Synthesis and characterization of linear and cyclic ... - EleA@UniSA

2.3. Conclusions
In this work, we have constructed two orthogonally protected N --carboxy alkylated units. The
successful insertion in PNA-based decamers, through standard solid-phase synthesis protocols, and the
following hybridization studies, in the presence of DNA antiparallel strand, demonstrate that the N -
substitution with negative charged groups is compatible with the formation of a stable PNA:DNA
duplex. The present study also extends the observation that correlates the efficacy of the nucleic acids
hybridization with the length of the N alkyl substitution, 5a expanding the validity also to N --negative
charged side chains. The newly produced structures can create new possibilities for PNA with
functional groups enabling further improvement in their ability to perform gene-regulation.
2.4 Experimental section
2.4.1 General Methods.
All reactions involving air or moisture sensitive reagents were carried out under a dry argon or
nitrogen atmosphere using freshly distilled solvents. Tetrahydrofuran (THF) was distilled from LiAlH4
under argon. Toluene and CH2Cl2 were distilled from CaH2. Glassware was flame-dried (0.05 Torr)
prior to use. When necessary, compounds were dried in vacuo over P2O5 or by azeotropic removal of
water with toluene under reduced pressure. Starting materials and reagents purchased from commercial
suppliers were generally used without purification unless otherwise mentioned. Reaction temperatures
were measured externally; reactions were monitored by TLC on Merck silica gel plates (0.25 mm) and
visualized by UV light, I2, or by spraying with H2SO4-Ce(SO4)2, phosphomolybdic acid or ninhydrin
solutions and drying. Flash chromatography was performed on Merck silica gel 60 (particle size: 0.040-
0.063 mm) and the solvents employed were of analytical grade. Yields refer to chromatographically and
spectroscopically ( 1 H- and 13 C-NMR) pure materials. The NMR spectra were recorded on Bruker DRX
400, ( 1 H at 400.13 MHz, 13 C at 100.03 MHz), Bruker DRX 250 ( 1 H at 250.13 MHz, 13 C at 62.89 MHz),
and Bruker DRX 300 ( 1 H at 300.10 MHz, 13 C at 75.50 MHz) spectrometers. Chemical shifts () are
reported in ppm relatively to the residual solvent peak (CHCl3, = 7.26, 13 CDCl3, : = 77.0; CD2HOD,
= 3.34, 13 CD3OD, = 49.0) and the multiplicity of each signal is designated by the following
abbreviations: s, singlet; d, doublet; t, triplet; q, quartet; quint, quintuplet; m, multiplet; br, broad.
Coupling costants (J) are quoted in Hz. Homonuclear decoupling, COSY-45 and DEPT experiments
completed the full assignment of each signal. Elemental analyses were performed on a CHNS-O
FlashEA apparatus (Thermo Electron Corporation) and are reported in percent abundance. High
resolution ESI-MS spectra were performed on a Q-Star Applied Biosystem mass spectrometer. ESI-MS
analysis in positive ion mode was performed using a Finnigan LCQ Deca ion trap mass spectrometer
(ThermoFinnigan, San Josè, CA, USA) and the mass spectra were acquired and processed using the
Xcalibur software provided by Thermo Finnigan. Samples were dissolved in 1:1 CH3OH/H2O, 0.1 %
formic acid, and infused in the ESI source by using a syringe pump; the flow rate was 5 μl/min. The
capillary voltage was set at 4.0 V, the spray voltage at 5 kV, and the tube lens offset at -40 V. The
capillary temperature was 220 °C. MALDI TOF mass spectrometric analyses were performed on a
PerSeptive Biosystems Voyager-De Pro MALDI mass spectrometer in the Linear mode using -cyano-
4-hydroxycinnamic acid as the matrix. HPLC analyses were performed on a Jasco BS 997-01 series,
equipped with a quaternary pumps Jasco PU-2089 Plus, and an UV detector Jasco MD-2010 Plus. The
39