Imino proton spectra and assignments of (a, b) the modified 24-nt and (c, d) natural 23-nt human telomeric sequences in K+ solution. (a, c) Guanine imino proton spectra with assignments over the reference spectrum (ref). Imino protons were assigned in 15N-filtered spectra of samples, 2% 15N-labeled at the indicated positions. (b, d) Imino proton spectra after (b) 1 day and (d) 2 h in D2O at 25 °C.