Tubulin polymerization dynamics are influenced by magnetic isotope effects consistent with the radical pair mechanism.

Weak magnetic fields have been shown to influence biological processes; however, the underlying mechanisms remain unknown as the energies involved are far below thermal energies challenging classical explanations. Microtubule cytoskeletal fibers offer an ideal system to test weak magnetic field effects due to their self-assembling capabilities, sensitivity to magnetic fields, and their central role in cellular processes. In this study, we use a combination of experiments and simulations to explore how nuclear spin dynamics affect microtubule polymerization by examining interactions between magnesium isotope substitution and weak magnetic fields. Our experiments reveal an isotope-dependent effect explicitly arising from nuclear spin properties. This nuclear spin-driven effect is enhanced under an applied weak 3-millitesla magnetic field. Our theoretical radical pair model achieves quantitative agreement with our experimental observations. These results support a connection between quantum spin dynamics and microtubule assembly, providing further insights into how weak magnetic fields may influence biomolecular functions.