Theoretical investigation of the photocycle dynamics of the Archaerhodopsin 3 based fluorescent voltage sensor Archon2

Abstract

Previously the retinal photocycle dynamics of the fluorescent voltage sensor Archon2, a mutant of Archaerhodopsin 3, in pH 8 Tris buffer was studied experimentally at room temperature by photo-excitation (excitation wavelengths 590 nm and 632.8 nm) of the protonated retinal Schiff base (PRSB) Ret_586 (absorption maximum at λmax = 586 nm) and following the absorption spectra development during and after light exposure.In this paper the experimental results are simulated numerically with developed photocycle schemes and relevant dynamics parameters are extracted.Photo-excitation of fresh thawed (dark-adapted) Archon2 (Archon2da) caused photoisomerization of Ret_586 (Ret_586da) to Ret_553 (Ret_535da, main component, absorption maximum at 553 nm) and to Ret_485 (Ret_485da, minor component) in a primary excitation process (primary photocycle). Ret_485 does not absorb at λexc = 590 nm or 632.8 nm and therefore is not involved in further photo-excitation. Continued light exposure transformed Archon2da to light-adapted Archon2la with secondary photocycle dynamics changing Ret_553 (Ret_535da) to Ret_535 (Ret_535la, absorption maximum at 535 nm). Ret_535la backward photo-isomerized to Ret_586 in light-adapted state (named Ret_586la) and partly deprotonated to neutral retinal Schiff base (RSB) Ret_372 in light adapted state (named Ret_372la, same isomer form as Ret_535).After excitation light switch-off Archon2la recovered in the dark to Archon2da with recovery of Ret_372la to Ret_372da, Ret_535la to Ret_535da, and Ret_586la to Ret_586da. Ret_372da slowly re-protonated to Ret_535da, and Ret_535da slowly isomerized back to Ret_586da. The long-time dynamics in the dark at room temperature changed over to thermal Archon2 denaturation accompanied with protonated retinal Schiff base deprotonation.