Excited state structural dynamics of a dual-emission calmodulingreen fluorescent protein sensor for calcium ion imaging

This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by National Academy of Sciences of the United States of America and can be found at: http://www.pnas.org/.This article contains supporting information online.Fluorescent proteins (FPs) have played a pivotal role in bioimaging and advancing biomedicine.\ud The versatile fluorescence from engineered, genetically encodable FP variants greatly enhances\ud cellular imaging capabilities, which are dictated by excited state structural dynamics of the\ud embedded chromophore inside the protein pocket. Visualization of the molecular choreography\ud of the photoexcited chromophore requires a spectroscopic technique capable of resolving atomic\ud motions on the intrinsic timescale of femtosecond to picosecond. We use femtosecond stimulated\ud Raman spectroscopy to study the excited state conformational dynamics of a recently developed\ud FP-calmodulin sensor, GEM-GECO1, for calcium ion (Ca²⁺) sensing. This study reveals that, in\ud the absence of Ca²⁺, the dominant skeletal motion is a ~170 cm⁻¹ phenol-ring in-plane rocking\ud that facilitates excited-state proton transfer (ESPT) with a time constant of ~30 ps (6 times\ud slower than wild-type GFP) to reach the green fluorescent state. The functional relevance of the\ud motion is corroborated by molecular dynamics simulations. Upon Ca²⁺ binding, this in-plane\ud rocking motion diminishes and blue emission from a trapped photoexcited neutral chromophore\ud dominates because ESPT is inhibited. Fluorescence properties of site-specific protein mutants\ud lend further support to functional roles of key residues including proline 377 in modulating the\ud H-bonding network and fluorescence outcome. These crucial structural dynamics insights will\ud aid rational design in bioengineering to generate versatile, robust, and more sensitive optical\ud sensors to detect Ca²⁺ at physiologically relevant environments