Organic Letters
Letter
+
room temperature, like observed in wt-enzymes, strongly
suggest H-tunneling mechanism but cannot be explained by
the Bell model.
The above analyses suggest that the reactions of GPhXn of
EWGs with both DMPBIH and MAH, as compared to EDGs,
would have more narrowly distributed DADTRS’s. Correlations
of the trend of the DADTRS distributions with the observed
smaller ΔE ’s in the reactions with EWGs and larger ΔE ’s with
EDGs in both systems (Table 1) clearly indicate that a smaller
ΔE results from a greater rigidity of the donor−acceptor
centers. Moreover, the reactions of DMPBIH would produce
the more rigid TRS’s than the reactions of MAH due to the
greater steric requirement and higher electron/hydride
donating ability of the DMPBIH donor (see introduction).
We have reported the CT absorptions of many similar
systems that include DMPBIH and MAH as hydride donors as
a
a
2
5,27
well.
Although it appears reasonable to expect that EWGs
+
in GPhXn would favor a tighter CT-complex in the TRS than
a
EDGs (due to a more favorable ΔG°), we have determined the
+
substituent effect in GPhXn (G = CF , H, (CH ) N) on the γ-
3
3 2
2
° KIEs at the N,N-2CH /2CD position of DMPBIH for
3 3
their reactions to attempt to verify the expectation. The 2° KIE
originates from a decrease in negative hyperconjugation
between the lone-pair electrons on N and σ* orbital of the
attached C-H/D bond due to the loss of electron density on N
The observed smaller ΔE ’s in the reactions of DMPBIH (0−
a
0.55 kcal/mol) than in the reactions of MAH (0.85−0.96 kcal/
mol) also suggest that a more rigid system gives a smaller ΔE .
a
3
0,31
in the reaction.
This process with electron density loss
All of these correlations between the reaction center rigidity
tightens the C-H/D bonds, leading to an inverse 2° KIE. It is
expected that an EWG would make a tighter CT complex so
that the DMPBIH moiety at the TRS ends up with more
electron density loss, equivalent to more positive charge gain,
producing a more inverse 2° KIE. On the other hand, we are
aware that the positive charge accumulation on DMPBIH is
not solely from the CT complexation, the hydride-transfer
from its 2-C-H bond cleavage also contributes to the
accumulation of the positive charge at DMPBIH. Under the
latter circumstances, however, according to the Hammond’s
and ΔE strongly support our hypothesis. Furthermore, we
a
note that the extent of change in ΔE is much greater in the
a
reactions of DMPBIH than in the reactions of MAH over the
same range of substituents (Table 1). This suggests that ΔE is
a
more sensitive to the electronic effect in a more rigid system.
Importantly, as expected, the ΔE ≈ 0 was found in the
a
+
reactions of DMPBIH with GPhXn of a strong EWG (CN or
CF ). While the ΔE ≈ 0 is rarely seen in solution reactions,
3
a
perhaps the more important discovery is that the result is
associated with the most rigid TRS among the reactions.
+
postulate, GPhXn with an EWG would form an early TRS so
that less positive charge would be developed on DMPBIH
To summarize, substituent/electronic effects on ΔE ’s for
a
+
the two series of NADH/NAD model reactions were studied
producing less inverse 2° KIE. Our results in Table 2 show that
to investigate the hypothesis that a more rigid system gives a
smaller ΔE . Reactions with a tighter CT complexation
a
Table 2. Γ-2CH /2CD 2° KIEs on DMPBIH and Charges
between H-donor and acceptor and more crowded reaction
3
3
a
at the DMPBIH Moiety of the TRS
centers give a smaller ΔE . ΔE ≈ 0 was found in the most
a
a
rigid system. Therefore, ΔE ≈ 0 is not unique to the wt-
a
γ-2CH /2CD 2° KIEs
charge (ζ) carried at
DMPBIH at the TRS
3
3
b
enzyme catalyzed H-transfer reactions, and modification of the
acceptor
on DMPBIH
system rigidity could make ΔE ≈ 0 for the reactions in
CF PhXn+
a
0.89 (0.01)
0.91 (0.01)
0.94 (0.02)
0.58+ (0.05)
0.47+ (0.05)
0.32+ (0.11)
3
solution. All of the results strongly support our hypothesis. The
PhXn+
change from ΔE ≈ 0 for a highly rigid system to ΔE > 0 for
+
a
a
(
CH ) NPhXn
3 2
systems with reduced rigidities in solution well replicates the
a
b
At 25 °C. Numbers in parentheses are standard deviations.
trends of ΔE ’s observed in wt-enzymes versus variants. This
a
supports the explanations in terms of the DAD
sampling
TRS
the 2° KIEs are indeed inverse and the value increases from
difference in relation to the densely packed active site in wt-
enzymes and impaired loosely packed active site in their
variants within the VA-AHT model. One other prediction from
+
GPhXn with CF (0.89) to H (0.91) to (CH ) N (0.94).
3
3 2
+
They strongly suggest that the EWGPhXn forms a tighter CT
complexation in the TRS structure (Scheme 3). By comparison
the latter model is that a longer DAD
leads to a larger
TRS
32,33
KIE.
This has indeed been observed in this work. In both
series of reactions, both DADTRS and KIE increase from EWGs
to EDGs (Table 1). Studies of the other predictions from the
DMPBIH with GPhXn+a
3
4−37
model is continuing in this lab.
Note that other
contemporary H-transfer/tunneling theories have also been
2
0,32,38−40
used to simulate the ΔE ’s observed in enzymes,
but
a
a
none of them could predict a straightforward structure−ΔE
relationship beforehand. We have not excluded the possibility
that our results could be explained by these latter theories, but
they can certainly add to the current debates on the
appropriateness of models to describe H-transfer reactions in
enzymes and solution.
a
Only the reactive rings of the reactants are drawn. The oval-shaped
H represents a H-wave packet.
ASSOCIATED CONTENT
sı Supporting Information
of the 2° KIEs with the equilibrium isotope effect (2° EIE =
■
+
0
.81) for the conversion from DMPBIH to DMPBI that
*
reflects a gain of a full positive charge on N, the partial positive
charge carried by the DMPBIH moiety at the TRS is calculated
3
1
(
ζ = (1−2° KIE)/(1−2° EIE)) and listed in Table 2 as well.
+
General procedures including syntheses and kinetic
determinations, detailed kinetic data (PDF)
It decreases from the reactions of GPhXn with CF (0.58+) to
3
H (0.47+) to (CH ) N (0.32+).
3
2
C
Org. Lett. XXXX, XXX, XXX−XXX
Adhikari, Pratichhya
