Origins of curvature in meso-tetra(4-sulfonatophenyl) porphine aggregation: molecular dynamics and electronic spectroscopy

meso-Tetra(4-sulfonatophenyl) porphine (TPPS4) is a significant theranostic agent for photodynamic therapy (PDT) and a model system of molecular nanowires. The zwitterionic forms of TPPS4 tend to form large chiral nanotubes in acidic conditions at pH ≈1. However, it is still not clear how these aggregates are structured at the molecular level. We describe a computational strategy to model the TPPS4 aggregation of small clusters using a molecular dynamics (MD) approach. Two possible forms of zwitterionic TPPS4 tetramers were considered, and their absorption and circular dichroism (CD) spectra were calculated using the Frenkel exciton model. Possible molecular packing is suggested as a candidate for the formation of large aggregates.