicating that both phosphate groups of lipid A are neces sary for electrostatic interactions by the terminal amidininium groups of the drug, Based on these data, a structural model is proposed for the pentamidine-lipid A complex, which may be of value in designing en dotoxin antagonists from first principles.