Conformational flexibility of phycocyanobilin: Monte-Carlo and DFT study

A scan of the potential-energy surface (PES) for phycocyanobilin has been performed by Monte-Carlo Multiple Minimum (MCMM) searching method using the force field of MMFFs. The resulting most stable/populated structures are then reoptimized with ab initio methods and density functional theory (DFT) using HF/6-31G and B3LYP/6-31G levels of theory, and all possible conformers of phycocyanobilin are investigated with these methods. Results indicate that the most stable structure is all-syn, all-Z conformation. Other local minima are also obtained, such as ZZZass, EZZass and ZZZssa, which are stabilized by intramolecular hydrogen bonds and favorable geometric structures minimizing steric interactions.