Direct measurement of enol ether radical cation reaction kinetics

Full text of DOI:10.1021/ja015602c

J. Am. Chem. Soc. 2001, 123, 6445-6446
6445
Scheme 1
Direct Measurement of Enol Ether Radical Cation
Reaction Kinetics
Martin Newcomb,*,† Neil Miranda,† Mousumi Sannigrahi,‡
Xianhia Huang,‡ and David Crich*,‡
Department of Chemistry
Wayne State UniVersity, Detroit, Michigan 48202
Department of Chemistry, UniVersity of Illinois at Chicago
Chicago, Illinois 60607
ReceiVed January 29, 2001
ReVised Manuscript ReceiVed May 8, 2001
Radicals produced at the C4′ position of a DNA sugar are
implicated in the degradation of DNA by antitumor antibiotics
such as bleomycin.1 Under anaerobic conditions, C4′ DNA
radicals and models fragment heterolytically to enol ether radical
cations and phosphate anions.2,3 Rate constants for heterolyses
of DNA model radicals3 and DNA radicals4 were determined by
indirect kinetic methods, and heterolysis rate constants for DNA
model radicals were measured directly.5 A sugar-centered enol
ether radical cation produced by heterolysis of a DNA C4′ radical
can oxidize a guanosine base in DNA, and Giese’s group has
employed this entry to DNA radical cations in their elegant studies
of electron (or hole) transfers in DNA.4,6 Despite the importance
of enol ether radical cations in DNA oxidations, kinetic studies
of these intermediates are limited, although the kinetics of
electron-transfer reactions to guanosine derivatives have been
reported.4,7 We report here direct kinetic studies of enol ether
radical cation reactions with water and with purine 2′-deoxy-
nucleosides using laser flash photolysis (LFP) methods.
We described direct measurements of the rate constants for
formation of enol ether radical cations via radical heterolyses using
an LFP reporter method for detecting enol ether radical cations
that lack useful chromophores.5 A variation of that method was
used here to measure rate constants for enol ether radical cation
reactions with water and with purine nucleosides (Scheme 1).
Enol ether radical cations were produced rapidly either from an
R-alkoxy-â-phosphatoxyalkyl radical (from a PTOC ester8) that
fragmented heterolytically or by oxidation of an enol ether with
chloranil triplet, Chl*.
competition with the amine, the observed rate constant (kobs) for
formation of triarylaminium cation radical was given by eq 1 ,
kobs ) k0 + kT[Trap]
(1)
where k0 is the sum of the pseudo-first-order rate constants for
all background reactions consuming the radical cation (including
reactions with Ar3N), kT is the second-order rate constant for the
competing reaction of interest, and [Trap] is the concentration of
the agent of interest. Multiple reactions were conducted with
varying concentrations of “Trap” to give plots with slopes of kT
and intercepts of k0. Typical data are shown in Figure 1.
The series of enol ether radical cations 1-6 was studied.
Reactions were conducted in mixed solvents of acetonitrile (ACN)
and 2,2,2-trifluoroethanol (TFE) or mixtures of ACN and TFE
containing water. Radical cations 2, 3, and 6 were produced by
oxidation of the corresponding enol ethers with Chl* in reactions
The enol ether radical cations oxidized triarylamines in
diffusion-controlled reactions to give triarylaminium cation
radicals that absorb in the long wavelength visible region. When
another agent was present that reacted with the radical cation in
† Wayne State University.
that were complete within 100 ns. Radical cations 4 and 5 were
formed by heterolysis of the corresponding R-alkoxy-â-(phos-
phatoxy)alkyl radicals that were produced by 355 nm irradiation
‡ University of Illinois at Chicago.
(1) Stubbe, J.; Kozarich, J. W.; Wu, W.; Vanderwall, D. E. Acc. Chem.
Res. 1996, 29, 322-330.
(2) (a) Behrens, G.; Koltzenburg, G.; Ritter, A.; Schulte-Frohlinde, D. Int.
J. Radiat. Biol. 1978, 33, 163-171. (b) Behrens, G.; Koltzenburg, G.; Schulte-
Frohlinde, D. Z. Naturforsch. C 1982, 37, 1205-1227. (c) Koltzenburg, G.;
Behrens, G.; Schulte-Frohlinde, D. J. Am. Chem. Soc. 1982, 104, 7311-7312.
(d) Peukert, S.; Giese, B. Tetrahedron Lett. 1996, 37, 4365-4368. (e) Peukert,
S.; Batra, R.; Giese, B. Tetrahedron Lett. 1997, 38, 3507-3510.
(3) Gugger, A.; Batra, R.; Rzadek, P.; Rist, G.; Giese, B. J. Am. Chem.
Soc. 1997, 119, 8740-8741.
(4) Meggers, E.; Kusch, D.; Spichty, M.; Wille, U.; Giese, B. Angew. Chem.,
Int. Ed. Engl. 1998, 37, 460-462. Meggers, E.; Dussy, A.; Scha¨fer, T.; Giese,
B. Chem. Eur. J. 2000, 6, 485-492.
(5) Newcomb, M.; Miranda, N.; Huang, X. H.; Crich, D. J. Am. Chem.
Soc. 2000, 122, 6128-6129.
(6) Giese, B.; Beyrich-Graf, X.; Erdmann, P.; Petretta, M.; Schwitter, U.
Chem. Biol. 1995, 2, 367-375. Giese, B. Chimia 1999, 53, 198-200. Giese,
B. Acc. Chem. Res. 2000, 33, 631-636.
(7) Steenken, S.; Jovanovic, S. V.; Bietti, M.; Bernhard, K. J. Am. Chem.
Soc. 2000, 122, 2373-2374.
(8) The acronym PTOC is for pyridine-2-thioneoxycarbonyl. PTOC esters
were originally designed as radical precursors for synthetic applications. See:
Barton, D. H. R.; Crich, D.; Motherwell, W. B. Tetrahedron 1985, 41, 3901-
3924.
Figure 1. Kinetic traces at 650 nm from reactions of radical cation 3
with Ph3N in the presence of (from the top) 0, 1 × 10-3, 2 × 10-3, and
3 × 10-3 M dA. The inset shows the observed rate constants (right axis)
plotted against the mM concentration of dA (bottom axis).
10.1021/ja015602c CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/08/2001

6446 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Communications to the Editor
Table 1. Rate Constants for Reactions of Enol Ether Radical
reaction pathway appears to be electron transfer on the basis of
the agreement between the rate constants for reactions of the
“dialkyl” enol ether radical cations 3-5 and the results reported
by Giese and co-workers.4 They observed that rate constants for
oxidations of dG positions in polymeric duplex DNA containing
radical cation sites on T-based sugars fit a logarithmic distance-
dependent function, and extrapolation of that function to a distance
of 3-4 Å gives ET rate constants in the range of k ) (1-4) ×
Cationsa
radical cation
water
dA
dI
dG
1b
1c
2c
3c
4b
5b
6c
10 × 105
15 × 108
10 × 108
5 × 108
17 × 108
14 × 108
26 × 108
11 × 108
1.8 × 108
87 × 108
8 × 105
10 × 105
6 × 105
92 × 108
48 × 108
8 × 108
3.4 × 108
0.8 × 108
0.9 × 108
1.1 × 108
0.9 × 105
2.5 × 105
1.4 × 105
109 s-1
.
10 × 108
38 × 108
4.6 × 108
The relative rate constants for oxidations of the purines can
be evaluated with Marcus theory by using the simplified form in
eq 2 (because one reactant is not charged), where ∆Gq is the free
a Second-order rate constants in units of M-1 s-1. Experimental
details and error limits are given in the Supporting Information. b From
the phosphatoxy radical. c From triplet chloranil oxidation of the enol
ether.
∆Gq ) ∆Gqint + ∆G0/2 + (∆G0)2/16 ∆Gq
(2)
int
energy of activation, ∆Gq is the intrinsic barrier, and ∆G0 is
int
the free energy change of the electron-transfer reaction.11 The
intrinsic barrier is not known, but we assume that it is similar for
the different purines. Oxidation potentials for G, I, and A are
of PTOC ester precursors; these reactions were “instant” on the
nanosecond time scale.5 Radical cation 1 was produced by both
methods to test for a large systematic difference in the two
methods. Triphenylamine was the reporter for 1-5, and tris-(p-
methoxyphenyl)amine was used as the reporter for 6.
available,12 and the E0 values for oxidations of simple enol
1/2
ethers13 can be used as approximations of the oxidation potentials.
The oxidation potential of G is about 150 mV lower than that
of I, and one calculates that a given enol ether radical cation
should oxidize dG about an order of magnitude faster than it
oxidizes dI. Thus, the results for dI in Table 1 might be consistent
with electron transfer as a major pathway for reactions of dI. They
are not consistent with predominant oxidation of dA, however,
which is calculated to be a factor of 2000 less rapid than oxidation
of dG.14
Rate constants for reactions of the enol ether radical cations
with water are listed in Table 1. In these reactions, water acts as
a nucleophile to give a distonic radical cation that is then
deprotonated and/or as a base that deprotonates the radical cation
to give an allyl radical. Addition reactions should occur with low
regioselectivity.2b In general, an expected decrease in rate
constants with increasing alkyl substitution on the radical cation
was observed. The rate constants for reactions of the “dialkyl”
radical cations 3-5 with water are somewhat smaller than that
determined indirectly for reaction of water with a duplex DNA
radical cation centered on a T residue (k ) 2 × 106 M-1 s-1).4,9
They are similar in magnitude to the rate constants for reactions
of alcohols with styrene radical cations.10
Preliminary work demonstrated the feasibility of kinetic studies
of enol ether radical cation reactions with purine nucleosides.
Specifically, oxidations of 2′-deoxyadenosine (dA), 2′-deoxy-
inosine (dI), and 2′-deoxyguanosine (dG) by Chl* gave the
corresponding radical cations that did not oxidize Ph3N. This is
a necessary condition for application of the reporter kinetic
method. For tris(p-methoxyphenyl)amine, used as the reporter for
radical cation 6, no oxidation of the amine by the dG radical cation
was observed, and a slow growth of signal from the aminium
radical cation was observed from reaction of the amine with the
radical cation from dA. The radical cation produced by oxidation
of thymidine by Chl* oxidized Ph3N.
The enol ether radical cations most likely react with dA in
acid-base reactions or as acceptors in nucleophilic addition
reactions. A recent study15 suggested that the diethyl phosphate
anion rapidly deprotonated an enol ether radical cation structurally
similar to 5 within an ion pair with an estimated rate constant of
1 × 109 s-1, and dA is a stronger base than diethyl phosphate. It
seems likely, therefore, that a major reaction pathway for dA is
the acid-base reaction.
In summary, we have demonstrated a direct kinetic method
for studies of reactions of enol ether radical cations that can be
employed in the presence of a number of chromophores. The
method should also be useful for intramolecular enol ether radical
cation reactions including complex enol ether radical cations such
as those produced in fragmentations of pyrimidine nucleoside
radicals.
Acknowledgment. We thank the National Institutes of Health
(GM56511 to M.N. and CA60500 to D.C.) for support.
Rate constants for reactions of dA, dI, and dG with the enol
ether radical cations were measured in the same manner as those
for reactions of the radical cations with water (Table 1). The most
easily oxidized species, dG, apparently reacted in diffusion-
controlled processes with radical cations 1 and 2, but not in
diffusion-controlled reactions with the other radical cations. None
of the dI or dA reactions was diffusion-controlled.
Supporting Information Available: Details of the syntheses of new
compounds, details of kinetic studies, and kinetic results with standard
deviations (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA015602C
(11) Eberson, L. Electron-Transfer Reactions in Organic Chemistry;
Springer-Verlag: Berlin, 1987.
The purines can react with the enol ether radical cations by
electron transfer or as bases or nucleophiles. For dG, the major
(12) Seidel, C. A. M.; Schulz, A.; Sauer, M. H. M. J. Phys. Chem. 1996,
100, 5541-5553. Lewis, F. D.; Letsinger, R. L.; Wasielewski, M. R. Acc.
Chem. Res. 2001, 34, 159-170.
(9) The pseudo-first-order rate constant for reaction of the DNA enol ether
radical cation with water was determined to be 1.1 × 108 s-1 from competition
between reaction of water and KI with the radical cation and an assumption
that the electron-transfer reaction with KI was diffusion-controlled with a rate
constant of ca. 5 × 109 M-1 s-1 (ref 4).
(13) Schepp, N. P.; Johnston, L. J. J. Am. Chem. Soc. 1996, 118, 2872-
2881.
(14) This estimate is consistent with the observation that the thioanisole
radical cation oxidizes dG 6600 times as fast as it oxidizes dA. See: Steenken,
S.; Jovanovic, S. V. J. Am. Chem. Soc. 1997, 119, 617-618.
(15) Horner, J. H.; Newcomb, M. J. Am. Chem. Soc. 2001, 123, 4364-
4365.
(10) Johnston, L. J.; Schepp, N. P. J. Am. Chem. Soc. 1993, 115, 6564-
6571.