A periodic DFT study of the adsorption of glycine on an amorphous silica surface is presented. It is shown that glycine adsorbs via a specific interaction with the geminal silanols. Geminal silanols are the most reactive sites for glycine adsorption and the adsorption on these sites is irreversible. Vicinal and isolated silanols are much less reactive and should only give rise to reversible glycine adsorption. Vibration frequencies of the adsorbed glycine are calculated and compared to experimental data.