Chemistry Reference
In-Depth Information
(Kyogoku et al., 1967b). As in DNA, other noncovalent interactions such as p-p
stacking are often present with these heterocyclic bases.
In a similar fashion to DNA, synthetic nucleobase containing polymers exhibited
“melting” in solution that was attributable to the unraveling of cooperatively associ-
ated chains (Lutz, Thunemann, Rurack 2005). Note, however, that the structure of the
associated species was not elucidated and likely possessed little resemblance to the
double helix of DNA. Lutz and colleagues observed this melting phenomenon in
mixtures of random copolymers of adenine and thymine functionalized styrenic
monomers, 9-(4-vinylbenzyl)adenine and 1-(vinylbenzyl) thymine, with dodecyl-
methacrylate (Lutz, Thunemann, Nehring 2005; Lutz, Thunemann, Rurack 2005).
These copolymers organized in relatively low dielectric constant organic
solvents (chloroform and dioxane) at concentrations near 3
10
25
M
21
. A decrease
in the UV absorbance near 260 nm for the complex compared to the individual
copolymers suggests the formation of stacked nucleotide base pairs. The hypochro-
mic effect, which is also observed in DNA, increased with solution concentration
because of a shift to a more associated state. The hypochromic effect diminished
with heating, as a result of thermal disruption of the hydrogen bonded complex,
and a characteristic “melting” temperature was measured at 325 K, which is similar
to the melting of A-T DNA oligonucleotides at 325 K.
Marsh and colleagues (1999) reported on the use of ATRP for the synthesis of
uridine- and adenosine-functionalized methacrylate homopolymers as well as end
functionalization using uridine and adenosine functional initiators. These uridine
and adenonsine monomers consisted of nucleotide bases uracil and adenine con-
nected to the five membered deoxyribose sugars present in DNA and subsequently
bonded to methacrylate groups. In their study, silyl protecting groups were employed
on the hydroxyl groups of the uridine and adenosine monomers to improve their
solubility in organic solvents. Narrow polydispersities were obtained; however, the
molecular weights of the hydrogen bonding homopolymers were typically low
(,10 000 g/mol). The T
g
values for the hydrogen bonding methacrylate homopoly-
mers were near 140 8C, even at these low molecular weights.
Mather et al. (2007a) recently utilized nitroxide mediated polymerization from a
novel difunctional initiator to synthesize adenine- and thymine-functionalized tri-
block copolymers. Hydrogen bonding interactions were observed for blends of the
complementary nucleobase-functionalized block copolymers in terms of increased
specific viscosity as well as higher scaling exponents for specific viscosity as a func-
tion of solution concentration. In the solid state, the blends exhibited evidence of a
complementary A-T hard phase, which formed upon annealing, and dynamic mech-
anical analysis revealed higher softening temperatures. Morphological development
of the block copolymers was studied using small-angle X-ray scattering (SAXS)
and AFM, which revealed intermediate interdomain spacings and surface textures
for the blends compared to the individual precursors.
Spijker and colleagues (2005) synthesized nucleobase-functionalized block
copolymers containing thymine via ATRP of a thymine methacrylate monomer
from a poly(ethylene glycol) (PEG) macroinitiator. This polymer was introduced
into the polymerization of
an adenine
containing an alkyl methacrylate
