Coumarin Partitioning in Model Biological Membranes: Limitations of log P as a Predictor

Time-resolved fluorescence measurements were used to quantify partitioning of three different 7-aminocoumarin derivatives into DPPC vesicle bilayers as a function of temperature. The coumarin derivatives were structurally equivalent except for the degree of substitution at the 7-amine position. Calculated log P (octanol: water partitioning) coefficients, a common indicator that correlates with bioconcentration, predict that the primary amine (coumarin 151 or C151) would experience a ∼40-fold partition enrichment in polar organic environments (log P_C151 = 1.6) while the tertiary amine's (coumarin 152 or C152) concentration should be >500 times enhanced (log P_C152 = 2.7). Both values predict that partitioning into lipid membranes is energetically favorable. Time-resolved emission spectra from C151 in solutions containing DPPC vesicles showed that within detection limits, the solute remained in the aqueous buffer regardless of temperature and vesicle bilayer phase. C152 displayed a sharp uptake into DPPC bilayers as the temperature approached DPPC's gel-liquid crystalline transition temperature, consistent with previously reported results ([ J. Phys. Chem. B 2017, 121, 4061-4070]). The secondary amine, synthesized specifically for these studies and dubbed C151.5 with a measured log P value of 1.9, partitioned into the bilayer's polar head group with no pronounced temperature dependence. These experiments illustrate the limitations of using a gross descriptor of preferential solvation to describe solute partitioning into complex, heterogeneous systems having nanometer-scale dimensions. From a broader perspective, results presented in this work illustrate the need for more chemically informed tools for predicting a solute tendency for where and how much it will bioconcentrate within a biological membrane.