Explicit Ion Modeling Predicts Physicochemical Interactions for Chromatin Organization.

Area of Science:

• Molecular Biophysics
• Computational Biology
• Chromatin Dynamics

Background:

• Understanding in vivo chromatin organization requires elucidating molecular mechanisms, particularly the contribution of intrinsic interactions.
• Nucleosome-nucleosome binding strength is a key metric, with prior experimental estimates varying widely (2-14 kBT).

Approach:

• Developed an explicit ion model to improve residue-level coarse-grained modeling accuracy across diverse ionic concentrations.
• Enabled de novo predictions of chromatin organization and efficient large-scale conformational sampling for free energy calculations.
• Validated model energetics against protein-DNA binding and nucleosomal DNA unwinding experiments.

Key Points:

• The model accurately reproduces experimental data, including the differential effects of monovalent and divalent ions on chromatin.
• Reconciles disparate experimental estimations of nucleosomal interactions, explaining previous discrepancies.
• Predicts a physiological nucleosome-nucleosome binding strength of 9 kBT, sensitive to DNA linker length and linker histones.

Conclusions:

• Physicochemical interactions play a crucial role in chromatin aggregate phase behavior and nuclear organization.
• The enhanced modeling approach provides a unified framework for understanding chromatin interactions and organization.