Binding of phospholipid vesicles to erythrosin B results in a shift of
the absorption maximum of the dye from 528 to 549 nm. This effect was
employed to develop a simple, rapid, and sensitive quantification met
hod for unilaminar phospholipid vesicles. At room temperature the colo
r development of the phospholipid-dye complex at 549 nm is essentially
complete in 5 min and only a slight decrease is observed in the follo
wing hours. The pH optimum of 4.5 for the assay is related to the tigh
t binding at this pH (K-d = 3.6 mu g/ml). A maximal binding of one ery
throsin to seven phospholipid molecules is found. The sensitivity of t
he assay is high for zwitterionic phospholipids (e.g., 0.023 Delta A(5
49) X ml/mu g for dioleoyl phosphatidyl choline) and lower for anionic
phospholipids. The assay depends on the size of the phospholipid lipo
somes, indicating the importance of the phospholipid surface area for
binding. (C) Academic Press, Inc.