Brominated and iodinated nucleosides are used in crystallography studies of oligonucleotide structure. They are also photolabile and are used for cross-linking studies to probe the structure of protein-DNA complexes. Antibodies exist to Br-dU and oligonucleotides containing Br-dU can be used as probes.
Coupling: No changes needed from standard method recommended by synthesizer manufacturer.
Deprotection: Ammonium Hydroxide: room temperature as required by nucleobases.
The most widely used method seems to be using Br-dU and, more recently, I-dU (1). The substitution of photoreactive Br and I for the 5-Me group of thymidine is attractive since their radii are similar to that of the methyl group. These modifications do not significantly affect the binding of oligonucleotides with proteins. Br-dU is irradiated at 308nm and crosslinking is typically not greater than 40%. Crosslinking of I-dU at 308nm is higher but is optimal at 325nm. Laser excitation is preferred.After the synthesis of oligonucleotides containing Br-dU or I-dU, care must be taken to avoid loss of the halogens. Even though both modifications are quite stable to ammonium hydroxide, it is sensible to play safe and carry out the deprotection at room temperature for 24 hours. Even better, use the UltraMild monomers and deprotect with potassium carbonate in methanol at room temeperature. The oligonucleotide products should be protected from light - plastic tubes and amber vials are safe. Gel electrophoresis should be carried out protected from light.||REFERENCE(S):(1) M.C. Willis, B.J. Hicke, O.C. Uhlenbeck, T.R. Cech and T.H. Koch, Science, 1993, 262, 1255.||