Figure 4. Analysis of the RBC/aspirin complexes.

(a) Shows all reflectivity curves for complexes containing between 0 and 3 mM ASA. (b) The pattern for the 2.5 mM sample is well fit by three series of peaks corresponding to l_o, l_d and peptide domains. (c) The location of the ASA molecule is determined by comparing the electron density of a pure RBC membrane with a low concentration of 1 mM ASA. Aspirin is found to partition the l_o lipid domains of RBC membranes and locate in the head group region, at |z|-values of 22.8 Å. (d) Small partitioning of aspirin is observed in l_d lipid domains, suggesting that aspirin preferably interacts with l_o domains. (e) Lamellar spacing, d_z, and membrane thickness, d_HH, of the l_o lipid domains decrease significantly with increasing ASA concentration until thickness of l_o and l_d domains coincide. We note that due to the absence of reflectivity peaks in the 3 mM ASA curve in part (a) no d-spacings could be determined for this concentration in part (e).