Chemistry Reference
In-Depth Information
viscosity, and so forth. In order to tune the strength of the association constant,
numerous molecular parameters are accessible. A common approach for increasing
the association strength is the incorporation of additional hydrogen bonding sites
within a single hydrogen bonding array, which is called “multiple hydrogen
bonding.” The association of two species is unfavorable entropically, but this loss
of entropy is not greatly increased from the association of additional pairs of
donors and acceptors on the same two molecules. Single to quadruple hydrogen
bonding units exist, and some larger arrays containing six (Zeng et al. 2000) and
eight (Folmer et al. 1999) hydrogen bonds were synthesized and studied. In addition,
the strength of the hydrogen bonding interaction is tuned via the pattern of hydrogen
bonding groups. Research has shown that a number of factors are important in the use
of multiple hydrogen bonded systems (Beijer et al. 1996; Brunsveld et al. 2001).
Alternation of donor and acceptor groups in a multiple hydrogen bonding array
reduces the strength of the association via repulsive interactions of opposing adjacent
donors or acceptors (Jorgensen and Pranata 1990; Beijer et al. 1998). Thus, Meijer's
quadruple hydrogen bonding (DDAA) UPy group is designed with minimum alter-
nation of donor and acceptor units (Beijer et al. 1998). When multiple hydrogen
bonding groups are considered, the possibility of self-complementarity arises. The
simplest case of self-complementarity is probably the carboxylic acid dimer, which
contains two hydrogen bonds. In contrast, complementary hydrogen bonding
arrays are found in DNA, which possesses both adenine-thymine (A-T) and
cytosine-guanine (C-G) pairs. Another factor to consider in heterocylic bases is
the ability of the base to tautomerize into different forms (Lee and Chan 1972).
This can lead to complexity in the behavior of the hydrogen bonding group, and
different hydrogen bonding guests can shift
the equilibrium between tautomers
(Ligthart et al. 2005).
The reversible nature of the hydrogen bond is not limited to thermoreversibility,
although that is the most commonly exploited and studied property of these
systems. Hydrogen bonds are also sensitive to other environmental factors, such as
the polarity of the medium and the presence of competitive solvents and water.
Deans et al. (1999) observed that solvent polarity has a strong effect on the hydrody-
namic volume for self-associating polymers. Thus, humidity and exposure to polar
media are two other factors that are considerations in the application of hydrogen
bonded materials. Another factor that strongly affects hydrogen bonding, as
suggested in Eq. (4.1), is concentration. Lower concentrations of monomeric
units lead to lower equilibrium concentrations of complexed units. Solution pH
can also affect hydrogen bonding systems, leading to pH controlled thickening
(Sotiropoulou et al. 2003) in cases of interpolymer complexes between
poly(acrylic acid) and polyacrylamide. Complexes of poly(acrylic acid) and
poly(N-vinylpyrrolidone) exhibit molecular weight dependent critical pHs below
which stable complexes are formed because of a transition to polyelectrolyte
species at higher pH (Nurkeeva et al. 2003).
The hydrogen bond is extensively used in nature, particularly in the construction
of proteins (Aggeli et al. 1997), DNA (Voet and Voet 1995), and RNA. In all cases, it
performs the role of establishing a reversible structure that allows such processes as
