Circularly Polarized Luminescence in Terbium-Lanmodulin: Disentangling the Effects of Primary Coordination Sphere and Chromophore Geometry.

The rational design of highly efficient circularly polarized luminescence (CPL) emitters requires a detailed understanding of the mechanisms governing chiroptical metrics. In particular, the ability to deconvolute the contributions from static and dynamic coupling is critical in the CPL of lanthanide ions, the most successful CPL emitters. In this work, we utilized the lanthanide-binding protein, lanmodulin (Al-LanM), as a robust and tunable scaffold to disentangle these effects. We generated a series of terbium(III)-bound protein variants, featuring single and double tryptophan (W) antennae (W84, W100, W84/W100), along with a variant with one EF-hand inactivated (W84-EF2inact). These mutations enabled the precise reorientation and the evaluation of the sensitizing effect of the chromophore(s) with negligible disturbance to the primary coordination environment of the metal centers. Spectroscopic measurements revealed highly resolved CPL signals with dissymmetry factors (glum) reaching up to 0.144, which is among the highest values reported for any terbium-bound protein to date. Our results demonstrate that the dominance of coupling mechanisms in terbium(III) is transition-specific; whereas the 5D4 → 7F6 and 5D4 → 7F4 transitions are primarily governed by dynamic coupling (position/orientation of the antenna), the 5D4 → 7F5 and 5D4 → 7F3 transitions are primarily governed by static coupling (local coordination geometry). These findings help establish a clear understanding of different CPL generation mechanisms, providing a strategic framework for the engineering of advanced chiroptical materials.