Primary photochemical event in vision: proton translocation

Area of Science:

• Biophysics
• Photochemistry
• Molecular Biology

Background:

• Rhodopsin initiates the visual cascade upon light absorption.
• The initial steps involve rapid formation of transient intermediates.
• Understanding these early events is key to visual photochemistry.

Purpose of the Study:

• To directly measure the formation rate of prelumirhodopsin.
• To investigate the temperature dependence of this process.
• To elucidate the molecular mechanism of prelumirhodopsin formation.

Main Methods:

• Picosecond spectroscopy of rhodopsin in low-temperature glasses.
• Temperature-dependent kinetic measurements from 4 K to 20 K.
• Comparison of native and deuterium-exchanged rhodopsin.

Main Results:

• Prelumirhodopsin formation is extremely rapid (36 psec at 4 K).
• The rate shows non-Arrhenius behavior at low temperatures, indicative of tunneling.
• Deuterium substitution significantly slows formation, showing an isotope effect (kH/kD ≈ 7).

Conclusions:

• Proton tunneling, not simple thermal motion, drives prelumirhodopsin formation.
• The proton likely translocates to the Schiff base nitrogen of the retinal chromophore.
• This quantum mechanical event is fundamental to the initial stage of vision.