Anders Frederiksen

Photoactivated intermolecular electron transfer (ET) in cryptochromes proceeds along chains of aromatic residues and creates a spatially separated pair of radical electrons. Ultrafast time-dependent spectroscopy can provide experimental insight into this process and theoretical estimates of charge transfer rates are commonly obtained via Marcus theory. Here, we present a new perspective on the ET in European robin cryptochrome 4a (ErCry4a) that synthesizes insights from real-time ET calculations, ultrafast spectroscopic measurements and analytical derivations. The simulations exemplify that molecular vibrations play an essential role in enabling the ET dynamics, which was further rationalized through analytical derivations. Ultrafast pump-probe spectroscopy provided experimental access to the first 1.5 ns of the ET cascade, where multiple radical pair recombination rates arise due to the dynamic equilibrium along the ET chain. We show that the motions of the protein environment and the ET dynamics are inseparably coupled, violating the timescale separation required for Marcus theory. The presented results highlight that non-equilibrium coupling between electronic and nuclear motion dominates ET kinetics in ErCry4a during the first nanosecond after photo-excitation. The findings exemplify the limits of Marcus theory and refine the interpretation of ultrafast spectroscopic signatures in cryptochromes.

Marine fish migrate long distances up to hundreds or even thousands of kilometers for various reasons that include seasonal dependencies, feeding, or reproduction. The ability to perceive the geomagnetic field, called magnetoreception, is one of the many mechanisms allowing some fish to navigate reliably in the aquatic realm. While it is believed that the photoreceptor protein cryptochrome 4 (Cry4) is the key component for the radical pair-based magnetoreception mechanism in night migratory songbirds, the Cry4 mechanism in fish is still largely unexplored. The present study aims to investigate properties of the fish Cry4 protein in order to understand the potential involvement in a radical pair-based magnetoreception. Specifically, a computationally reconstructed atomistic model of Cry4 from the Atlantic herring (Clupea harengus) was studied employing classical molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) methods to investigate internal electron transfers and the radical pair formation. The QM/MM simulations reveal that electron transfers occur similarly to those found experimentally and computationally in Cry4 from European robin (Erithacus rubecula). It is therefore plausible that the investigated Atlantic herring Cry4 has the physical and chemical properties to form radical pairs that in turn could provide fish with a radical pair-based magnetic field compass sensor.

Migratory birds possess remarkable accuracy in orientation and navigation, which involves various compass systems including the magnetic compass. Identifying the primary magnetosensor remains a fundamental open question. Cryptochromes (Cry) have been shown to be magnetically sensitive, and Cry4a from a migratory songbird seems to show enhanced magnetic sensitivity in vitro compared to Cry4a from resident species. We investigate Cry and their potential involvement in magnetoreception in a phylogenetic framework, integrating molecular evolutionary analyses with protein dynamics modelling. Our analysis is based on 363 bird genomes and identifies different selection regimes in passerines. We show that Cry4a is characterized by strong positive selection and high variability, typical characteristics of sensor proteins. We identify key sites that are likely to have facilitated the evolution of an optimized sensory protein for night-time orientation in songbirds. Additionally, we show that Cry4 was lost in hummingbirds, parrots and Tyranni (Suboscines), and thus identified a gene deletion, which might facilitate testing the function of Cry4a in birds. In contrast, the other avian Cry (Cry1 and Cry2) were highly conserved across all species, indicating basal, non-sensory functions. Our results support a specialization or functional differentiation of Cry4 in songbirds which could be magnetosensation.

Migratory birds possess remarkable accuracy in orientation and navigation, which involves various compass systems including the magnetic compass. Identifying the primary magnetosensor remains a fundamental open question. Cryptochromes (Cry) have been shown to be magnetically sensitive, specifically Cry4 shows enhanced magnetic sensitivity in migratory songbirds compared to resident species. Here, we investigate cryptochromes and their potential involvement in magnetoreception in a phylogenetic framework, integrating molecular evolutionary analyses with protein dynamics modeling. We base our analysis on 363 bird genomes and associate different selection regimes with migratory behaviour. We show that Cry4 is characterized by strong positive selection and high variability, typical characteristics of sensor proteins. We identify key sites that likely facilitated the evolution of a highly optimized sensory protein for night time compass orientation in songbirds and a potential functional shift or specialisation. Additionally, we show that Cry4 was lost in hummingbirds, parrots and Tyranni (Suboscines) and thus identified a natural comparative gene knockout, which can be used to test the function of Cry4 in birds. In contrast, the other two cryptochromes Cry1 and Cry2, were highly conserved in all species, indicating basal, non-sensory functions. Our results strengthen the hypothesised role of Cry4 as sensor protein in (night)-migratory songbirds.

Cryptochromes are a class of light-absorbing proteins that have been shown to be a part of the circadian rhythm of many animals but seem to play a central role for the magnetosensing of migratory birds. Following a documented difference in the sensitivity to an external magnetic field of cryptochrome 4a proteins from migratory and non-migratory birds, a detailed analysis of inter- and intra-protein energetics is called for. The present study relies on classical molecular dynamics simulations of cryptochrome 4a from five avian species to reveal if any of the cryptochromes feature peculiarities in their internal energetics. The five avian cryptochrome 4a proteins from pigeon, European robin, zebra finch, chicken, and Eurasian blackcap are found to be highly similar in respect of their intra-energetic behaviors, while some minor differences between the cryptochromes can be ascribed to the site of specific structural differences. Particular attention has been paid to account for the interaction of the protein with the solvent, and it has been revealed that the solvent could lead to significant stabilization of the chromophore flavin adenine dinucleotide inside of the cryptochrome 4a scaffold.

The primary step in the elusive ability of migratory birds to sense weak Earth-strength magnetic fields is supposedly the light-induced formation of a long-lived, magnetically sensitive radical pair inside a cryptochrome flavoprotein located in the retina of these birds. Blue light absorption by a flavin chromophore triggers a series of sequential electron transfer steps across a tetradic tryptophan chain towards the flavin acceptor. The recent ability to express cryptochrome 4 from the night-migratory European robin (Erithacus rubecula), ErCry4, and to replace the tryptophan residues individually by a redox-inactive phenylalanine offers the prospect of exploring the role of each of the tryptophan residues in the electron transfer chain. Here, we compare ultrafast transient absorption spectroscopy of wild type ErCry4 and four of its mutants having phenylalanine residues in different positions of the chain. In the mutants we observe that each of the first three tryptophan residues in the chain adds a distinct relaxation component (time constants 0.5, 30 and 150 ps) to the transient absorption data. The dynamics in the mutant with a terminal phenylalanine residue are very similar to those in wild type ErCry4, excepted for a reduced concentration of long-lived radical pairs. The experimental results are evaluated and discussed in connection with Marcus-Hopfield theory, providing a complete microscopic insight into the sequential electron transfers across the tryptophan chain. Our results offer a path to studying spin transport and dynamical spin correlations in flavoprotein radical pairs.

The sixth edition of the Physics League Across Numerous Countries for Kick-ass Students (PLANCKS) competition was held in the city of Odense, Denmark. The purpose of PLANCKS is to find the most prominent theoretical physics student team in the world. The competition was part of a three-day programme, featuring scientific as well as social events. The scientific programme featured three symposia all containing talks from prominent physicists, most notably the 2001 Nobel Prize laureate Wolfgang Ketterle of MIT. Both the competition and the symposia will enlighten the students about fields of physics which may not be present at their home university. The competition contained a total of 34 teams from 18 countries including the winning team ‘The Four Vectors’ from Germany, and the runners-ups ‘CV5 Irreducibles’ and ‘Komfur’ from Serbia and Denmark, respectively. This year’s problems featured 10 exciting exercises. Here we present two problems from PLANCKS 2019 and the line of reasoning behind making a PLANCKS problem, which is an opportunity for a scientist to highlight an interesting part of their field to young students.

The mechanisms of cellular growth have attracted scientists’ attention for a long time, leading to recent efforts in establishing cellular growth on specific functionalized substrates. In order to fully understand the supported cellular growth mechanisms, one needs first to comprehend how individual amino acids interact with the substrate material as cells are known to attach to surfaces through specific proteins designed to improve adhesion. In this study, we have considered graphene as a candidate material for support-assisted cellular growth and simulated the interaction of all 20 naturally occurring amino acids deposited on graphene. Investigations utilized classical molecular dynamics (MD) for amino acids in aqueous solution and in vacuo, in tandem with quantum chemical calculations. The MD simulations were carried out for classical and polarizable CHARMM force fields. The simulations performed with the polarizable force field confirmed that adhesion of amino acids to the graphene surface may be significantly enhanced due to the polarization forces, which was further supported by quantum chemical calculations. The performed analysis thus revealed the role of polarization on amino acids’ adhesion to the graphene surface. Graphical