A single CH/pi weak hydrogen bond governs stability and the photocycle of the photoactive yellow protein.

Importance of the CH/pi interaction on the structure and function of the photoactive yellow protein (PYP) was substantiated. Focusing on the phenyl ring of Phe6 adjacent to the alkyl chain of Lys123, the mutants for these amino acid residues were characterized. The results demonstrated that the mutants lacking the pi-electron at position 6 or the alkyl chain at position 123 show substantial malfunction. This is a clear example that single CH/pi weak interaction plays a crucial role in the normal action of the protein.