From the occurrences, it can be seen for the center position that hydroxyl substituents are practically absent in any structure with purple excitation wavelengths. However, as the excitation wavelength increases, they become much more prevalent. Similarly, the amine substituent is only present in about 12 % of center positions for low wavelengths, but can be found in almost 50 % of all center positions in the longer wavelength region. These findings can be interpreted as balancing electron withdrawing / donating effects in the chromophore, but may also be connected to a potential formation of intramolecular hydrogen bonds with the quinone oxygen atoms for those two groups, specifically. To confirm this, a detailed analysis of natural bond orbitals (NBO) may prove or disprove the hydrogen bond formation, while .