Experimental EVV 2DIR ratio spectra of 1 mM Myc2345 in the presence of 100 mM K + divided by 1 mM Myc2345 in the absence of ions. b, Computational EVV 2DIR ratio spectrum of G-quadruplex / Gquartet. The calculated spectra include only electrical coupling, but a similar overall change is observed when mechanical coupling is included. Alternating increase and decrease in coupling observed around ~1700 cm -1 (dashed lines) are due to the red-shifts of guanine carbonyl stretch frequencies. Positive features in the experimental difference spectrum at 𝜔 𝛼 = 1691 cm -1 , 𝜔 𝛽 -𝜔 𝛼 = 1132 cm -1 and 𝜔 𝛼 = 1705 cm -1 , 𝜔 𝛽 -𝜔 𝛼 = 1675 cm -1 (black crosses) are due to the appearance of new peaks 92 and 101, respectively. The remaining difference features are due to intensity changes in cross-peaks as a result of structural changes in the DNA as potassium ion is introduced. The experimental ratio spectrum is consistent with structural differences between a G-quartet and a G4. Alternating columns of red and blue features on the right-hand side of both spectra is a qualitative indication that the essential features associated with those columns are reflected in both calculation and experiment as is the absence of that pattern in the corresponding rows.