We estimate the frequency distribution of chiral water superstructures (Figure ) by adding the absolute values of the computationally predicted symmetric and asymmetric stretch components. We find that the first hydration shell subset has a single-peak frequency distribution. This is consistent with the IR spectrum in Figure , which has a single peak around 3400 cm -1 , and is very similar to that of bulk water. This is consistent with our previous argument that water molecules around a protein experience a similar environment to bulk water in terms of hydrogen bonding environment. Whatever environment a water molecule experiences, it tries to form three to four hydrogen bonds. Furthermore, our calculations and fitting reveal that in the subset of water hydrogen-bonded to the peptide backbone N-H, the peak at the highest frequency is due solely to the asymmetric stretch component (Figure ). We previously argued that this highfrequency peak is due to water molecules containing one O-H group pointing into the air and another hydrogen bonded to an N-H group on the backbone. We now understand that this peak is due to the asymmetric-stretch half of a pair centered at ~3650 cm -1 (Figure , right). This interpretation indicates that the water molecules producing this pair of water peaks are not hydrogen-bond free but most likely experience some degree of hydrogen bonding with both of their O-H groups, although this hydrogen bonding is weak. Hence, the water molecules in this subset do not have O-H groups pointing directly toward the air, as that would most likely push the peak pair's central frequency above 3700 cm -1 . The corresponding symmetric stretch peak in the high-frequency pair (Figure , right) is negative and buried in the total line shape. This analysis illustrates a subtlety of heterodyne chiral SFG -peaks can cancel and obscure each other due to the nature of the positive-and negative-phase response. In addition, a peak in the frequency distribution (Figure ) does not necessarily indicate a peak pair center near the peak's frequency.