duplex effects

The design of primers is frequently complicated by the degeneracy of the genetic code. Three strategies are now available to confront this problem. In the first, a mixed base addition (N) is used to form the degenerate site. This approach is best if the number of degenerate sites is small. A second option is the use of 2’-deoxyInosine or 2’-deoxyNebularine which exhibit low, but unequal, hydrogen bonding to the other four bases. The third option is the use of a universal nucleoside. In this strategy, the base analog does not hybridize significantly to the other four bases and makes up some of the duplex destabilization by acting as an intercalating agent. 3-Nitropyrrole 2’-deoxynucleoside (M) is the first example of a set of universal bases. Subsequently, 5-nitroindole was determined to be an effective universal base and to be superior to 3-nitropyrrole, based on duplex melting experiments.

The modified bases designated P and K show considerable promise as degenerate bases. The pyrimidine derivative P, when introduced into oligonucleotides, base pairs with either A or G, while the purine derivative K base pairs with either C or T. A dP+dK mix also can serve as a mixed base with much less degeneracy than dA+dC+dG+dT (N).

Unnatural base pairs display unique abilities in duplex DNA and in nucleic acid and protein biosyntheses. A standard Watson and Crick base pair is formed between iso-C and iso-G, but the hydrogen bonding pattern is quite different from the natural base pairs A-T and C-G. (The 5-methyl analogue was chosen as the synthetic target due to the reported instability of 2’-deoxyisocytidine caused by deamination during oligonucleotide synthesis or deprotection.)

The unnatural base pair between 7-(2-thienyl)-imidazo[4,5-b]pyridine (Ds) and pyrrole-2-carbaldehyde (Pa) is formed by specific hydrophobic shape complementation. The shape of the Ds-Pa pair is different from those of the natural A-T and G-C pairs, but the Ds-Pa pair works together with the natural pairs in in vitro replication and transcription. Pa also functions as a template base for incorporating another unnatural base, 2-amino-6-(2-thienyl)purine (s), into RNA. The s base also acts as a unique fluorescent base analog in DNA and RNA fragments. dDss is strongly fluorescent and is useful as a fluorescent tag for DNA detection. dDss also forms a base pair with dPa. Biotin PaTP can be site-specifically incorporated into RNA, opposite dDs at a desired position in DNA templates, by T7 transcription. Similarly, the fluorescent s base can be site-specifically incorporated into RNA opposite dPa in DNA templates.

chain terminators

In situations where ligation must be blocked at the 5’ terminus, 5’-OMe-dT may be used. 5’-OMe modification of a strand of siRNA using 5’-OMe-T can control guide strand selection and targeting specificity.¹ 5’-Amino-dT terminates an oligonucleotide with a 5’-amino group which may be used for attaching a peptide or a PNA sequence. To avoid polymerase extension at the 3’ terminus, 2’,3’-dideoxynucleoside and 3’-deoxynucleoside CPGs have proved to be effective. 2’,3’- Phosphoramidites are designed to be used with the 5’-phosphoramidites and supports. Since these phosphoramidites have no DMT group, they are not compatible with purification by the DMT-on technique. Ion exchange HPLC or PAGE should be used to purify these dideoxy terminated oligos to ensure that shorter sequences (containing 3’-OH) groups are removed. (3’-Termination can also be effected using a 3’-3’ linkage formed using 5’-supports, or 3’-spacer C3 CPG.)