Self-assembly of org. mols. at metal surfaces is of greatest importance in nanoscience; in fact, it opens new perspectives in the field of mol. electronics and in the study of biocompatible materials. Combining an exptl. low-temp. scanning tunneling microscopy investigation with ab initio calcns., we succeeded to describe in detail (S)-glutamic acid adsorption on Ag(100) at T = 350 K. We find that (S)-glutamic acid organizes in a squared structure and, at variance with the majority of cases reported in literature, it adsorbs in the neutral form, 4.6 Å above the surface plane. The interaction with the poorly reactive Ag substrate is only due to weak van der Waals forces, while H-bonds between carboxyl groups and the formation of a OCOH-OCOH-OCOH-OCOH cycle at the vertex of the squares are the main responsible for the self-assembly. [on SciFinder(R)]