Quantitative replacement of methionine with its non-natural amino acid analogues, norleucine, selenomethionine and telluro-methionine in human recombinant annexin V, is applied to study conformational and folding properties in solution. This procedure replaces each methionine sulphur atom with Se, Te or -CH 2 -, pro-viding single-atom exchanges, or »atomic mutations«. Using guani-dine chloride as denaturant, the estimated stabilities of protein variants are not significantly changed. The denaturation midpoints are shifted towards lower values owing to the increase in the hydro-phobicity of exchanged residues. Co-operativity expressed in terms of m-values is also affected by such exchanges and is highly corre-lated with the physical properties of methionine and its analogues in solution. This approach can contribute to a detailed understand-ing of interactions of particular amino acids and their implications on protein folding and protein-ligand interactions.