Fluorescent supramolecular fibers and sheets built from doubly hexa-His tagged mCherry proteins conjugated by divalent metal cations

Doubly His₆-tagged mCherry red fluorescent proteins are observed to form fibers and sheets at neutral pH in the presence of no more than equimolar amounts of Zn²⁺ or Ni²⁺. These architectures, on the order of 10 μm in extent, are detected with scanning transmission electron microscopy imaging. Far ultraviolet circular dichroism spectroscopy attests to the preservation of the native secondary structure of mCherry, while the emission spectrum reveals the maintenance of the chemical environment of the fluorophore site. Two-dimensional, fluorescence microscopy images provide evidence for our assertion that the mechanism underlying protein assembly relies on [metal:chelator] conjugation, i.e., between a His₆-tag and divalent cations: (a) Conjugation is reversible when competing water-soluble chelators (e.g., 5 mM EDTA, histidine or imidazole) are present; (b) Conjugation depends on pH. Below pH 6, when more than 50 % of the imidazole rings in the His₆-tag are protonated, protein conjugation is suppressed. The straightforward chemistry with which our approach can be implemented, combined with its potential generality and non-denaturing properties, suggests that these fluorescent biopolymers may be suitable for enhancing the sensitivity of immunoassays and histology staining studies.