Adenine biotin derivative

Biotin-dUTP derivatives are formed by modification of the C-5 position of uridine. This location is not involved in hydrogen bonding activity with complementary DNA strands, thus hybridization efficiency is not immediately compromised. By contrast, biotin-dCTP or biotin-dATP derivatives involve modification of the bases at the N-4 position of cytosine and the N-6 position of adenine, locations directly involved in hydrogen bond formation with complementary bases. Thus, DNA biotinylation through the use of modified deoxynucleoside triphosphates to be incorporated into existing DNA strands may result in better activity of the probe if dUTP is used over dATP or dCTP. 

Adrian, Jr., J. C., Wilcox, C. S. (1989). Orderly functional group dyads. Recognition of biotin and adenine derivatives by a new synthetic host, J. Am. Chem. Soc., Ill ... 

Figure A6.1 Structures of three of the vitamin-derived cofactors. NAD, like several other cofactors, has a handle consisting of adenine (blue) and ribose phosphate (black), but the actual chemistry of oxidation and reduction is done by the nicotinamide (red), which is derived from the vitamin nicotinic add or niacin. Note the positive charge on the ring nitrogen in the oxidised form, NAD+, which is shown here. Pyridoxal phosphate, derived from vitamin carries out its chemical contribution via the aldehyde group (mauve). Biotin likewise contributes its own characteristic chemistry, with the nitrogen atom, shown in blue, readily able to pick up CO2 as a carboxyl group.

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