Vibronic coupling is key to efficient energy flow in molecular systems and a critical component of mechanisms invoking quantum effects in biological processes. Despite increasing evidence for coherent coupling of electronic states mediated by vibrational motion, no evidence has been put forward on how and to what degree properties associated with vibrational coherence can impact the efficiency of light-induced processes under natural, incoherent illumination.<br/>Isotope effects have traditionally been used to provide unique critical information about the mechanism of chemical reactions. With this motivation, we synthesized the 11-D, 12-D and 11,12-D2 isotopomers of 11-cis retinal and regenerated bovine rhodopsin to examine the role of these isotopes on the quantum yield and rate of the 200 fs primary isomerization reaction that initiates vision. Surprisingly, we found that these heavy hydrogen substitutions about the isomerizing C11=C12 bond had no effect on the isomerization rate. The primary photoproduct absorption for each derivative was completely formed in 200 fs within the 20 fs experimental error. However, we found that the quantum yield for the primary photochemistry was in fact isotope dependent: While the symmetric isotopomers 11,12-H2 and 11,12-D2 had the expected quantum yields of 0.65 and 0.69, respectively; the monodeuterio species 11-D and 12-D exhibited a significant DECREASE in the photoisomerization quantum yield to 0.45 and 0.48, respectively.<br/>This first observation of a 30% isotope effect on the primary step in vision reveals that the vibrational dynamics of the 11 and 12 hydrogens play a critical role in transiting the internal conversion/conical intersection (CI) that connects the Franck-Condon excited state to the primary photoproduct. This observation can be understood if the efficiency of successful internal conversion depends on the relative displacement phase of the 11 and 12 hydrogens (deuterons) at the CI. If these hydrogen wags oscillate in–phase so that they are displaced on opposite sides the local ethylenic plane, then reactive internal conversion is enhanced. The mono-deuterio derivatives necessarily destroy this critical phase coherence relationship and reduce the quantum yield. Our results point to the existence of a vibrational phase-dependent isotope effect where coherent vibronic coupling functionally impacts a photochemical process even with incoherent excitation.

Evidence for a vibrational phase isotope effect on the photochemistry of vision

253rd National Meeting (2017)

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