Light-induced conversion of ethane-1,2-diol or 2-aminoethanol into acetaldehyde in the presence of 8-methoxy-5′-deoxy-5′-adenosyl-cobalamin or a simpler cobalt(<scp>III</scp>) alkyl

Hartshorn, A. J.; Johnson, A. W.; Kennedy, Shaun M.; Läppert, Michael F.; MacQuitty, Jonathan J.

doi:10.1039/c39780000643

Cited by 9 publications

(3 citation statements)

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“…We previously synthesized and characterized 8-MeOAdoCbl for just this purpose and have demonstrated that 8-MeOAdoCbl thermolysis in ethylene glycol solution yields Co(II)Cbl • and 8-MeOAdo • . As desired, 8-MeOAdo • does not cyclize (does not exhibit path 1, Scheme ) when thermolyzed in ethylene glycol. , This in turn allowed a more direct determination of the KIE by analysis of the ratio of 8-MeoAdo-H to 8-MeoAdo-D.…”

Section: Resultsmentioning

confidence: 91%

The First Experimental Test of the Hypothesis that Enzymes Have Evolved To Enhance Hydrogen Tunneling

2003

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The literature hypothesis that "the optimization of enzyme catalysis may entail the evolutionary implementation of chemical strategies that increase the probability of quantum-mechanical tunneling" is experimentally tested herein for the first time. The system employed is the key to being able to provide this first experimental test of the "enhanced hydrogen tunneling" hypothesis, one that requires a comparison of the three criteria diagnostic of tunneling (vide infra) for the same, or nearly the same, reaction with and without the enzyme. Specifically, studied herein are the adenosylcobalamin (AdoCbl, also known as coenzyme B(12))-dependent diol dehydratase model reactions of (i). H(D)(*) atom abstraction from ethylene glycol-d(0) and ethylene glycol-d(4) solvent by 5'-deoxyadenosyl radical (Ado(*)) and (ii.) the same H(*) abstraction reactions by the 8-methoxy-5'-deoxyadenosyl radical (8-MeOAdo(*)). The Ado(*) and 8-MeOAdo(*) radicals are generated by Co-C thermolysis of their respective precursors, AdoCbl and 8-MeOAdoCbl. Deuterium kinetic isotope effects (KIEs) of the H(*)(D(*)) abstraction reactions from ethylene glycol have been measured over a temperature range of 80-120 degrees C: KIE = 12.4 +/- 1.1 at 80 degrees C for Ado(*) and KIE = 12.5 +/- 0.9 at 80 degrees C for 8-MeOAdo(*) (values ca. 2-fold that of the predicted maximum primary times secondary ground-state zero-point energy (GS-ZPE) KIE of 6.4 at 80 degrees C). From the temperature dependence of the KIEs, zero-point activation energy differences ([E(D) - E(H)]) of 3.0 +/- 0.3 kcal mol(-)(1) for Ado(*) and 2.1 +/- 0.6 kcal mol(-)(1) for 8-MeOAdo(*) have been obtained, both of which are significantly larger than the nontunneling, zero-point energy only maximum of 1.2 kcal mol(-)(1). Pre-exponential factor ratios (A(H)/A(D)) of 0.16 +/- 0.07 for Ado(*) and 0.5 +/- 0.4 for 8-MeOAdo(*) are observed, both of which are significantly less than the 0.7 minimum for nontunneling behavior. The data provide strong evidence for the expected quantum mechanical tunneling in the Ado(*) and 8-MeOAdo(*)-mediated H(*) abstraction reactions from ethylene glycol. More importantly, a comparison of these enzyme-free tunneling data to the same KIE, (E(D) - E(H)) and A(H)/A(D) data for a closely related, Ado(*)-mediated H(*) abstraction reaction from a primary CH(3)- group in AdoCbl-dependent methylmalonyl-CoA mutase shows the enzymic and enzyme-free data sets are identical within experimental error. The Occam's Razor conclusion is that at least this adenosylcobalamin-dependent enzyme has not evolved to enhance quantum mechanical tunneling, at least within the present error bars. Instead, this B(12)-dependent enzyme simply exploits the identical level of quantum mechanical tunneling that is available in the enzyme-free, solution-based H(*) abstraction reaction. The results also require a similar, if not identical, barrier width and height within experimental error for the H(*) abstraction both within, and outside of, the enzyme.

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Section: Resultsmentioning

confidence: 91%

The First Experimental Test of the Hypothesis that Enzymes Have Evolved To Enhance Hydrogen Tunneling

2003

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“…While many studies of alkylcobalamin photolysis jiave focused on the rates and products of light-induced bond cleavage (Hogenkamp et al, 1962;Hogenkamp, 1963Hogenkamp, ,1966Johnson et al, 1962Johnson et al, ,1963; Dolphin et al, 1964;Pratt, 1964;Yamada et al, 1966a,b;Schrauzer et al, 1970; Endicott & Ferraudi, 1977;Pailes & Hogenkamp, 1968;Hartshorn et al, 1978;Rudakova et al, 1978;Chemaly & Pratt, 1980a), only a few have measured the quantum yields of photolysis of the Co-Ca bond. Quantum yields have been reported for several cobalamins on continuous-wave (CW) (Pratt & Whitear, 1971; Taylor, 1973) and ultrafast time scales (Endicott & Netzel, 1979;.…”

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confidence: 99%

Continuous-wave quantum yields of various cobalamins are influenced by competition between geminate recombination and cage escape

Chen¹,

Chance²

1993

Biochemistry

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Quantum yields of photolysis of the cobalt-carbon bond for three cobalamin compounds were measured with a continuous-wave laser at 442 nm under both aerobic and anaerobic conditions. Aerobically, the initial homolysis product, Co(II) cobalamin, is trapped by oxygen to form aquocobalamin. Use of an excess of the radical trapping reagent 2,2,6,6-tetramethyl-1-piperidinyloxyl, under anaerobic conditions, scavenges the carbon radical and allows detection of the cobalt(II) photoproduct. Quantum yields measured under anaerobic conditions for 5'-deoxyadenosylcobalamin (phi (Co-C alpha),442 = 0.20 +/- 0.03) and methylcobalamin (phi (Co-C alpha),442 = 0.35 +/- 0.03) are in agreement with the values obtained under aerobic conditions (phi (Co-C alpha),442 = 0.19 +/- 0.04 and phi (Co-C alpha),442 = 0.36 +/- 0.04, respectively). Additionally, the quantum yield values for 5'-deoxyadenosylcobalamin and its base-off derivative (phi (Co-C alpha),442 = 0.045 +/- 0.015) match those obtained on a nanosecond time scale [Chen, E., & Chance, M. R. (1990) J. Biol. Chem. 256, 12987-12994]. A comparison of quantum yields obtained anaerobically for 5'-deoxyadenosylcobalamin and methylcobalamin in H2O versus ethylene glycol shows a 4-fold decrease for the former cobalamin and no change for the latter. These quantum yields are evaluated in terms of time-independent radical separation distances.

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“…The spectra at pH 5.8 (Figure 7) are nearly identical with those observed in the 2-propanol system at the same pH (Figure 4). Thus, the results seem to suggest that the following sequence of reactions best describes this system: Co11 + -CH(OH)CH2OH -Com-CH(OH)CH2OH (41) Com-CH(OH)CH2OH -Co1 + CH2OHCHO + H30+ (42) Com-CH(OH)CH2OH -* Com-H CH(OH)==CHOH(CH2OHCHO) (43) followed by reactions 33 and 34 and some decomposition products of Com-H.…”

Section: Resultsmentioning

confidence: 86%

Reactions of B12r with aliphatic free radicals: a pulse-radiolysis study

Mulac¹,

Meyerstein²

1982

J. Am. Chem. Soc.

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Light-induced conversion of ethane-1,2-diol or 2-aminoethanol into acetaldehyde in the presence of 8-methoxy-5′-deoxy-5′-adenosyl-cobalamin or a simpler cobalt(III) alkyl

Cited by 9 publications

References 0 publications

The First Experimental Test of the Hypothesis that Enzymes Have Evolved To Enhance Hydrogen Tunneling

The First Experimental Test of the Hypothesis that Enzymes Have Evolved To Enhance Hydrogen Tunneling

Continuous-wave quantum yields of various cobalamins are influenced by competition between geminate recombination and cage escape

Reactions of B12r with aliphatic free radicals: a pulse-radiolysis study

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