Jl. Mergny et al., INTRAMOLECULAR FOLDING OF PYRIMIDINE OLIGODEOXYNUCLEOTIDES INTO AN I-DNA MOTIF, Journal of the American Chemical Society, 117(35), 1995, pp. 8887-8898
At slightly acidic or even neutral pH, oligodeoxynucleotides which inc
lude a stretch of cytidines form a tetrameric structure in which two p
arallel-stranded duplexes have their hemiprotonated C.C+ base pairs fa
ce to face and fully intercalated, in a so-called i-motif, first obser
ved for d-TCCCCC. Cytosine-rich pyrimidine oligodeoxynucleotides, 14-3
0 bases long, can form an intramolecular ''i-motif', in a manner simil
ar to that observed in strands of telomeric repeat sequences (Leroy et
al. Nucleic Acids Res. 1994, 22, 1600-1606). This intramolecular i-mo
tif can accommodate as few as eight cytosines (four intercalated C.Cbase pairs), Evidence for intramolecular pairing was provided by therm
al denaturation, nondenaturing gel electrophoresis, and gel filtration
experiments and was characterized by specific NMR cross-peaks, The st
ability of a C.C+ base pair is very high at pH 5 and still detectable
at neutral pH. Formation of the i-motif, whether intramolecular or int
ermolecular, should be taken into account as a competing structure in
the design of triplex-forming oligodeoxynucleotides.