Photocycle of dried acid purple form of bacteriorhodopsin

The photocycle of dried bacteriorhodopsin, pretreated in a 0.3 M HCl soluti on, was studied. Some properties of this dried sample resemble that of the acid purple suspension: the retinal conformation is mostly all-trans, 15-an ti form, the spectrum of the sample is blue-shifted by 5 nm to 560 nm, and it has a truncated photocycle. After photoexcitation, a K-like red-shifted intermediate appears, which decays to the ground state through several inte rmediates with spectra between the K and the ground state. There are no oth er bacteriorhodopsin-like intermediates (L, M, N, O) present in the photocy cle. The K to K' transition proceeds with an enthalpy decrease, whereas dur ing all the following steps, the entropic energy of the system decreases. T he electric response signal of the oriented sample has only negative compon ents, which relaxes to zero. These suggest that the steps after intermediat e K represent a relaxation process, during which the absorbed energy is dis sipated and the protein returns to its original ground state. The initial c harge separation on the retinal is followed by limited charge rearrangement s in the protein, and later, all these relax. The decay times of the interm ediates are strongly influenced by the humidity of the sample. Double-flash experiments proved that all the intermediates are directly driven back to the ground state. The study of the dried acid purple samples could help in understanding the fast primary processes of the protein function. It may al so have importance in technical applications.