Photobleaching of a Cyanine Dye in Solution and in Membranes

Abstract— N,N′‐bis(2‐ethyl‐1,3‐dioxolane)‐kryptocyanine (EDKC), a lipophilic dye with a delocalized positive charge, photosensitizes cells to visible irradiation. In phosphate‐buffered saline (PBS), EDKC absorbs maximally at 700 nm (ε= 1.2 × 10 5 M −1 cm −1 ) and in methanol, the absorption maximum is at 706 nm (ε= 2.3 × 10 5 M −1 cm −1 ). EDKC partitions from PBS into small unilamellar liposomes prepared from saturated phospholipids and into membranes prepared from red blood cells (RBC) and binds to human serum albumin (HSA). The EDKC fluorescence maximum red shifts from 713 nm in PBS to 720–725 nm in liposomes and RBC membranes and the fluorescence intensity is enhanced by factors of 14–35 compared to PBS (φ= 0.0046). EDKC is thermally unstable in PBS (T 1/2 = 2 h at 1.3 × 10 −5 M EDKC), but stable in methanol. In liposomes and RBC membranes, EDKC is 10 times more stable than in PBS, indicating that it is only partially exposed to the aqueous phase. Quenching of EDKC fluorescence in liposomes and RBC membranes by trinitrobenzene sulfonate also indicates that EDKC is not buried within the membranes. Photodecomposition of EDKC was oxygen‐dependent and occurred with a low quantum yield (6.4 × 10 −4 in PBS). Singlet oxygen was not detected upon irradiation of EDKC in membranes or with HSA since the self‐sensitized oxidation of EDKC occurred at the same rate in D 2 O as in H 2 O and was not quenched by sodium azide or histidine.