The experimental 2DIR spectra of 1 mM Myc2345 in the presence of 0, 5,10 and 100 mM K + are presented in Figure . In EVV 2DIR spectroscopy, one infrared pulse excites one vibrational band while the other excites another. In this work, we consider only the situation where, in a normal-mode factorization of the vibrational wavefunction, the first pulse induces a one-quantum excitation to a singly excited state and the second induces a two-quantum excitation to a doubly excited state in which at least one of the mode indices corresponds to that of the singly excited state. The coupling is read-out by a visible pulse which Ramanscatters from the coherence if the two excited bands are coupled and does not scatter if they are not. The measured four-wave mixing signals are plotted with the Raman excitation frequency 𝜔 𝛾 subtracted, yielding a difference frequency axis 𝜔 𝛽 -𝜔 𝛼 (Y-axis) as a function of 𝜔 𝛼 (X-axis). As 𝜔 𝛽 excites a combination band, the two axes approximately correspond to the frequencies of the two fundamental vibrations which contribute to the combination band. This leads to rows and columns of features being observable in the data. These correspond to a particular vibrational mode, coupling to multiple other modes. Because the coupling density is high, multiple coupling peaks overlap in these spectra which means that peak positions do not necessarily represent the positions of couplings but instead are sometimes the peaks caused by the presence of multiple couplings. All spectra have been normalised with respect to the highest intensity peak at ω α = 1525 cm -1 , ω β -ω α = 1450 cm -1 . In EVV 2DIR spectroscopy, when considering the situation where combination bands and overtones are excited by one IR pulse at frequency ω β while a one-quantum transition is excited by a second IR pulse at frequency ω α , the coupling of one quantum transitions to their overtones is expected to fall on the dotted "overtone diagonal" line, although due to other factors such as anharmonicity, they do sometimes fall slightly below it. Off-diagonal features arise from the coupling of vibrational modes at frequencies ω α and ω β -ω α , with ω β exciting combination bands with frequencies that are approximately the sum of the two coupled modes. Horizontal and vertical dashed lines in (a) highlight the row and column of cross-peaks with frequencies which are dependent on potassium ion concentration, which have been assigned to the coupling of a guanine carbonyl stretch mode with various other modes. The guanine carbonyl stretch mode giving rise to the row of cross-peaks (GCO I) is higher in frequency than the guanine carbonyl stretch mode giving rise to the column of cross-peaks (GCO II). of a fundamental mode at 𝜔 𝛼 and its overtone at 𝜔 𝛽 . Off-diagonal features arise from the coupling of vibrational modes at frequencies 𝜔 𝛼 and 𝜔 𝛽 -𝜔 𝛼 . Spectral changes observed as a result of K + -induced structural changes in the DNA include a decrease in cross-peak intensity (blue circles); an increase in cross-peak intensity (red circles); the red-shifting of guanine carbonyl stretch frequencies at ~1700 cm - 1 (green diamonds); and the appearance new vibrational couplings (purple circles).