Autofocusing molecular simulations for integrative structural biology of broad conformational ensembles

Integrative structural biology seeks molecular descriptions consistent with complementary experimental measurements. A particular challenge arises for molecules whose broad conformational ensembles resist description by a few representative structures, such as natively unfolded proteins and fluid-phase biomembrane lipids. Although molecular simulations can generate such ensembles, their simultaneous agreement with a large number of experimental constraints is commonly prevented by model inaccuracies. Here we introduce autofocusing as an approach that integrates experimental information by automated finetuning of the physical model generating the ensemble. We demonstrate it for a fluid bilayer of the phospholipid POPC, for which no existing force field has simultaneously reproduced the complete set of 82 NMR C-H bond order parameters. Autofocusing brings all the 82 within experimental uncertainty while preserving agreement with independent small-angle X-ray scattering measurements and retaining or improving agreement with untargeted NMR relaxation rates and effective correlation times, thus supporting its description of membrane structure and dynamics beyond the calibration targets. The resulting simulations challenge a long-standing two-state interpretation of phosphatidylcholine headgroup structure, instead supporting a broad conformational ensemble poorly represented by crystal-derived conformers. These results establish autofocusing as a practical approach to constructing experimentally grounded conformational ensembles while retaining the computational tractability of classical molecular dynamics.