dspacer is also known as abasic site. It is a derivative of tetrahydrofuran and consists of a 1’,2’-Dideoxyribose without any nucleic basis attached. In nature, abasic sites are generated following depurination/depyrimidination events due to UV light or as an intermediate of base excision repair processes. Whereas naturally occurring abasic sites are rather fragile, synthetic spacers are considerably more stable and endure procedures like oligo synthesis and purification.
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M. Takeshita, C.N. Chang, F. Johnson, S. Will, A.P. Grollman. Oligodeoxynucleotides containing synthetic abasic sites. Model substrates for DNA polymerases and apurinic/apyrimidinic endonucleases. J. Biol. Chem. (1987), 262: 10171-10179.