The importance of loop length on the stability of i-motif structures† †Electronic supplementary information (ESI) available: Experimental details, UV melting curves at 295 nm, and CD spectra at pH 5 and 8. See DOI: 10.1039/c4cc07279k Click here for additional data file.

Experimental All chemicals and oligonucleotides were purchased from Sigma-Aldrich. Oligonucleotides were purified by reverse phase column by Sigma-Aldrich and characteristic mass spectroscopy data is provided in table S1. UV spectroscopy was carried out using an Agilent Cary 100 with a temperature controlled 6 x 6 cell changer. CD spectra were collected on beamline B23 at Diamond Light Source. Table S1 – Characteristic mass spectrum data for oligonucleotides used in this study. Oligonucleotide Concentrations The concentration of all oligonucleotides were calculated from the absorbance value at 260 nm using extinction coefficients calculated using the nearest neighbour method. UV melting experiments Oligonucleotides were dissolved in 50 mM sodium cacodylate buffer of the appropriate pH to a final concentration of 1 µM, and were annealed by heating to 90°C and slowly cooling to room temperature. UV spectra were recorded at 260 and 295 nm at 1°C intervals between 20-90 °C, with a temperature change rate of 1°C/min in a 1cm pathlength cuvette. UV pH titrations Oligonucleotides were dissolved in 50 mM sodium cacodylate at pH 8, and a spectrum recorded between 200-350 nm. The pH was adjusted by the addition of small aliquots (approximately 1 µL) of HCl and measured in the cuvette using a Thermo Scientific Orion Star A11 pH meter equipped with a Name Sequence 5'-3' Theorectical MW Observed MW C3T333 d(CCCTTTCCCTTTCCCTTTCCCT)