Macromolecules
Article
with a Waters Acquity BEH C-18 (1.7 μm) column at a flow rate of
0.6 mL min−1. UPLC samples were dissolved and passed through a
220 nm nylon filter prior to injection.
7.60−7.52 (m, 2 H), 7.40−7.34 (m, 3 H), 5.28 (s, 1 H), 3.79 (s, 3 H)
(for full spectrum, see Supporting Information Figure S1-3).
Synthesis of Methyl Bromoisobutyrate, MBiB. Bromoisobutyric
acid (4.17 g, 25 mmol) was dissolved in a solution of concentrated
sulfuric acid (0.75 mL) and methanol (50 mL) and stirred under
reflux for 2 h. The solution was then cooled to room temperature and
concentrated under vacuum. The residue was dissolved in diethyl
ether and washed with 10% aqueous sodium bicarbonate, washed with
brine, dried under magnesium sulfate, and concentrated again under
vacuum. A colorless oil was obtained (4.2 g, 23 mmol, 93% yield) 1H
NMR (300 MHz, CDCl3) δ(ppm): 3.78 (s, 3H), 1.93 (s, 6H) (for full
Solid-Phase Peptide Synthesis of BiB-SPKYS-OH. To pretreat the
resin for peptide synthesis, the CTC resin (500 mg, max reactive units
0.666 mmol, 1.33 mmol/g) was agitated in a solution of thionyl
chloride in DCM (8 mL, 5% v/v) at room temperature for 2 h after
which was washed with a solution of DIPEA in DCM (7 × 5 mL) and
dichloromethane (3 × 5 mL).
A portion of the CTC resin (100 mg, 0.13 mmol reactive sites, 1.33
mmol/g) was then agitated in a solution of FMoc-Ser(tBu)-OH (204
mg, 0.53 mmol, 4 equiv) and DIPEA (137 mg, 1.06 mmol, 8 equiv) in
DCM (4.3 mL of 0.125 M amino acid) for 16 h at room temperature
and then washed with DCM (5 × 5 mL). The resin was then agitated
in a solution of DCM, DIPEA, and methanol (5 mL, 17:2:1 v/v/v,
respectively) for 30 min and then washed again with DCM (5 × 5
mL) and diethyl ether (3 × 5 mL) and then dried under vacuum.
Resin loading was found to be 48% by the conditions outlined
previously.
The following deprotection, coupling, and capping cycle was
repeated for each amino acid. The rest of the coupled resin was
swelled in DCM at room temperature and then washed with DMF (5
× 5 mL). This was then agitated in a solution of piperidine in DMF
(∼3 mL, 20% v/v) for 15 min and then washed with DMF (3 × 3
mL). The deprotected resin was then agitated in a solution of FMoc-
protected amino acid (0.27 mmol, 4 equiv, 124 mg for FMoc-
Tyr(tBu)-OH, 126 mg for FMoc-Lys(Boc)-OH, 91 mg for FMoc-
Pro-OH, 102 mg for FMoc-Ser(tBu)-OH), DIC (34 mg, 0.27 mmol,
4 equiv), and OxymaPure (38 mg, 0.27 mmol, 4 equiv) in DMF (1.3
mL) at room temperature for 1 h. This was then washed with DMF (5
× 5 mL) and DCM (3 × 5 mL). The coupled resin was then agitated
in a solution of acetic anhydride and DIPEA in DMF (3 mL, 2.5 and
5% v/v, respectively) for 15 min.
Cyclic voltammetry (CV) was performed using a Bioanalytical
Systems Electrochemical Analyzer. The electrolyte solution ((Bu4N)-
PF6, 0.1 M in acetonitrile) was sparged with argon before use. The
voltammogram was recorded using a glassy carbon working electrode
(1.5 mm diameter), a platinum wire as the counter electrode, and an
Ag/Ag+ wire reference electrode was used. Ferrocene was added as an
internal standard upon completion of each experiment. All potentials
are quoted in mV vs Fc+/Fc (calibrated as E° = 450 mV vs Ag/Ag+).
Electrospray ionization mass spectrometry (ESI-MS) was per-
formed on a Thermo VelosPro Orbitrap mass spectrometer. Samples
were infused via a syringe pump at 10 μL min−1 with the standard
electrospray ionization source. The capillary was set to a temperature
of 330 °C, a potential of 4.5 kV, and with the nebulizer pressure at 10
psi. Orbitrap was run in full scan mode (or high mass mode when
scanning up to 4000 m/z) with a resolving power of 100,000, and a
lock mass of dioctylphthalate ([M + H]+ 391.284286 m/z, [M + Na]+
413.266230 m/z) was used for calibration.
Resin loading calculation was performed via UV−vis spectropho-
tometry. To calculate amino acid loading, the dried resin (∼1 mg)
was added to a solution of piperidine in DMF (3 mL, 20% v/v). This
was left to react for at least 15 min. The UV−visible spectrum of the
filtrate was then obtained on an Agilent Technologies Cary 5000
spectrometer at room temperature. The absorbance value at 290 nm
was taken against a 20% piperidine blank (dibenzofulvene−piperidine
adduct has an experimentally determined value for molar absorptivity
in DMF @301 nm, ε = 7800 L mol−1 cm−1).
Nonlinear curve fitting of molecular weight distributions was
performed on OriginPro 8.6 software using the Levenberg−
Marquardt algorithm against bi-Gaussian curves.
Synthesis of Chain-Transfer Agents and Initiators. Synthesis
of Bis(dodecylsulfanylthiocarbonyl) Disulfide, BisTTC. n-Dodeca-
nethiol (5.13 g, 25 mmol) was added dropwise to a stirred suspension
of sodium hydride (60% dispersion in mineral oil) (1.16 g, 26 mmol)
in dry diethyl ether (50 mL) over 10 min at around 5 °C. The
evolution of hydrogen production was observed, and a thick white
slurry of sodium thiododecylate was generated. The reaction mixture
was cooled to 0 °C, and carbon disulfide (2.0 g, 26 mmol) was added
dropwise. Iodine (2.1 g, 25 mmol) was then added portionwise, and
the suspension was stirred for 1 h at room temperature. The yellow
filtrate was washed with a solution of sodium thiosulfate to remove
excess iodine, dried over with sodium sulfate, and concentrated under
vacuum. The residue was then purified under flash chromatography
on silica gel (hexanes). A yellow solid was obtained (12.1 g, 83%
yield); mp 32−35 °C; 1H NMR (300 MHz, CDCl3) δ(ppm): 3.32 (t,
J = 7.5 Hz, 4H), 1.71 (p, J = 6.9 Hz, 4H), 1.55−1.15 (m, 36H), 0.90
(t, J = 7.2 Hz, 6H) (for full spectrum, see Supporting Information
Synthesis of Dixanthogen, BisXan. Potassium ethyl xanthogenate
(2.00 g, 12.5 mmol) was dissolved in methanol (50 mL, 0.25 M), and
iodine (1.60 g, 6.25 mmol) was added portionwise at room
temperature. The solution was then concentrated under vacuum
and then precipitated in water. The residue was washed with saturated
aqueous sodium thiosulphate and then water and dried under
vacuum. A light-yellow solid was obtained (1.5 g, quantitative); mp
28−34 °C; 1H NMR (300 MHz, CDCl3) δ(ppm): 4.64 (q, J = 7.2 Hz,
4H), 1.37 (t, J = 7.2 Hz, 6H) (for full spectrum, see Supporting
Upon coupling of the final amino acid, capping, and N-terminus
deprotection, the resin was agitated in a solution of bromoisobutyryl
bromide (BiBB) and DIPEA in DMF (5 mL, 2.5 and 5% v/v,
respectively) for 1 h at room temperature and then washed with DMF
(5 × 5 mL) and DCM (5 × 5 mL).
The resin-functionalized, side chain-protected product was washed
with DCM (10 × 5 mL). This was then agitated in a solution of TFA
and TIPS in deionized water (5 mL, 90 and 5% v/v, respectively) at
room temperature for 2 h. The filtrate was then concentrated under
nitrogen flow, and the product was precipitated twice in diethyl ether.
Upon purification by HPLC and lyophilization, a colorless solid was
obtained (32 mg, 70% resin yield, 93% stepwise yield) 1H NMR (500
MHz, D2O) δ(ppm): 7.09 (d, J = 8.5 Hz, 2H), 6.77 (d, J = 8.5 Hz,
2H), 4.69−4.66 (m, 1H), 4.62 (dd, J = 9.3, 5.7 Hz, 1H), 4.38 (t, J =
4.5 Hz, 1H), 4.33 (dd, J = 8.3, 6.0 Hz, 1H), 4.16 (dd, J = 8.4, 6.4 Hz,
1H), 3.94−3.76 (m, 5H), 3.66 (dt, J = 10.1, 6.9 Hz, 1H), 3.09 (dd, J =
14.0, 5.7 Hz, 1H), 2.92−2.83 (m, 3H), 2.23−2.12 (m, 1H), 1.95 (p, J
= 6.8 Hz, 2H), 1.88 (d, J = 5.3 Hz, 6H), 1.76−1.66 (m, 1H), 1.66−
1.52 (m, 4H), 1.36−1.16 (m, 2H) (for full spectrum, see Supporting
Polymerization. General Procedure for the Synthesis of Poly(n-
butyl acrylate), PnBA. In a typical experiment, a solution of
bis(dodecylsulfanylthiocarbonyl) disulfide (1 equiv) in n-butyl
acrylate (40 equiv) was added to a solution of alkyl bromide (2
equiv) in DMF (10 M monomer concentration, 0.8 monomer/solvent
mole ratio) in a 4 mL vial. Bismuth oxide (0.2 equiv) was added to
the solution. The reaction vial was sealed with a septum and degassed
Synthesis of Methyl Bromophenylacetate, MBPA. Bromophenyl-
acetic acid (5.38 g, 50 mmol) was dissolved in a solution of
concentrated sulfuric acid in methanol (50 mL, 1.5% v/v) and stirred
under reflux for 1 h. The solution was then cooled to room
temperature and concentrated under vacuum. The residue was then
dissolved in diethyl ether and washed with 10% aqueous sodium
bicarbonate, washed with brine, dried under magnesium sulfate, and
concentrated again under vacuum. A slightly yellow oil was obtained
1
(5.5 g, 24 mmol, 96% yield) H NMR (300 MHz, CDCl3) δ(ppm):
C
Macromolecules XXXX, XXX, XXX−XXX