We study the energetics of CH2O + NH3 -> CH2NH H2O in the gas phase at the B3LYP and CCSD(T) levels. This reaction is shown to have a sizeable barrier. Ionization of NH? reduces the barrier to about 5 kcal/mol. We also show that in water, a proton-catalyzed mechanism yields no barriers in excess of the reaction endothermicity. Since this reaction has been proposed as one of the steps in interstellar synthesis of glycine, the simplest amino acid, this work suggests that the formation of amino acids is occurring in and/or on interstellar water ice grains, and not in the gas phase.