Fluorescent proteins are widely used in modern experimental biology, but much controversy exists regarding details of maturation of different types of their chromophores. Here we studied possible mechanisms of DsRed-type red chromophore formation using synthetic biomimetic GFP-like chromophores, bearing an acylamino substituent, corresponding to an amino acid residue at position 65. We have shown these model compounds to readily react with molecular oxygen to produce a highly unstable DsRed-like acylimine, isolated in the form of stable derivatives. Under the same aerobic conditions an unusual red-shifted imide chromophore - a product of 4-electron oxidation of Gly65 residue - is formed. Our data showed that GFP chromophore is prone to autoxidation at position 65 Cα by its chemical nature with basic conditions being the only key factor required.