Solution-state conformational ensemble of a hexameric porphyrin array characterized using molecular dynamics and X-ray scattering.

Solution-phase X-ray scattering measurements in combination with coordinate-based modeling have been used to characterize the conformational ensemble of a hexameric, diphenylethyne-linked porphyrin array in solution. Configurationally broadened X-ray scattering patterns measured at room temperature for dilute toluene solutions of the porphyrin array were compared to scattering patterns calculated from structural ensembles in constant pressure and temperature molecular dynamics simulations. Thermal fluctuations sampled at picosecond intervals within nanosecond time scale dynamic simulations show large-amplitude motions that include porphyrin ring "tipping" around the porphyrin linkage axes and extended hexameric porphyrin array "breathing" motions involving torsional distortions collectively distributed along porphyrin and diphenylethyne groups. Each type of group motion produced characteristic, angle-dependent dampening of scattering features that are needed to reproduce dampening features in the experimental X-ray scattering. However, mismatches in the magnitudes of experimental and simulated dampening of high-angle X-ray scattering patterns show that large-amplitude hexamer array breathing-type motions are significantly under-represented in the simulated ensembles. This comparison between experiment and simulation provides a means not only to interpret scattering data in terms of an explicit atomic model but more generally demonstrates the use of solution X-ray scattering as an experimental benchmark for the development of simulation methods that more accurately predict configurational dynamics of supramolecular assemblies.