A study of changes in surface area and molecular interactions in phospholipid vesicles by condensed phase radioluminescence.

(1) The radioactive decay of tritium was used to excite 12-(9-anthroyloxy)-stearic acid. The resulting radioluminescence was observed by single-photon counting. A signal can only be observed if the emitting tritium is close enough to the absorbing fluorophore. This is accomplished by condensing the emitter and absorber into a lipid membrane. Therefore, we call the technique Condensed phase radioluminescence (CPR). (2) We present the theoretical background for the observed CPR signals. (3) We observed a large CPR signal when tritiated oleic acid was added to 12-(3-anthroyloxy)stearic acid micelles. (4) The phase transition of dimyristoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine in unilamellar vesicles can be monitored with CPR, and the relative intensity change observed is directly related to the relative surface change at the centre of the bilayer. (5) Oleic acid and 12-(9-anthroyloxy)stearic acid exchange between dimyristoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine vesicles. The labels show no real preference for the fluid phase of the dimyristoyl phosphatidylcholine vesicles. (6) CPR is a powerful method for studying distance and binding relationships in membranes.