J . Org. Chem. 1996, 61, 9599-9602
9599
analyzed reactions for which the microwave effect was
proposed were either solid phase7 or plainly out of the
realm of biological models (high temperature and pres-
sure).8
Effect of Micr ow a ve Ra d ia tion on
Cop p er (II) 2,2′-Bip yr id yl-Med ia ted
Hyd r olysis of Bis(p-n itr op h en yl)
P h osp h od iester a n d En zym a tic Hyd r olysis
of Ca r boh yd r a tes
True biological reactions are very complex and can be
difficult to mimic in vitro.9 This led us to use model
reactions of natural hydrolytic processes. In nature, the
stability of phosphodiester linkages is of critical impor-
tance.10 All organisms contain RNA and most DNA,
which in turn are polymers whose backbone phosphate
units link sugars. The omnipresence of microwave
radiation (radio, TV, cellular phones, radar, etc.) in
today’s environment makes it all the more critical to
understand its influence on DNA and RNA stability.
These macromolecules (DNA, RNA) are prone to a variety
of mutagenic factors. For example, irradiation with
microwaves of the hydrogen-bonded strands of DNA could
lead to modifications in chain superstructure. On the
other hand, the phosphoester moiety itself, despite the
fact that it possesses rather polar portions usually
associated with microwave uptake, showed little or no
microwave absorption over a wide range of frequencies
(1000-10 000 MHz), as was reported by two independent
groups of scientists at Uppsala University in Sweden and
King’s College in London.11 The model reaction we have
studied is the hydrolysis of bis(p-nitrophenyl) phosphodi-
ester with copper(II) 2,2′-bipyridyl complex, as shown in
Scheme 1. In this reaction, a hydrolysis of the bis(p-
nitrophenyl) phosphodiester occurs with the release of
p-nitrophenoxy anion. This reaction mimics the cleavage
of the phosphodiester backbone of the DNA molecule.
The second model reaction pertains to the enzymatic
backbone scission of polycarbohydrates. It utilizes cel-
lobiose hydrolysis to model â-glycosidic bond scission
(Figure 1). We have chosen cellulase ((1,4-[1,3:1,4]-â-D-
glucan-4-glucanohydrolase) from Penicillinum funiculo-
sum) as a model enzyme since it acts as an exo-â-
glycosidase. Thus, despite the fact that polycarbohydrates
differ immensely from a disaccharide like cellobiose, units
of glucose are liberated by the action of the enzyme the
same way as in the model reaction (Figure 1). The
industrial importance of this process lies in the potential
rapid decomposition of cellulosic waste. Annual produc-
tion of cellulosic waste reaches 120 million tons.12 Mi-
crowave enhancement of cellulose degradation reactions
could lead to obvious benefits in sewage treatment and
waste management in the industrial realm. From an
academic point of view, the hydrolysis of cellobiose by
cellulase is one of the first reports of microwave-moder-
ated enzymatic reactions.
Konrad G. Kabza,* J ason E. Gestwicki,
J essica L. McGrath, and H. Michael Petrassi
Department of Chemistry, SUNY College at Fredonia,
Fredonia, New York 14063
Received J uly 17, 1996
In tr od u ction
The influence of microwave radiation on chemical
conversions is well documented1 but much less under-
stood at the present time. The common opinion is that
microwaved media absorb energy in two general modes,
i.e., by dipole rotation and ionic movement.2 However,
these proposed mechanisms do not explain some unusu-
ally high yields and quick conversions reported in the
literature.3 Some researchers have proposed the exist-
ence of a special “microwave effect” that causes these
anomalies. Others have questioned the existence of such
an effect and have provided preliminary data suggesting
that, if a microwave-irradiated reaction is compared to
a similar thermally compensated reaction, the rates of
both processes are nearly identical.4
The body of published work concerning heat-limited,
microwave-assisted chemistry is small. Such processes
(using electromagnetic radiation to amplify chemical
conversions while eliminating the heating of the irradi-
ated media) would be of great importance, if feasible,
especially applied to biological reactions. This is because
biological reactions typically proceed in aqueous media
(water, due to its polar nature, is prone to rapid micro-
wave heating) but have a strict temperature regime. In
fact, most of the enzymes utilized in biochemical reactions
will undergo denaturing at elevated temperatures.5
Thus, the choice of an enzymatic reaction is a good one
for heat-limited, microwave-assisted chemistry, because
such a reaction has an inherent temperature self-check
and will not yield any rate enhancement due to elevated
temperature. If the heating microzones are responsible
for the microwave effect, as was suggested by some
researchers,6 then the progress of an aqueous enzymatic
reaction should be severly limited if not completely
inhibited in the presence of microwaves. Localized zones
of high temperature would irrevesibly denature the
protein structure of the enzyme and thus destroy its
enzymatic function. In fact, some of the previously
To study these microwave-mediated model reactions
under conditions where the temperature can be controlled
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