Constant-pH MD Simulations of Lipids

Constant pH Molecular Dynamics (CpHMD) simulations represent a cutting-edge computational approach for studying biological systems with remarkable realism. Recent advancements have enhanced the accessibility and efficiency of CpHMD, significantly reducing the performance overhead compared to traditional constant-protonation MD simulations. This chapter guides the reader through the application of CpHMD to investigate the pH-dependent behavior of Cationic Ionizable Lipids (CILs) -- a critical component of Lipid Nanoparticles (LNPs), which are among the most promising platforms for drug delivery. LNPs, including those employed in mRNA-based vaccines, played a pivotal role in the global response to the SARS-CoV-2 pandemic, underscoring their potential in modern medicine. The chapter begins with a comprehensive introduction to the fundamental concepts of LNPs and provides a step-by-step protocol for setting up simulations of membranes containing CILs to calculate their apparent pKa. This parameter is crucial for governing the in vivo behavior of LNPs, where precise control is essential to optimize delivery efficiency while minimizing toxicity. By showcasing the ability of CpHMD simulations to unravel the intricate relationship between pH-dependent protonation, membrane structure, and lipid distribution, this chapter highlights their potential to inform the rational design of novel LNP formulations.