On the amine-catalyzed isomerization of vitamin A aldehydes

Lukton, David; Rando, Robert R.

doi:10.1021/ja00313a054

Cited by 10 publications

(6 citation statements)

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“…Assuming a prefactor of 10 13 s −1 , we use a half time of 7 min for isomerization of 11-cis retinyl PSB measured by Rando and Lukton (28) at 25°C to extrapolate the rate to higher temperatures, which yields an Arrhenius plot (Fig. 5G, blue solid line) with a barrier ΔG ‡ = 20 kcal/mol that intersects the one in the upper experimental temperature range at 67.3°C, close to T m .…”

Section: Resultsmentioning

confidence: 99%

Unusual kinetics of thermal decay of dim-light photoreceptors in vertebrate vision

Guo

Sekharan

Liu

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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We present measurements of rate constants for thermal-induced reactions of the 11-cis retinyl chromophore in vertebrate visual pigment rhodopsin, a process that produces noise and limits the sensitivity of vision in dim light. At temperatures of 52.0-64.6°C, the rate constants fit well to an Arrhenius straight line with, however, an unexpectedly large activation energy of 114 ± 8 kcal/mol, which is much larger than the 60-kcal/mol photoactivation energy at 500 nm. Moreover, we obtain an unprecedentedly large prefactor of 10 72±5 s −1 , which is roughly 60 orders of magnitude larger than typical frequencies of molecular motions! At lower temperatures, the measured Arrhenius parameters become more normal: E a = 22 ± 2 kcal/mol and A pref = 10 9±1 s −1 in the range of 37.0-44.5°C. We present a theoretical framework and supporting calculations that attribute this unusual temperature-dependent kinetics of rhodopsin to a lowering of the reaction barrier at higher temperatures due to entropy-driven partial breakup of the rigid hydrogen-bonding network that hinders the reaction at lower temperatures.non-Arrhenius | dim-light vision | transition state theory | isomerization rate R hodopsin is a vertebrate dim-light photoreceptor. Molecular studies of rhodopsin in recent decades have largely focused on its photochemistry and photoactivation (1-3). However, complete understanding of rhodopsin's function requires characterization of its thermal properties because thermal isomerization of the 11-cis retinyl chromophore can trigger the same physiological response as photo-isomerization, generating false visual signals as dark noise that jeopardizes photosensitivity (4-6). To enhance dim-light vision, rhodopsin has evolved to acquire remarkable thermal stability with a half-life of 420 y as determined by electrophysiological experiments using the outer segments of rod cells at 36°C (4). However, the molecular mechanism for the thermal stability has remained unclear. Here, we have addressed this by exploring the temperature dependence of thermal decay rate constants k(T) associated with isomerization of the retinyl chromophore and hydrolysis of the chromophore protonated Schiff-base (PSB) linkage, for temperatures ranging from the physiological temperature of 37.0°C to 64.6°C. In the upper range of temperatures from 52.0°C to 64.6°C, we find that the rate constants determined by UV-visible spectroscopy follow a linear Arrhenius model, k(T) = A pref exp(−E a /k B T), where k B is the Boltzmann constant (Fig. 1A). The slope, however, is very steep, giving an elevated activation energy, E a = 114 ± 8 kcal/mol. Surprisingly, this value is much higher than the photoactivation energy at visible wavelengths (60 kcal/mol at 500 nm). E a is also much higher than the reaction enthalpy change (32-35 kcal/mol) (7, 8) (Fig. 1B). In the lower temperature range of our rate constant measurements, 37.0-44.5°C, the slope of the Arrhenius plot decreases abruptly, as shown in Fig. 1A. Fitting a straight line through the low temperature points prod...

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Section: Resultsmentioning

confidence: 99%

Unusual kinetics of thermal decay of dim-light photoreceptors in vertebrate vision

Guo

Sekharan

Liu

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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“…6(b), the reaction barrier for the disordered melted state for both WT and mutants was taken from an extrapolation of a prior measurement of the thermal rate of isomerization of 11-cis retinyl protonated Schiff base in solution, ∆H ‡ D = 20.6 kcal/mol. 48 For WT the final parameter, the reaction barrier at low temperature, was taken from our low temperature measurements, ∆H ‡ O = 27.5 kcal/mol. Similarly, for the mutants we obtain an estimate of ∆H ‡ O = 40 kcal/mol from our low temperature data.…”

Section: Discussionmentioning

confidence: 99%

Probing the remarkable thermal kinetics of visual rhodopsin with E181Q and S186A mutants

Guo

Hendrickson

Videla

et al. 2017

The Journal of Chemical Physics

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We recently reported a very unusual temperature dependence of the rate of thermal reaction of wild type bovine rhodopsin: the Arrhenius plot exhibits a sharp "elbow" at 47 °C and, in the upper temperature range, an unexpectedly large activation energy (114 ± 8 kcal/mol) and an enormous prefactor (10 s). In this report, we present new measurements and a theoretical model that establish convincingly that this behavior results from a collective, entropy-driven breakup of the rigid hydrogen bonding networks (HBNs) that hinder the reaction at lower temperatures. For E181Q and S186A, two rhodopsin mutants that disrupt the HBNs near the binding pocket of the 11-cis retinyl chromophore, we observe significant decreases in the activation energy (∼90 kcal/mol) and prefactor (∼10 s), consistent with the conclusion that the reaction rate is enhanced by breakup of the HBN. The results provide insights into the molecular mechanism of dim-light vision and eye diseases caused by inherited mutations in the rhodopsin gene that perturb the HBNs.

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“…Table 4 summarizes the results. One word of caution, when HCI is used as the protonating agent, substantial dark isomerization can quickly occur (Donahue and Waddell, 1984;Lukton and Rando, 1984). HCI is a powerful catalyst for dark isomerization of Schiff bases of 11cis retinal whereas TCA is very much less promoting (of dark isomerization).…”

Section: Retinal and Polyene Schiff Bases And Protonated Schiff Basesmentioning

confidence: 99%

THE VISUAL PROCESS: PHOTOPHYSICS and PHOTOISOMERIZATION OF MODEL VISUAL PIGMENTS and THE PRIMARY REACTION

Becker

1988

Photochem & Photobiology

119

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Abstract-A systematic comprehensive consideration of thc emission spectroscopy, emission lifetimes, transient spectroscopy, as well as quantum yields of fluorescence. triplet occupation and photoisomerization is given for a wide variety of polyene derivatives including retinyl and longer, as well as shorter chainlength homologs. Absorption spectral properties and the results and significance of theoretical calculations are also included. Chainlength, solvent and temperature effects on state order and photophysical as well as photoisomerization properties are evaluated. The mechanism for the primary light step in vision is considered in light of photophysical arid photoisomerization data on model visual pigments and rhodopsin

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On the amine-catalyzed isomerization of vitamin A aldehydes

Cited by 10 publications

References 0 publications

Unusual kinetics of thermal decay of dim-light photoreceptors in vertebrate vision

Unusual kinetics of thermal decay of dim-light photoreceptors in vertebrate vision

Probing the remarkable thermal kinetics of visual rhodopsin with E181Q and S186A mutants

THE VISUAL PROCESS: PHOTOPHYSICS and PHOTOISOMERIZATION OF MODEL VISUAL PIGMENTS and THE PRIMARY REACTION

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