A 2'-5'-linked isoTBA 15 mer sequence with (232) loop composition formed stable antiparallel quadruplex structures similar to the SELEX derived 15 mer TBA sequence with (232) loop composition. A parallel versus antiparallel topology of 3'-5'-G-quadruplexes is largely dictated by the loop length, and it is known that the truncated loops favour parallel quadruplexes. In contrast to TBA, systematic reduction of the loop length in isoTBA from (232) to (222), (131) or even (111) did not alter the antiparallel topology of the resulting 14 mer, 13 mer and 11 mer G-rich modified isoTBA-like sequences.