Abstract
Acetic acid-ferric oxide hydrosol was bound as a monolayer of particles on the outer surface of plasma membranes of various cells. Substitution of propanoic for acetic acid produced novel binding patterns, the aggregation of colloid at focal sites being independent of in situ negative surface charge. This pattern was observed on some cells (leukocytes, platelets, malaria-infected erythrocytes and malaria parasites) but not on others (normal erythrocytes) and on regional areas of certain cells (i.e., midpiece of sperm). Alteration of the aliphatic portion of lipids abolished aggregation. Hydrolysis of zwitterions of phospholipids or neuraminic acid and partial enzymatic degradation of membrane proteins had no effect on the phenomenon. We propose that aggregation of positively charged colloid particles depends on the association of the aliphatic tail of propanoic acid with the lipid region of the membrane. Positioning of carboxyl groups of propanoic acid in the hydrophilic region of the membrane would increase negative surface charge. Focal aggregation of ferric oxide in butanoic but not in lactic acid was consistent with this hypothesis.