Incorporation of a hydration layer in the `dummy atom' ab initio structural modelling of biological macromolecules

Ab initio algorithms for the restoration of biomacromolecular structure from small-angle scattering data have gained popularity in the past 15 years. In particular, `dummy atom' models that require minimal information about the system under study have been proven capable of recovering the low-resolution shape of proteins and nucleic acids in many published works. However, consideration of solvated biological molecules as particles of uniform electron density contrast relative to the solvent neglects the presence of a hydration layer around their surface, leading to an overall apparent swelling of the obtained models and to a large overestimation of the volume of the particle. Here this problem is addressed by the introduction of an additional type of `dummy atom', representing the hydration layer. Successful applications of this new approach are illustrated for several proteins, and related results are compared with those from the program DAMMIN [Svergun (1999). Biophys. J. 76, 2879-2886].