Formation of reactive catalipid conjugates
To demonstrate catalipid-conjugation, a Cy3-labeled pentamer 3'-phosphate was allowed to react with the cationic lipid model 9a in the presence of EDC. Conjugate 12 was identified by MALDI-MS (Figure 16). Other lipids, of the DOSH and DOPH type were tested in similar reactions but did not show up in the MALDI-MS when the reactions were carried out under conditions that showed phosphorimidazolide of the models. We finally found that the temperature has a remarkable influence - conjugation took place above 40°C finally leading to the desired conjugate for MOH (Figure 17):
Figure 16
Figure 17
In a typical experiment 1 µmol of MOH (8a) dissolved in
chloroform was transferred into an Eppendorf tube and evaporated. A
volume of 200 µl of a 0.15 mM solution of a Cy3/Cy5-labeled
pentamer 3'-phosphate in buffer (0.1M HEPES, pH 7.2, 20% acetonitrile)
containing 0.1M EDC was added to the MOH-coated Eppendorf tube and was
sonificated for 10 minutes at RT. This reaction mixture was incubated
for 3 hours at 40 °C. Monitoring of product formation and following
purification was achieved using RP-HPLC. Due to high ammonium
concentrations during the evaporation process of the isolated product
fractions increased hydrolysis of the oligonucleotide-catalipid
conjugates was observed. Thus, product-containing eluates from HPLC
were desalted using Sep-PAC cartridges before they were concentrated in
a vacuum centrifuge. Yields were estimated to 70-80% according to HPLC
signal integration. The conjugates were characterized by MALDI-MS which
showed a significant co-ocurrence with the starting material/hydrolysis
product.
To investigate the ligation reaction via phosphorimidazolide
activation (Figure 2) pentameric template building blocks were
used.[37]
A solution of 1 mM 5'-amino-oligonucleotide NH2-CGGAA (15)
and 30 mol% 5'-Cy3 labeled template molecule Cy3-TTCCGCGGAA
(16) in 10 µl ligation buffer (0.1 M HEPES, pH 7.2, 50 mM NaCl,
20% acetonitrile) was added to 10 nmol of oligonucleotide-catalipid
conjugate 13. The reaction mixture was incubated for several hours at
20 °C and the reaction process was studied by RP-HPLC. The
formation of the ligation product Cy5-TTCCGpnCGGAA (17)
could not be detected however the reaction solution turned immediately
turbid after addition of 13.
Thus the reaction temperature was increased to 35 °C to dissolve 13 and to accelerate the reaction rate of product formation. This temperature rise caused only a faster hydrolysis of 13 but no ligation product 17 could be detected.