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The rise of quantum information science has spurred chemists to prepare new molecules that serve as useful building blocks in quantum technologies of the future. Implementation of molecular spin-based qubits requires new methods to induce high spin polarization of samples. Herein, we report design criteria to develop axially symmetric spin-1/2 molecules amenable to optically induced magnetization (OIM), a technique using circularly polarized (CP) excitation to deliver spin polarization. We apply these criteria to develop a series of tungsten(V) chalcogenide complexes that are demonstrated to have large spin-sensitive responses to CP light using magnetic circular dichroism (MCD) that could allow up to ∼20% spin polarization through OIM. Pulsed electron paramagnetic resonance (EPR) spectra reveal these systems have improved relaxation times over molecules like K2IrCl6, a species recently investigated by OIM, and field-swept electron spin−echo (FS-ESE) experiments show they have a remarkable lack of anisotropy in their phase-memory Tm times. The design criteria are general and point toward future ways to improve OIMinitializable qubits. 

Measurement of the conformation of DNA in protein-DNA complexes is important to decipher the role of DNA conformation and dynamics in protein recognition and function. In this work we report a rigid nucleotide-independent spin label that places paramagnetic Cu(II) within the DNA helix. The labeling strategy exploits the chelation of Cu(II) to two 8-aminoquinoline moieties, one in each strand. Because the rigidity of the probe avoids potentially confounding motions of the label itself, EPR signals can resolve 2-3 Å changes in interspin distance; the breadth of the distance distribution reports dy-namic fluctuations from the most probable conformation. Continuous wave and pulsed electron paramagnetic resonance spectroscopy (EPR) show that the Cu(II) coordinates properly to the labeling sites. Measurements of the interspin dipolar interaction on DNA oligonucleotides with two labels placed at various distances demonstrate that the label provides accurate and narrow distance distributions sensitive to DNA conformation and flexibility. Molecular dynamics simulations support these interpretations. We utilize this label to measure the conformations of DNA when type II restriction endonuclease EcoRV binds to its specific recognition sequence. The results provide in-solution evidence that EcoRV endonuclease induces axial DNA bending in the absence of metal ions, contrary to long-standing belief. Furthermore, the distance distribution nar-rows upon protein binding and even further on subsequent metal binding, implying that bound protein constrains the bend-ing dynamics of DNA. This method provides a novel and accurate approach to assess DNA conformation and dynamics in solution. 

In‐cell measurements of the relationship between structure and dynamics to protein function is at the forefront of biophysics. Recently, developments in EPR methodology have demonstrated the sensitivity and power of this method to measure structural constraints in‐cell. However, the need to spin label proteins ex‐situ or use noncanonical amino acids to achieve endogenous labeling remains a bottleneck. In this work we expand the methodology to endogenously spin label proteins with Cu(II) spin labels and describe how to assess in‐cell spin labeling. We quantify the amount of Cu(II)‐NTA in cells, assess spin labeling, and account for orientational effects during distance measurements. We compare the efficacy of using heat‐shock and hypotonic swelling to deliver spin label, showing that hypotonic swelling is a facile and reproducible method to efficiently deliver Cu(II)‐NTA into E. coli. Notably, over six repeats we accomplish a bulk average of 57 μM spin labeled sites, surpassing existing endogenous labeling methods. The results of this work open the door for endogenous spin labeling that is easily accessible to the broader biophysical community. 

Abstract Electron paramagnetic resonance (EPR) has become a powerful probe of conformational heterogeneity and dynamics of biomolecules. In this Review, we discuss different computational modeling techniques that enrich the interpretation of EPR measurements of dynamics or distance restraints. A variety of spin labels are surveyed to provide a background for the discussion of modeling tools. Molecular dynamics (MD) simulations of models containing spin labels provide dynamical properties of biomolecules and their labels. These simulations can be used to predict EPR spectra, sample stable conformations and sample rotameric preferences of label sidechains. For molecular motions longer than milliseconds, enhanced sampling strategies and de novo prediction software incorporating or validated by EPR measurements are able to efficiently refine or predict protein conformations, respectively. To sample large‐amplitude conformational transition, a coarse‐grained or an atomistic weighted ensemble (WE) strategy can be guided with EPR insights. Looking forward, we anticipate an integrative strategy for efficient sampling of alternate conformations by de novo predictions, followed by validations by systematic EPR measurements and MD simulations. Continuous pathways between alternate states can be further sampled by WE‐MD including all intermediate states.