An ab initio conformational study on 2,3-dihydrobilin-1,19(21H,24H)-dione, a model compound for open-chain tetrapyrroles

The molecule 2,3-dihydrobilin-1,19(21H,24H)-dione (DHB) was studied as a model of the fully conjugated linear open-chain tetrapyrroles phycocyanobilin (PCB), phycoerythrobilin (PEB) and phytochrome (PC) as well as biliverdin (BV) and bilirubin (BR). The rotations around the single bonds of the exocyclic methine bridges were investigated for all possible cis and trans, E and Z isomers of DHB. The geometries and energies of conformers were investigated with semiempirical and ab initio methods using AM1 and RHF/3-21G levels of theory. Results indicate that geometries with a central syn-cis configuration are preferred to other conformations around the central methine bridge. Four lowest energy conformations stabilized by hydrogen bonding and favorable geometric arrangements minimizing steric strain were predicted. This model elucidated the trends and identified variables associated with tetrapyrrole conformation and energy and thus may serve as a preliminary basis for studying other open-chain tetrapyrrole structures.