Excited-State Dynamics of a Photoacid: A Potential Probe for Recognizing Transition from Lamellar to Nonlamellar...

Liposomes of a cationic lipid dioctadecyldimethylammonium bromide (DODAB) are efficient nanocarriers of nucleic acids. Incorporation of a neutral lipid monoolein (MO) in excess (x_MO >0.5) changes the lamellar organization of DODAB liposomes into non-lamellar inverted structures of DODAB/MO liposomes facilitating nucleic acid delivery to cells. Photoexcitation of 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS), a photoacid, initiates an excited state proton transfer (ESPT) reaction in its protonated form (ROH*) generating the deprotonated anionic form (RO^- *). The fluorescence intensity ratio (I_ROH* /I_RO-* ) of these two forms is governed by the ESPT dynamics, and increases with increasing MO content (x_MO ) in the cationic liposomes of DODAB. Transition from lamellar organization of DODAB liposomes into non-lamellar inverted structures of DODAB/MO liposomes, due to incorporation of MO (x_MO ~0.7), is manifested by a significant increase of ESPT time (τ_PT ) and the time constant of wobbling motion (τ_W ) of HPTS. Thus, the lamellar organizations of DODAB or DODAB-rich (x_MO 0.2) liposomes and the non-lamellar organizations of MO-rich (x_MO ~0.7) liposomes are recognized by significantly different excited state dynamics of the photoacid.