In the book applied photochemistry: when light meets molecules, the authors specifically state that "Natural dissolved organic matter in surface waters consists of both autochthonous material, mainly made up of aliphatic chains that do not absorb radiation (polysaccharides, complex carbohydrates, peptides, proteins), and of allochtonous compounds that derive from soil erosion such as fulvic and humic substances. The latter contain a significant percentage of aromatic groups that absorb sunlight, at such an extent that DOM is the most important radiation absorber in surface waters." - I am under the impression that carbohydrates and peptides can absolutely absorb radiation, despite their aliphatic groups, could anyone please clarify what the authors mean with this statement? Many peptides have aromatic moieties that absorb light, thereby the peptides as a whole become excited... Carbohydrates have carbonyl groups that absorb light and become excited... So I really don't understand why the authors are making this distinction? Help?