On the importance of π-π interactions in the structural stability of phycocyanins

The influences of ?-? interactions in phycocyanin proteins and their environmental preferences were analyzed. The observations indicate that the majority of the aromatic residues in phycocyanin proteins are involved in ?-? interactions. Phe and Tyr residues were found to be involved in ?-? interactions much more frequently than Trp or His. Similarly, the Phe-Phe and Tyr-Tyr ?-? interacting pair had the highest frequency of occurrence. In addition to ?-? interactions, the aromatic residues also form ?-networks in phycocyanins. The ?-? interactions are most favorable at the pair distance range of 5.5-7 ?, with a clear preference for T-shaped ring arrangements. Using ab initio calculations, we observed that most of the ?-? interactions have energy from 0 to -10 kJ mol-1. Stabilization centers for these proteins showed that all residues found in ?-? interactions are important in locating one or more such centers. ?-? interacting residues are evolutionary conserved. The results obtained from this study will be beneficial in further understanding the structural stability and eventual development of protein engineering of phycocyanins.