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

The influences of pi-pi 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 pi-pi interactions. Phenylalanine (Phe) and tyrosine (Tyr) residues were found to be involved in pi-pi interactions much more frequently than tryptophan (Trp) or histidine (His). Similarly, the Phe-Phe and Tyr-Tyr pi-pi interacting pair had the highest frequency of occurrence. In addition to pi-pi interactions, the aromatic residues also form pi networks in phycocyanins. The pi-pi interactions are most favourable at the pair distance range of 5.5-7 A, with a clear preference for T-shaped ring arrangements. Using ab initio calculations, we observed that most of the pi-pi interactions possess energy from 0 to -10 kJ mol(-1). Stabilization centres for these proteins showed that all residues found in pi-pi interactions are important in locating one or more such centres. pi-pi 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.