In order to disentangle these complex spectra as much as possible, the 2DIR spectra are fitted with two-dimensional Gaussian functions, which provide a good approximation to the shape of typical EVV 2DIR coupling feature. In this case, a minimum of 102 2D Gaussian functions are required, revealing the presence of at least 102 distinct couplings. The fitted 2DIR spectra are presented in Figure , with superimposed markers denoting cross-peak central frequencies from the fit. As expected, these do not always perfectly coincide with the peaks of the raw, unfitted data. Vertical line-cuts comparing raw and fitted spectra are presented in Figure which demonstrates the high signal to noise ratio of this data set and the goodness of the fit. Comparisons of raw and fitted data at other frequencies can be found in Table in the Supplementary Information along with Chi-squared values. In this section we now focus on the frequency shifts, intensity changes and new vibrational couplings that occur as a result of structural changes on K + ion addition. It is observed that multiple peaks in the region of in-plane base vibrations at around ~ 1700 cm -1 red-shift with increasing K + concentration, as shown in Figures . Peaks 12, 30, and 58 red-shift along the 𝜔 𝛽 -𝜔 𝛼 axis; peaks 74, 77, 85 and 87 red-shift along the 𝜔 𝛼 axis; and peak 80 red-shifts along both axes. The observed red shifts are consistent with the weakening of the guanine carbonyl bonds as Hoogsteen hydrogen bond formation and K + coordination occurs. Interestingly, the 𝜔 𝛽 -𝜔 𝛼 frequency representing guanine carbonyl stretching appears at a higher frequency than the 𝜔 𝛼 frequency. In the absence of potassium ions, the 𝜔 𝛽 -𝜔 𝛼 frequency associated with guanine carbonyl stretching is in the range 1694-1707 cm -1 , whereas the 𝜔 𝛼 guanine carbonyl stretching frequency is observed between 1671-1688 cm -1 . Interpretation of this highly unusual observation can be found in the Discussion section. Additionally, the guanine carbonyl stretching frequencies observed in the absence of potassium ions is higher than the single strand guanine frequency typically observed between 1660-1673 cm -1 29 , and this observation hints at the possibility of some pre-existing structure in the absence of cations. It is well known that nucleic acids exhibit changes in absorbance (hypochromism and hyperchromism) in the UV/Vis region of the spectrum, which is thought to be caused by base stacking interactions. Hypochromism and hyperchromism are reflected in the Raman line intensities of ring modes, which has previously been used to monitor conformational changes in nucleic acids. In the EVV 2DIR spectra, we focus on peaks which exhibit a monotonic increase/decrease in intensity and those which also show a minimum intensity change of 20% between the 0 K + and 100 mM K + spectra. We identify 29 peaks which decrease in intensity as K + concentration increases (Figure ). The majority of these peaks appear at frequencies characteristic of guanine ring vibrations (1476-1495 cm -1 , 1564-1568 cm -1 and 1575-1590 cm - 1 ). Additionally, five peaks have been observed to increase in intensity as K + concentration increases (Figure ). Assignment of these features and a description of the mechanisms for coupling increases and decreases are given in the discussion. The intensity traces of the remaining peaks as a function of K + ion concentration are shown in Figure in the Supplementary Information.