Aqueous solutions of the bioprotectant proline are simulated for solute molar fractions ranging from 2.0 x 10(-3) to 2.3 x 10(-1). Statistical analyses show that proline affects the water structure more strongly than glycine betaine and trimethylaniine-N-oxide, two of the most effective bioprotectants widely diffuse in nature, and as strongly as tert-butyl alcohol, a protein denaturant which at high concentration self-aggregates. No evidence is found, however, that proline self-aggregates as it has been previously suggested to explain experimental findings on concentrated proline solutions. Nevertheless, the behavior of the diffusion coefficients of proline and water vs. solute concentration qualitatively agrees with such results. (c) 2005 Elsevier B.V. All rights reserved.