Surface and solution properties of steroid antibiotics: 3-acetoxylfusidic acid, cephalosporin P1 and helvolic acid.

The colloid/chemical properties of the fusidane antibiotics, 3-acetoxylfusidic acid, cephalosporin P1, and helvolic acid, and their sodium salts, were investigated. The sodium salts of 3-acetoxylfusidic acid and cephalosporin P1 were found to be detergent-like molecules with micellar properties comparable to the parent compound sodium fusidate and the bile salt sodium cholate. Critical micellar temperatures (cmt) were less than 0 degrees C except for sodium helvolate which being sparingly soluble did not form micelles between 0 and 50 degrees C. Potentiometric titrations of dilute solutions gave apparent pK values (5.2-6.5) in the range expected for carboxylated steroid detergents. The apparent pK values increased significantly once the detergent concentration exceeded the critical micellar concentration (cmc). Micellar properties were determined by surface tension, titration with a water-soluble dye (Rhodamine 6G), light scattering, and solubilization of lecithin and cholesterol. Cmc's, in the range of 1.5 to 5.6 mM, were found which varied slightly depending on the method employed and in all cases fell slightly in the presence of added NaCl. The number of monomers per micelle (aggregation number) in concentrations well above the cmc was extrapolated from Debye light scattering plots in 0.15 M NaCl. The values varied from 6 for fusidate to 14 for 3-acetoxylfusidate with sodium cephalosporin P1 having an intermediate value. Each detergent readily solubilized the phospholipid lecithin.