New cross peaks which appear as the potassium ion concentration is increased are potential markers of a fully folded G4. We have identified two new peaks formed on potassium addition: Peak 92 (𝜔 𝛼 = 1691 cm -1 , 𝜔 𝛽 -𝜔 𝛼 = 1132 cm -1 ) and Peak 101 (𝜔 𝛼 = 1705 cm -1 , 𝜔 𝛽 -𝜔 𝛼 = 1675 cm -1 ). Neither of the peaks appear in the calculated G4 spectrum, which suggests that both peaks could involve vibrational modes outside of the core G4 structural elements included in the calculation (Figure ). Using the calculated vibrational frequencies of the G4 structure (Figure ), the 𝜔 𝛽 -𝜔 𝛼 frequencies of both peaks have been tentatively assigned to vibrations of the core G4 structure: Namely, guanine ring deformations of terminal quartets in peak 92 and bends and C=O stretches of all quartets in peak 101. Animations of these G4 vibrations at 1132 cm -1 (peak 92) and 1675 cm -1 (peak 101) can be found in the Supplementary Information. The 𝜔 𝛼 frequency of both peaks is characteristic of a G C6=O6 stretch or a T C2=O2 stretch, which could be located within G/T in the end caps of Myc2345 (Figure ) that were not included in the calculation. It is already proposed by other work that bases in the end caps of G4s interact with the core G4 structure, further stabilising it. The NMR structure of Myc2345 (PDB: 7KBV) is consistent with this idea. 42