Because some close contacts in this dataset do not contain CH-p interactions, we categorized each contact into one of the following three categories: CH-p stacking interactions, hydrogen bonding interactions, or all other non-specific contacts (Figure ). CH-p stacking interactions are defined as instances in which the galactose stacks on top of the amino acid and three or more CH bonds are localized over the aromatic ring system (Figure ). CH bonds are considered localized over the aromatic ring when the carbon atom is positioned within 4.15 Å of the heavy atoms on the aromatic residue (Figure ). The resulting dataset contained 272 tryptophan, 69 tyrosine, and 69 phenylalanine CH-p stacking interactions. Hydrogen bonding interactions formed between the galactose and the aromatic side chain were identified after hydrogen geometries were optimized with DFT by using the polar contacts function in PyMOL, which annotates potential hydrogen bonding interactions that have a maximum acceptor-donor distance of 3.6 Å and a minimum acceptor-hydrogen-donor angle of 120° (Figure and Supporting Information Figure ). There were 29 tryptophan and 4 tyrosine sidechains that formed hydrogen bonds that met these criteria. In these cases, the N-H and O-H atoms on the sidechains primarily acted as hydrogen bond donors to oxygen atoms on the galactose. The remaining 50 tryptophan, 124 tyrosine, and 130 phenylalanine side chains formed non-specific interactions that did not meet either criterion. Thus, these sidechains had two or fewer C-H bonds localized over the aromatic ring system and no hydrogen bonds (Figure ). Thus, from 550 native close contacts, 62% of the close contacts form a CH-p stacking interaction, 6% form a hydrogen bond, and the other 32% are in proximity but form non-specific close contacts (Supporting Information Figure and Table ). The close contacts in this dataset are initially derived from 499 protein structures that have a non-covalently bound β-galactoside. Analysis of the types of protein structures contained in the set reveals that 42% were carbohydrate-binding proteins, 20% hydrolases, 16% viral proteins, 7% toxins, 7% transferases, and 8% other miscellaneous types. For 169 of these structures, we did not observe close contacts between galactose and an aromatic amino acid with good density support (i.e., from EDIA scores), whereas we identified 550 well-resolved close contacts for the other 330 structures (i.e., 1 or more per protein). All unique close contacts were retained, including those where multiple amino acids interact with the same carbohydrate (i.e., multiple close contacts), and cases where contacts were found on repeated protein subunits (Supporting Information Figure and Table ).