Gas-phase structures of neutral serine and of neutral cysteine were optimized by means of density functional theory. Moreover, three zwitterion structures stabilized by four water molecules were found for each system. Optical rotation of each conformer of neutral and zwitterion structures was calculated using Hartree-Fock and density-functional theory (with Coulomb attenuated functional), employing analytical response theory. The optical rotation of both amino acids is found to be very sensitive to the molecular conformation, to the extent that it changes sign for the different conformers. Dependence of optical rotation on the side-chain conformation is similar for serine and cysteine. (c) 2005 Elsevier B.V. All rights reserved.