Influence of pH, temperature and the cationic porphyrin TMPyP4 on the stability of the i-motif formed by the 5′-(C3TA2)4-3′ sequence of the human telomere

The influence of pH, temperature and the cationic porphyrin TMPyP4 on the conformational equilibria of the cytosine-rich strand of the human telomeric sequence 5'-(C(3)TA(2))(4)-3', as well as those of the sequence 5'-(C3TT2)(4)-3', was studied. The presence of adenine bases at the loops causes the formation of two different intramolecular i-motif structures with a pH-transition midpoint around 4.6, which stability is lower than the i-motif formed by the sequence 5'-(C3TT2)(4)-3'. The stoichiometries of the complexes formed by these i-motif structures with TMPyP4 are also influenced by the presence of adenine at the loops. (C) 2011 Elsevier B.V. All rights reserved.