Selenocysteine has a structure similar to that of cysteine, but with an atom of selenium taking the place of the usual sulfur, forming a selenol group which is deprotonated at physiological pH. (Like other natural proteinogenic amino acids, cysteine and selenocysteine have chirality in the older / notation based on homology to - and -glyceraldehyde. In the newer "R"/"S" system of designating chirality, based on the atomic numbers of atoms near the asymmetric carbon, they have "R" chirality, because of the presence of sulfur or selenium as a second neighbor to the asymmetric carbon. The remaining chiral amino acids, having only lighter atoms in that position, have "S" chirality.)