The C-nucleoside 2'-deoxypseudouridine, in contrast to dU, forms stable C:pseudoU-A triplets. 2-Aminopurine lacks groups critical for hydrogen bonding and is a mildly fluorescent base.
Demand for sulfur modified bases continues to expand for investigations of oligonucleotide structure, but primarily for cross-linking purposes. 6-Thio-dG, 4-Thio-dT and 4-thio-dU are very useful modifications for photo cross-linking and photoaffinity labelling experiments. Oligos containing 2-thio-dT are useful in examining protein-DNA interaction by acting as photosensitizing probes. The thiocarbonyl group in 2-thio-dT is especially interesting in that it is available to react with compounds associating with the minor groove of DNA. 2-Amino-A forms a very stable base pair with T containing three hydrogen bonds but the stability of the base pair with 2-thio-T is greatly diminished. Due to steric interactions between the 2-thio group of thymidine and the 2-amino group of 2-amino-A, the base pair contains only a single hydrogen bond. Oligos containing 2-amino-dA and 2-thio-dT exhibit high affinity for natural oligonucleotides but show little affinity for other similar oligos even of a complementary sequence.
Coupling: No changes needed from standard method recommended by synthesizer manufacturer.
Deprotection: No changes needed from standard method recommended by synthesizer manufacturer.
Response: While AMA (Ammonium hydroxide/40% Methylamine 1:1 v/v) is considered compatible with fluorescein, the use of methylamine when deprotecting a Fluorescein-labeled oligo does lead to a small amount of degradation, which is characterized by a the appearance of a late-eluting peak by RP HPLC that shows no visible fluorescein absorbance. With standard deprotection conditions (AMA 10 minutes at 65 C) the amount of degradation is approximately 5%|||