We next apply the new fitting strategy (equation ) to analyze our previously published experimental heterodyne chiral SFG spectra. The O-H-stretch peaks were identified using isotopic labeling with H2 18 O, which is expected to make O-H-dominated peaks red-shift by ~12 cm -1 . 2, 14 Peaks due to the N-H stretch were fit with conventional Lorentzians (one peak per feature). Figure shows fits for experimental chiral SFG spectra of LK7b in H2O (left column), LK7b in H2 18 O (central column), and LE7b in H2O (right column). The experimental spectra and their overall fits are shown in Figure , the fitted Lorentzians for N-H stretches in Figure , the fitted pairs of Lorentzians for O-H stretches in Figure , and the estimated frequency distribution in Figure . The residuals from the overall fits are plotted in yellow above Figure . As seen from Figure and the residuals, the fits are quite satisfactory. Also, the H2 18 O-induced red shifts are clearly captured. To obtain a similarly good fit for LK7b using the standard approach, a total of seven Lorentzian peaks and 21 free parameters were needed (Figure , Tables ), whereas here only 14 free parameters were used. Thus, our new fitting strategy can reduce the ambiguity of spectral analysis, improving the molecular understanding of the biomolecules and their hydration structures. We obtain the approximate frequency distribution by adding the absolute values of the symmetric and asymmetric stretch contributions (Figure ). The distribution appears to be more reliable than that estimated from the conventional fit (Figure ). The highly jagged nature of the frequency distribution obtained using the previous conventional fit (Figure ) does not match the smooth nature of the calculated IR response (Figure ), while the frequency distribution obtained using the new fitting strategy is smoother. Isotopic labeling with H2 O causes red shifts in the frequency distribution of the water around LK7b, as expected. By contrast, the conventional fits show drastically different frequency distributions for H2O and H2 18 O (Figure ), failing to reveal the expected isotopic shifts. This observation again validates the new fitting strategy for providing a molecular interpretation of chiral SFG spectra of water.