Using Prodan, Laurdan and Acdan to monitor the formation of the ethanol-induced lipid interdigitation

Prodon, Acdan and Laurdan are environmentally sensitive fluorescent probes. The abrupt decrease in the fluorescence intensity ratio of these proves at 435 nm to that at 510 nm, F435/F510, allows the authors to determine the critical ethanol concentration (Cr) for the formation of the interdigitated state from the noninterdigitated gel state of DPPC. The Cr for DPPC at 20 C has been determined to be 1.04 M by Acdan, 1.13 M by Prodan, and 1.33 M by Laurdan. These values are close to the Cr previously reported by others. However, the molecular mechanism for the abrupt change in F435/F510 is different for different probes. Using a precipitation method, the authors have found that the percentage of Prodan in DPPC drops abruptly from 43% at 0 M ethanol to 19% at 2.4 M, and Laurdan in DPPC remains almost at the same level. This explains why the emission maximum of Laurdan and Acdan in DPPC remains almost unchanged over the ethanol concentration range of 0-2.4 M whereas the emission maximum of Prodan fluorescence undergoes a dramatic red shift at 1.1 M ethanol at 20 C. This suggests that probe relocation may contribute to the change in F435/F510 in the case ofmore » Prodan in DPPC. While little changes are observed in the partition of Laurdan in DPPC with ethanol, the abrupt change in the F435/F510 value of Laurdan should be attributed to other factors such as the tightness of lipid packing and the reduced electrostatic interactions in the interdigitated state.« less