Alkyl-Chain Modulation of Solvation and Microenvironment Probing in Model Lipid Membranes: A Density Functional...

The remarkable hydrophobicity of pyrene (Py) makes it one of the most widely used fluorophores for fluorescence probing in lipid systems. A variety of linear pyrene derivatives have been developed to tune the photophysical properties of pyrene-based probes. To investigate the effect of alkyl-chain modification on the solvation behavior of pyrene in lipid membranes, we performed density functional theory (DFT) calculations and nanoscale molecular dynamics (MD) simulations on Py, 2-n-butylpyrene (Py-C4), and 2-n-octylpyrene (Py-C8), embedded in a cell membrane-mimicking environment composed of hydrated synthetic glycolipids, n-dodecyl-β-d-maltoside (DDM). DFT results reveal that alkyl substitution enhances noncovalent interactions between Py and the hydrophobic core of DDM, leading to larger binding energies compared with interactions at the hydrophilic headgroup region. Moreover, alkyl-chain modification lowers the molecular symmetry of the Py core, thereby relaxing electric-dipole selection rules and enhancing the radiative decay rate, consistent with experimentally observed shorter fluorescence lifetimes. MD simulations demonstrate that Py, Py-C4, and Py-C8 experience distinct solvation environments within the DDM bilayer: both Py and Py-C4 preferentially reside closer to the hydrophilic headgroup region, whereas Py-C8 is predominantly solvated within the hydrophobic core. Free-energy surfaces describing probe migration along the bilayer normal (z axis) indicate that alkyl-chain elongation increases the kinetic barrier for migration. As a result, alkyl substitution reduces the conformational flexibility of the pyrene probes, rationalizing their region-selective solvation behavior within the bilayer. Overall, these findings provide valuable molecular-level insights that support the rational design of fluorescent probes with enhanced specificity toward distinct regions of lipid membranes.