ALPHA-DEUTERIUM AND C-13 ISOTOPE EFFECTS FOR A SIMPLE, INTER-MOLECULAR SULFUR-TO-OXYGEN METHYL-TRANSFER REACTION - TRANSITION-STATE STRUCTURES AND ISOTOPE EFFECTS IN TRANSMETHYLATION AND TRANSALKYLATION
ALPHA-DEUTERIUM AND C-13 ISOTOPE EFFECTS FOR A SIMPLE, INTER-MOLECULAR SULFUR-TO-OXYGEN METHYL-TRANSFER REACTION - TRANSITION-STATE STRUCTURES AND ISOTOPE EFFECTS IN TRANSMETHYLATION AND TRANSALKYLATION
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DOI：
10.1021/ja00509a051
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发表时间：
1979-01-01
影响因子：
15
通讯作者：
SCHOWEN, RL
中科院分区：
文献类型：
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作者：
GRAY, CH;COWARD, JK;SCHOWEN, RL
同位素标记的S-甲基二苯并噻吩鎓（~（12）CH_3-SC_1_2H_8 ~+，~（13）CH_3-SC）的甲基转移反应|2 Hg+和2CD 3-SC 12 H8+）四氟硼酸盐与甲醇中的甲醇离子在25 ℃下显示kiH/kD = 0.97±0.02和k12·k1 = 1.08±0.02。大碳同位素效应与酶促甲基转移的效应相似，并且与过渡态中中心的、大致平面的甲基一致。利用Hartshorn和Shiner的计算因子，通过估计相对于乙烷的过渡态分馏因子，将α-D效应与酶促和非酶促甲基转移和烷基转移反应的其它效应进行比较。对于22个转甲基化反应，0 T用反应物（r）和产物（0 P）因子通过= O密切描述。99（0R0P）70.转甲基化过渡态在结构上看起来是简单的，与甲基的化合价大致恒定，可能很高。相对于反应物和产物，烷基转移过渡态似乎比甲基转移过渡态更松散，而且结构上更具可塑性。酶的过渡态具有异常大的分馏因子，这与作为酶催化机制的过渡态压缩一致。
The transmethylation reaction of isotopically labeled S-methyldibenzothiophenium (l2CH3-SCi2H8+, 13CH3-SC| 2Hg+, and, 2CD3-SC12H8+) tetrafluoroborates with methoxide ion in methanol at 25 C shows kiH/k} D= 0.97±0.02 and k\2¡ k\i= 1.08±0.02. The large carbon isotope effect is similar to that for enzymic transmethylation and is consistent with a central, roughly planar methyl group in the transition state. The «-D effect is compared with others for enzymic and nonenzymic transmethylation and transalkylation reactions by estimating the transition-state fractionation factor-relative to ethane, using thecalculated factors of Hartshorn and Shiner. For 22 transmethylation reactions, 0T is closely described in terms of reactant (r) and product (0P) factors by= O. 99 (0R0P) 70. Transmethylation transition states appear structural-ly implasticwith a roughly constant, probably high valency to methyl. Transalkylation transition states appear to be looser, relative to reactants andproducts, than transmethylation transition states and also far more structurally plastic. The enzymic transition state has an unusually large fractionation factor, consistent with transition-state compression as a mechanism of en-zymic catalysis.