Substituent Effects on the Supramolecular Arrangement in Bio-Inspired Chlorin Nanotubes

The efficiency and adaptability of chlorosomal light-harvesting systems arise from their ability to form well-defined supramolecular nanotube architectures through pigment self-assembly. In this work, we carry out theoretical studies to investigate how peripheral substituents orchestrate the architectures of chlorin-type aggregates, as exemplified by natural bacteriochlorophyll c (BChl c) and a semisynthetic zinc chlorin (ZnChl) analogue. We find that long ester chains at the 17² position govern nanotube curvature and stability: comparatively smaller farnesyl tails in BChl c yield less curved assemblies with larger radii, while bulkier oligoethylene glycol chains in ZnChl produce tighter tubes with smaller radii and more ordered structures. We further show that the R/S configuration at position 3¹, together with the CH₃ group at position 2, collectively locks the hydroxyl into an anti or syn orientation, which strongly influences pi–pi stacking and packing motifs: R-epimers form more compact tubes, while S-epimers suffer steric hindrance and reduced aggregation. Spectral calculations using LC-TD-DFTB reveal that these substituent-induced structural variations lead to energetic shifts in the absorption spectra. Our results demonstrate that even small variations in substituent patterning lead to systematic and reproducible changes in tube morphology. These insights provide a foundation for understanding the structural design principles that govern both naturally evolved and synthetically engineered light-harvesting assemblies.