Aggregation of chlorophylls on plant thylakoid membranes using coarse-grained simulations

Chlorophyll a (CLA) molecules in light-harvesting complexes are the most essential pigments for photosynthesis. Coarse-grained molecular dynamics simulations of CLA are carried out in plant thylakoid membranes at 293 K by varying the total lipid-to-CLA ratio using our previously derived coarse-grained model of CLA and MARTINI force fields for lipids. Our simulations show that CLA molecules dynamically form aggregates that break and reform. The lifetime of the dimer and the waiting time of the dimer formation follow bi-exponential distributions for the higher concentrations of CLA. The number of aggregates increases with an increasing concentration of CLA, where the aggregation is governed by van der Waals interactions. Our simulations suggest that selective lipids promote the formation of CLA aggregates in plant thylakoid membranes. As the concentration of CLA increases, diacylglycerol and phosphatidylglycerol lipids with palmitoyl tails prefer to reside near the CLA aggregates, and the lipids with linolenoyl tails with higher levels of unsaturation move away from the aggregates. Such preferential locations of lipids result in increasing lateral heterogeneity in the order parameter and density with increasing CLA concentration. This induces more undulations in membranes, resulting in a lower bending modulus and area compressibility. Our work unfolds the mechanism of the formation of CLA aggregates and their effect on the structure of thylakoid bilayers. The study provides the foundation for a better understanding of more complex biophysical phenomena, such as photosynthesis and non-photochemical quenching, in the future.