Functionalized Silver‐Ion‐Mediated α‐dC/β‐dC Hybrid Base Pairs with Exceptional Stability: α‐d‐5‐Iodo‐2′‐Deoxycytidine and Its Octadiynyl Derivative in Metal DNA

Silver-mediated α-dC-Ag+ -β-dC hybrid base pairs decorated with 5-iodo- or 5-octadiynyl residues are well accommodated in duplex DNA. A strong Tm increase and favorable thermodynamic data for duplex DNA were observed after addition of silver ions. The phenomenon is particularly obvious when both nucleobases of the base pairs are functionalized. Neither the position of the base pair, nor the type of 5-substituent had a negative influence. On the contrary, functionalization of conventional silver-mediated β-dC-Ag+ -β-dC homo base pairs showed a negative impact induced by the bulky substituents. To this end, cytosine modified 12-mer oligodeoxynucleotides were prepared by solid-phase synthesis employing new α-anomeric 2'-deoxycytidine phosphoramidites. A multigram scale synthesis was developed for 5-iodo-α-d-2'-deoxycytidine (1) employing the direct glycosylation of cytosine with Hoffer's α-d-halogenose followed by separation of anomeric DMT nucleosides. Regarding base-pair stability and functionalization silver-mediated α/β-dC hybrid base pairs were found to be superior to β/β-dC homo pairs. According to their extraordinary properties, they might find applications in DNA diagnostics, material science, or nanotechnology.