Proton-coupled electron transfer (PCET) from tyrosine produces a neutral tyrosyl radical (Y•) that is vital to many catalytic redox reactions. To better understand how the protein environment influences the PCET properties of tyrosine, we have studied the radical formation behavior of Y32 in the α3Y model protein. The previously solved α3Y solution NMR structure shows that Y32 is sequestered ∼7.7 ± 0.3 Å below the protein surface without any primary proton acceptors nearby. Here we present transient absorption kinetic data and molecular dynamics (MD) simulations to resolve the PCET mechanism associated with Y32 oxidation. Y32 • was generated in a bimolecular reaction with [Ru­(bpy)3]3+ formed by flash photolysis. At pH > 8, the rate constant of Y32 • formation (k PCET) increases by one order of magnitude per pH unit, corresponding to a proton-first mechanism via tyrosinate (PTET). At lower pH < 7.5, the pH dependence is weak and shows a previously measured KIE ≈ 2.5, which best fits a concerted mechanism. k PCET is independent of phosphate buffer concentration at pH 6.5. This provides clear evidence that phosphate buffer is not the primary proton acceptor. MD simulations show that one to two water molecules can enter the hydrophobic cavity of α3Y and hydrogen bond to Y32, as well as the possibility of hydrogen-bonding interactions between Y32 and E13, through structural fluctuations that reorient surrounding side chains. Our results illustrate how protein conformational motions can influence the redox reactivity of a tyrosine residue and how PCET mechanisms can be tuned by changing the pH even when the PCET occurs within the interior of a protein.

Concerted electron-proton transfer (CEPT) reactions avoid charged intermediates and may be energetically favorable for redox and radical-transfer reactions in natural and synthetic systems. Tryptophan (W) often partakes in radical-transfer chains in nature but has been proposed to only undergo sequential electron transfer followed by proton transfer when water is the primary proton acceptor. Nevertheless, our group has shown that oxidation of freely solvated tyrosine and W often exhibit weakly pH-dependent proton-coupled electron transfer (PCET) rate constants with moderate kinetic isotope effects (KIE ≈ 2–5), which could be associated with a CEPT mechanism. These results and conclusions have been questioned. Here, we present PCET rate constants for W derivatives with oxidized Ru- and Zn-porphyrin photosensitizers, extracted from laser flash-quench studies. Alternative quenching/photo-oxidation methods were used to avoid complications of previous studies, and both the amine and carboxylic acid groups of W were protected to make the indole the only deprotonable group. With a suitably tuned oxidant strength, we found an ET-limited reaction at pH < 4 and weakly pH-dependent rates at pH > ∼5 that are intrinsic to the PCET of the indole group with water (H 2 O) as the proton acceptor. The observed rate constants are up to more than 100 times higher than those measured for initial electron transfer, excluding the electron-first mechanism. Instead, the reaction can be attributed to CEPT. These conclusions are important for our view of CEPT in water and of PCET-mediated radical reactions with solvent-exposed tryptophan in natural systems.

A first-year undergraduate course at Uppsala University has been redesigned in a process exploring different levels of student participation. In the first part of the project, the student voice was heard through interviews focusing on the role of the course in the degree program. In the second part, a student-teacher team was formed to develop course curriculum and teaching material in partnership. Among the implemented changes were new seminars focusing on conceptual understanding, redesign of all lectures to include active student participation, and a change of the course literature. The redesigned course significantly increased student satisfaction compared to previous years. Important success factors were involvement of the student organization to promote the project, institutional support, early selection of concrete development tasks, and allowing team members to choose what they wanted to develop according to their own expertise. Keywords Student engagement, Course curriculum, Student voice, Students as partners Creative Commons LicenseThis work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License. Cover Page FootnoteWe thank Maja Elmgren for continuous support and helpful comments on the manuscript, and Felix Ho for auditing a project meeting and gathering reflections from project members. We would also like to thank Johan Wickström, Ulrike Schnaas, Susanna Barrineau and Alexis Bergström for helpful discussions. Financial support was obtained from a TUFF pedagogical development grant from the Disciplinary Domain of Science and Technology, Uppsala University. A first-year undergraduate course at Uppsala University has been redesigned in a process exploring different levels of student participation. In the first part of the project, the student voice was heard through interviews focusing on the role of the course in the degree program. In the second part, a student-teacher team was formed to develop course curriculum and teaching material in partnership. Among the implemented changes were new seminars focusing on conceptual understanding, redesign of all lectures to include active student participation, and a change of the course literature. The redesigned course significantly increased student satisfaction compared to previous years. Important success factors were involvement of the student organization to promote the project, institutional support, early selection of concrete development tasks, and allowing team members to choose what they wanted to develop according to their own expertise.