Reports: DNI751850-DNI7: N-Alkyl Urea Peptoid Oligomers as a New Type of Self-Assembling, Highly Versatile Soft Materials for Applications Involving Organogelators
Neil Ayres, PhD, University of Cincinnati
The synthesis of N-alkyl urea oligomers is
simple to perform, however when preparing longer oligomers, this synthetic
route results in many steps. The purification and isolation of the synthetic
intermediates is often the most challenging component. For simple systems it
would be beneficial to only need one oligomer that can be prepared in larger
quantities. We designed an N-alkyl-N,N-linked urea
oligomer that contained hydroxyl functionalized N-alkyl groups to
accomplish this (Compound 7, Scheme 1).
We chose to use a hydroxyl group as (in addition
to being inherently useful) it is highly versatile and can easily be converted
into many other functional groups. However, hydroxyl groups are not compatible
with the oligomer synthesis route. As a result we used protecting group
chemistry for the hydroxyl group during the oligomer synthesis. The methoxy
methyl ether (MOM ether) group is advantageous as it is a common protecting
group and stable to the oligomer synthesis conditions. The MOM groups were deprotected using a 2 M HCl solution at
40 °C We demonstrated the
synthetic versatility of the hydroxyl-functionalized oligomer by converting the
OH group to three other functional groups, namely alkyl chloride, alkyl azide
and carboxylic acid (Scheme 1).
Scheme 1.
Functional group conversion of hydroxyl groups into alkyl chlorides, alkyl
azides, and carboxylic acids.
Having completed the universal N-alkyl urea
peptoid synthesis we focused our attention on using these oligomers as
organogelators. Our first molecules used ureidopyrimidone (UPy) terminal
moieties to promote gelation in polar aprotic solvents through a balance of the
insolubility of the UPy groups and the high solubility of the N-alkyl urea
functional group (Scheme 2).
Scheme2. Synthesis of Gelator 1 and 2.
The gelation abilities of both gelators in organic
solvents were investigated using the stable to inversion method. As
summarized in Table 1, both gelators formed gels in selected polar aprotic
solvents.
Table 1. Gelation ability of 1 and 2 in organic solvents. Critical
concentration value (wt % and concentration in mol/L (M)) is shown next to the
gel symbol.
Solvent
1
2
n-Hexane
P
P
Toluene
P
P
Anisole
P
P
Pyridine
G(7.5, 0.097 M)
G(5.8, 0.056 M)
Tetrahydrofuran
P
P
Ethyl acetate
P
P
Chloroform
P
S
DMSO
G(4.3, 0.061 M)
G(6.0, 0.065 M)
DMF
G(6.5, 0.082 M)
G(6.6, 0.065 M)
Heavy mineral oil
P
P
G: gel; P: precipitate; S: soluble.
The gel transition temperature (Tgel)
is the temperature at which a gel undergoes a gel-sol transition and the value
of Tgel reflects the thermodynamic stability of a gel. The Tgel
increased as the weight percentage of gelators was increased until a plateau
was reached. The organogels from gelator 1 show higher Tgel
values than those from gelator 2. The differences in Tgel
values between the two gelators presumably reflect differences in network
organization in the gelled state.
Rheological measurements provide determinative
evidence for gel formation. A weight ratio of 5 wt% of gelator 1 in DMSO
was used to make a gel. The storage modulus, G', and the loss modulus, G,
of the 5 wt% gelator solution were plotted against time upon quenching from 80 oC
to 25 oC (Figure 3a). The moduli increased continuously over the
time span of the experiment, and eventually reached equilibrium values meaning
formation of a stable gel.
Figure 3. (a) Time sweep of the storage
(G') and loss (G) modulus at 25°C after a quench from 80°C at frequency of 1
rad/s and strain of 1%. (b) Frequency sweep at 25°C at a strain of 1% (c)
Strain sweep at 25°C at a frequency of 1 rad/s. Symbols: G' black squares, G
red circles.
The gel was then subjected to a frequency sweep
over a frequency range of 100 to 0.01 rad/s. As shown in Figure 3b, G'
was invariant with frequency confirming the dominant elastic (i.e., gel)
character.
Having shown that UPy-functionalized N-alkyl urea
peptoid oligomers act as gelators we modified our universal oligomer with UPy
units and investigated their aggregation. When 60 mg of the UPy-end
functionalized oligomer B was dissolved in 1.0 mL of CHCl3,
the solution was observed to undergo a marked increase in viscosity. Indeed,
after two days the vial could be inverted and the solution would not flow
unless agitated. This result strongly suggested that the UPy-modified N-alkyl
urea peptoid oligomers were aggregating into higher molecular weight species Diffusion-ordered
1H NMR spectroscopy (DOSY) and concentration-dependent solution viscosity
experiments were performed. The viscosity corrected self-diffusion constant (Dc)
of solutions of oligomers becomes smaller with increasing concentration of
oligomer in solution, implying that polymeric species are forming at higher
concentrations. A double logarithmic plot of the specific viscosity against solution
concentration showed a marked change in the slope from 1.006 to 2.785 at
approximately 30 mM (Figure 4).
Figure 4. Solution viscosities of Compound B (top) and Compound 6
(bottom) at varying concentrations. The change in slope for Compound B
occurs at ~ 30 mM.The slopes are shown in the Figure as m = x.xx.The result shown in Figure 4 can be explained due
to the presence of equilibrium in solution between polymers and low molecular
weight species. We investigated
the viscosity of the oligomer containing MOM pendent groups without UPy groups (Compound
6) at different concentrations as a control experiment. The results are
also plotted in Figure 4.
We have modified the oligomer with carbazole to
investigate the afforded gels photochemistry (Scheme 3). These experiments are
currently ongoing.
Scheme 3.
Carbazole functionalized oligomers
This work has had a positive impact on my career and
the training of students (Xiaoping Chen; Xinjun Yu) who have participated in
the project. Xiaoping and Xinjun have presented their findings at national
meetings, garnering them exposure to other scientists and professional
networking opportunities. This work has lead to scientific discussions and
potential collaborations both internally at the University of Cincinnati and
with researchers at other institutions. We anticipate that the remainder on
this project will result in new findings that will be leveraged into several
new research directions.