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Harmony and Dissonance in the Concert of Proton Motions.

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Harmony and Dissonance in the Concert of Proton Motions
Richard L. Schowen"
The motion of a proton between donor and acceptor atoms is
one of the simplest chemical reactions, and an overall reaction
involving two such motions within the same molecular array
represents the simplest case in which to consider the coupling of
two chemical processes ; do they occur simultaneously in time
(concertedly) or serially in time (in a stepwise manner)?"' Reactions in which multiple (often two) proton-transfer events
among electronegative atoms such as 0, N, and S accompany
bonding changes in a framework of atoms heavier than H are of
interest in biology as well as chemistry, for example in the action
of enzymes[21and in tautomerization reactions in nucleic-acid
base pairs.[31Several recent studies have begun to illuminate the
manner in which the molecular choice between concerted and
stepwise motion of the protons is made.
The history of this subject began some time ago. In 1952
Swain and BrownL4]showed that reagents with both protondonor and proton-acceptor sites can accelerate with extraordinary power reactions that require both proton donation and
proton removal. They denoted the phenomenon polyfunctional
[*] Prof. R. L. Schowen
Department of Chemistry
University of Kansas
Lawrence, KS 66045-0046 (USA)
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catalysis, which i s exemplified by the catalysis of an elementary
ring-opening step in the mutarotation of glucose (or its 2,3,4,6tetramethyl derivative) by 2-pyridone. The ring-opening reaction of glucose derivatives involves more than the motion of the
two protons. The internal C-0 o bond of the pyranose ring is
cleaved, and a 71 bond added to the external C - 0 o bond. If the
formation and fission of C - 0 bonds (heavy-atom reorganization) were coupled to the transfer of the protons between the
glucose molecule and the catalyst (which need not be true),[']
such a reaction could proceed by any of three pathways from
reactant state 1 to product state 3, which can take place via the
activated complex 2 and the intermediate compounds 4 and 5.
In reactions through 4 and 5, the motions of the two protons
between their bonding partners occur in a stepwise manner via
two transition states that come before and after the intermediates along the reaction pathway. In a reaction through 2 the
motions are concerted, and there is no intermediate. Swain and
Brown concluded from the extraordinary catalytic activity of
bifunctional compounds that the concerted pathway was preferred. This follows from the fact that 2-pyridone would function only as a very weak general base in the reaction leading to
4 and only as a very weak general acid in the reaction leading to
5. Only in transition state 2 for the concerted reaction is the
cooperative acid-base enhancement characteristic of bifunctional catalysts capable of expression. Rony and co-worker~'~'
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later showed that all highly effective bifunctional catalysts are
indeed tautomeric catalysts, that is, they possess two tautomeric
forms of similar energy (such as 2-pyridone and 2-hydroxypyridine) between which they interconvert in the course of bifunctional catalysis.
In 1953, the year following Swain and Brown's discovery of
polyfunctional catalysis, Watson and Crickc6]proposed the
best-known hydrogen-bonding scheme in all of modern science
(Scheme 1). The Watson-Crick pattern that selectively binds
pairs of nucleic-acid bases within the double-helical DNA structure provides for specific molecular recognition between deoxyadenylate (A) and deoxythymidylate (T) in a double hydrogen bond complex, and between deoxyguanylate (G) and
deoxycytidylate (C) in a triple hydrogen bond complex. This
enforces the security of the genetic code in DNA and the fidelity
of the genetic code in its replication into DNA, its transcription
into RNA, and its translation into a protein sequence. Watson
and Crick also initiated a series of proposals, recently reviewed
by Goodmanr3]and by Douhal et al.,['] according to which
tautomerization within the hydrogen-bonded pairs could give
rise to base pairs incorrectly matched in terms of the genetic
code (see Scheme 1). Such tautomers might therefore produce
genetic mutations through misreading of the DNA code during
transcription into RNA or misreading of its RNA transcript
during translation into a protein structure.
Like the ring-opening reaction of glucose, cooperative basepair tautomerization in nucleic acids involves components of
both proton transfer and heavy-atom reorganization. The fairly
large-scale heavy-atom reorganization in glucose ring opening
involves fission of a C - 0 (J bond and formation of a C-0 x
bond, whereas base-pair tautomerization produces more modest interconversions of wholly or partially single and double
bonds between heavy atoms within the framework of the paired
The question of whether double proton transfer in the glucose
example is concerted or stepwise was addressed definitively in
1982 by Engdahl et al.[*l They measured the rate constant for
mutarotation of tetramethylglucose catalyzed by 2-pyridone in
benzene; the samples of sugar and catalyst had definite fractions n of deuterium replacing the protium at the two protonbridging sites. This test for simultaneous or sequential bonding
changes at two transition-state protonic sites by variation of the
degree of deuteration is known as the proton-inventory apAngew. Chem. Int. Ed. Engl. 1997, 36, No. 13/14
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Scheme 1. Hydrogen bonding between Watson-Crick base pairs: A recognizes T
(and vice versa) by the two hydrogen bond pattern (first row, left), G recognizes C
(and vice versa) by the three hydrogen bond pattern (second row, left). Double
proton exchange within A - T pairs can generate the tautomeric forms A and T'
(first row right), and double proton transfer within G-C pairs can generate the
tautomeric forms G and C (second row, right). The lower half of the Scheme shows
how misreading is possible by such tautomerizations. Each of the "wrong" tauwith a ccwrong..
T', G, and c' can successfully recognize and base
partner (c,G, T, and A, respectively)
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p r ~ a c h . [A~ ]rate constant is measured as a function of the atom
fraction of deuterium n in exchangeable positions from n = 0
(pure protium label) to n = 1 (pure deuterium label). A linear
k,(n) indicates that the isotope effect arises from bonding
changes at a single transition-state site (as in the transition states
leading to or from the intermediates 4 and 5), whereas a
quadratic dependence indicates simultaneous bonding changes
at two sites, as in the concerted reaction via transition state 2.
Engdahl and co-workers found a quadratic dependence with the
isotope effect of about 4 arising from effects of about 2 at each
of the transition-state sites. This confirms the Swain- Brown
conclusion of concerted double proton transfer.
Another related isotope-effect approach to the coupling of
proton motions was taken by Limbach and co-workers. From
the 1970s to the present time they have been developing NMR
methods for characterizing multiple hydrogen-transfer processes among tautomeric species, and have thereby elucidated the
detailed dynamics of numerous tautomerization reactions in the
liquid and solid states.
In very recent work, Braun and co-workers["I examined the
degenerate tautomerization of porphyrin molecules (6S).This
topes of hydrogen (naturally necessitating precautions with the highly tritiated material),
rate constants for the degenerate rearrangement of porphyrin by double proton transfer
were obtained for proton and deuteron motions between 100 and 350 K as well as triton
motions between 237 and 350 K. Rates and iso-
the isotope effect at a single site. The single-site isotope effect
k,,/k,, is then predicted to be 3.3. If the two hydrons instead
move in succession, the cis intermediate 9 must form and then
decompose through transition states of equal Gibbs free energy
(because the rearrangement is degenerate). In such a case
k,, = k, and k,, = k,. In the singly deuterated species, either
hydron may move first (with rate constant k, for H or k , for D)
to generate the singly labeled intermediate. A fraction of the
intermediate thus formed will proceed to product by motion of
the hydron that did not previously move. The ritte constant for
this fraction will therefore be given by [k,/(k, + k,)] if H moved
first and by [k,/(k,+k,)] if D moved first. Therefore
k,, = 2k,k,/(k, + k,) and kH,/k,, will approach 2 as k , becomes much larger than k,. In fact Braun et al. find that k,,/
k,, is approximately 2 at 298 K. This is just as expected for the
stepwise process, because k , is 11 times greater than k , on this
model. This result indicates that the porphyrin rearrangement
occurs by a stepwise mechanism through the low-concentration,
kinetically insignificant intermediate 9.
Along still another independent route of investigation,
Petrich and his collaborators[' examined the tautomerization
of 7-azaindole in MeOH and EtOH
(10$12). This chromophore forms the
side chain of 7-azatryptophan, an unnatural amino acid introduced and applied by
Petrich et al. to a wide variety of problems
in peptide and protein dynamics.['2] The
tautomerization of 7-azaindole occurs
\ '
readily in the excited state and constitutes
the major route of nonradiative relaxation; the rate constant can be measured
by fluorescence decay. Compared to the
rate constant for reaction in MeOH and
EtOH, the decay rate constant is decreased by a factor of about 2.7 for reaction in MeOD or EtOD (since exchange at
the N-H position is fast, 7-azaindole will
also be deuterated in the deuterated
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13 or 14 or both. Petrich and co-workers examined the question
of concertedness in the double proton transfer by means of a
proton-inventory experiment. The rate constant decreases as a
quadratic function of the atom fraction of deuterium in the
solvent, showing that the overall solvent isotope effect of 2.7
arises from roughly equal isotope effects of 1.6- 1.7 at two sites
in a single transition state such as 11. These two sites are therefore likely to be the N-H(D) site of 7-azaindole and the
0-H(D) site of the alcohol molecule mediating the tautomerization through a single transition state with concerted motion
of the two protons. The alcohol molecule is thus functioning as
a bifunctional catalyst.
The isomerization of excited 7-azaindole molecules also
occurs in the hydrogen-bonded dimer (15+17), with each
monomer serving as both substrate and bifunctional catalyst for
the taut~merization.[’~~
The reaction may occur through a
single transition state 16 for a concerted pathway or in a stepwise manner through intermediates such as 18 or 19. Douhal
et al.[’] examined the mechanism for this reaction in the gas
Q.--3 Q 2
a m
phase. They prepared dimers in a molecular beam with definite
quantities of excess vibrational energy and electronically excited
them with an initial 60-femtosecond laser pulse at 305-310 nm.
After variable periods of up to 20 picoseconds a second pulse at
620 nm ionized the excited molecules, and the ions were drawn
into the analyzer of a time-of-flight mass spectrometer. The ion
signal decreased with time because the ionization cross section
of the tautomer-product dimer was smaller than that of the
initial 7-azaindole dimer. Furthermore, the signal decreased as
a biexponential function of time, showing that an intermediate
compound (of intermediate ionization cross section) formed in
significant amounts in the course of the tautomerization reaction. At an excess vibrational energy of 1 kcalmol-’ the kinetic
= 10 for the initial fast phase of the reacisotope effect kHH/kDD
tion and 12 for the second slower phase. This suggests that
proton transfer occurs in both phases of the reaction. The most
reasonable model is that single proton transfer generates the
intermediate ion pair 18 or radical pair 19 or both in kinetically
signficant amounts. The second proton then transfers to form
the product. Thus, the gas-phase tautomerization of 7-azaindole
occurs in a stepwise manner through a relatively stable intermediate.
Table 1 summarizes the results of these four studies on the
question of concerted versus stepwise reaction pathways for
double proton transfer accompanied by heavy-atom reorganization. The gamut of possible outcomes is observed: two reactions exhibit concerted mechanisms, one reaction occurs along
a stepwise route through a high-energy, kinetically insignificant
intermediate, and one reaction follows a stepwise pathway
through an observable, accumulating intermediate.
It seems likely that these initially perplexing findings are in
fact an expression of a principle we owe to William P. Jencks,
which was summarized in 1980 in an article[141entitled “When
is an intermediate not an intermediate? Enforced mechanisms of
general acid- base catalyzed, carbocation, carbanion, and ligand exchange reactions”. The essence of Jencks’s concept is that
if an intermediate compound along a stepwise route must have
a very high-energy structure, then it is likely that a lower-energy
structure will be possible for the concerted-reaction transition
state. If this is the case, the reaction will naturally proceed
along the concerted route, a choice that Jencks describes as
“enforced” by the high energy required for the alternative,
stepwise process. Each of the cases we have considered obeys
Jencks’s principle.
The two examples that exhibit concerted mechanisms would
require high-energy species as intermediates. In the mutarotation of glucose the instability of intermediates 4 and 5 enforces
a concerted reaction through the transition state 2. Similarly, in
the alcohol-mediated tautomerization of 7-azaindole, the oxonium moiety of 13 and the alkoxide moiety of 14 confer instability
Table 1. Summary of the favored reaction pathways ana potential intermediates for double proton transfer reactions
Concertedness: evidence
Potential intermediate
mutarotation of tetramethylglucose with
2-pyridone as catalyst in benzene
concerted: quadratic proton inventory indicates simultaneous
bonding changes at two protonic sites
conjugate acid- base pairs for singly protonateddeprotonated sugar and catalyst (45)
porphyrin isomerization in DMF
stepwise: double-label isotope effect of 11, single-label isotope
effect of 2 (see text)
cis-porphyrin 9; steady-state intermediate not
accumulating in kinetically significant amounts
alcohol-mediated isomerization of excited
7-azaindoie in MeOH or EtOH
concerted : quadratic proton inventory indicates simultaneous
bonding changes at two protonic sites
conjugate acid-base pairs for singly protonateddeprotonated substrate and catalyst (13,141
isomerization of dimeric excited 7-azaindole in the gas phase
stepwise: intermediate accumulates in kinetically signtficant
amounts; large isotope effects for both formation and decomposition of intermediate
presumably ion pair 18 or radical pair 19 or both
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even if the conjugate acid and conjugate base of 7-azaindole
should be relatively stable. In both cases, the high energy of the
required intermediates along the potential stepwise pathway
may be considered to enforce reaction by the concerted route.
The two reactions that exhibit stepwise mechanisms, as required by the Jencks principle, can employ intermediate structures that may be of some stability. The porphyrin isomerization
can pass through the intermediate 9, which maintains the cyclic
conjugation of the porphyrin system. Although the energy of
this intermediate is lower than that of the potential transition
state 7 for the concerted reaction, the intermediate is nevertheless sufficiently unstable that it returns to the reactant state or
proceeds to the product state with such rapidity that it fails to
accumulate and cannot be detected in the NMR experiment.
However, in the gas-phase tautomerization reaction of the electronically excited 7-azaindole dimer, the stability of the intermediate state 18/19is so great that it not only surpasses the stability
of the potential transition state 16 for the concerted reaction but
also accumulates during the reaction, rendering the kinetics
biphasic. The stability of the intermediate species in the gas
phase, if its correct representation is the ion pair 18, may seem
paradoxical in comparison to the instability of intermediates 13
and 14 in the polar liquid phase. However, both ionic moieties
of 18 are stabilized by charge delocalization (unlike the oxonium
moiety of 13 and the alkoxide moiety of 14), and the stability of
the complex may also benefit from the polarizability of the two
relatively large partners. Furthermore, intramolecular electron
transfer may occur readily to generate the radical pair 19, which
could be the accumulating form of the intermediate even if a
minor population of 18 is the reaction-pathway intermediate
that arises from reactants and generates products.
The reactions of Table 1 occur in widely different media
ranging from the gas phase through benzene to DMF and alkanols. The molecular choices made in each of the cases (concerted
reactions in benzene and alkanols, stepwise reactions in the gas
phase and DMF) cannot be understood in terms of coarse generalizations but instead require a more refined consideration of
the structure of the intermediate and its interaction with the
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medium. The power of Jencks’s principle thus transcends the
facile simplicity of .traditional thinking.
A question that has not been treated here, but will be addressed elsewhere, concerns the means by which the hydrons
achieve translocation, whether along the concerted or stepwise
pathway. For example, is quantum tunneling involved,;or, is the
hydron motion coupled into the reaction coordinate for heavyatom reorganization? Whatever the answer to this question, the
main molecular selection between concerted and stepwise pathways appears to follow from Jencks’s principle of avoiding highenergy intermediates.
German version: Angew Chem. 1997, 109, 1502-1506
Keywords: carbohydrates heterocycles
isotope effects nucleic acids
hydrogen transfer
The terms concerted and stepwise are used here in the simplest sense of occurring via a single transition state (concerted) or different transition states (commonly with an intermediate state having a lifetime of more than one molecularvibration period intervening between the transition states; stepwise).
Considerably finer distinctions in the timing of events during chemical reactions are possible and have been reviewed: C. F. Bernasconi, Adv. Phys. Org.
Chem. 1992,27, 119-238.
R L. Schowen in Mechanistic Principles of Enzyme Activity (Eds.: J. F. Liebman, A. Greenberg), VCH, New York, 1988, pp. 119-168.
M. F. Goodman, Nufure 1995,378, 237-238.
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