The natural optical activity of two chiral amino acids, alanine and proline, has been calculated using Hartree-Fock and density-functional theory with the Becke three-parameter Lee-Yang-Parr (B3LYP) functional employing analytical response theory. The dependence of the optical activity on the molecular conformation in the gas phase was investigated. In the case of proline, calculations were also carried out for the protonated and deprotonated molecules. The increase of the optical rotation of proline with increasing pH, found experimentally, is reproduced by our calculations. The optical rotation of both amino acids is found to be very sensitive to the molecular geometry, to the extent that it changes sign for the different conformers. For alanine, the sign of the optical rotation varies with the rotation of the amino or carbonyl groups.