Distance distributions of end-labeled curved bispeptide oligomers by electron spin resonance

The world as viewed by and with unpaired electrons

Recent advances in electron paramagnetic resonance (EPR) include capabilities for applications to areas as diverse as archeology, beer shelf life, biological structure, dosimetry, in vivo imaging, molecular magnets, and quantum computing. Enabling technologies include multifrequency continuous wave, pulsed, and rapid scan EPR. Interpretation is enhanced by increasingly powerful computational models.

Evidence for water-tuned structural differences in proteins: an approach emphasizing variations in local hydrophilicity

We present experimental evidence for the significant effect that water can have on the functional structure of proteins in solution. Human (HSA) and Bovine Serum Albumin (BSA) have an amino acid sequence identity of 75.52% and are chosen as model proteins. We employ EPR-based nanoscale distance measurements using double electron-electron resonance (DEER) spectroscopy and both albumins loaded with long chain fatty acids (FAs) in solution to globally (yet indirectly) characterize the tertiary protein structures from the bound ligands’ points of view. The complete primary structures and crystal structures of HSA and as of recently also BSA are available. We complement the picture as we have recently determined the DEER-derived solution structure of HSA and here present the corresponding BSA solution structure. The characteristic asymmetric FA distribution in the crystal structure of HSA can surprisingly be observed by DEER in BSA in solution. This indicates that the BSA conformational ensemble in solution seems to be narrow and close to the crystal structure of HSA. In contrast, for HSA in solution a much more symmetric FA distribution was found. Thus, conformational adaptability and flexibility dominate in the HSA solution structure while BSA seems to lack these properties. We further show that differences in amino acid hydropathies of specific structural regions in both proteins can be used to correlate the observed difference in the global (tertiary) solution structures with the differences on the primary structure level.

Pulsed dipolar spectroscopy distance measurements in biomacromolecules labeled with Gd(III) markers

This work demonstrates the feasibility of using Gd(III) tags for long-range Double Electron Electron Resonance (DEER) distance measurements in biomacromolecules. Double-stranded 14- base pair Gd(III)-DNA conjugates were synthesized and investigated at K(a) band. For the longest Gd(III) tag the average distance and average deviation between Gd(III) ions determined from the DEER time domains was about 59±12Å. This result demonstrates that DEER measurements with Gd(III) tags can be routinely carried out for distances of at least 60Å, and analysis indicates that distance measurements up to 100Å are possible. Compared with commonly used nitroxide labels, Gd(III)-based labels will be most beneficial for the detection of distance variations in large biomacromolecules, with an emphasis on large scale changes in shape or distance. Tracking the folding/unfolding and domain interactions of proteins and the conformational changes in DNA are examples of such applications.

Flexibility of shape-persistent molecular building blocks composed of p-phenylene and ethynylene units

Ethynylene and p-phenylene are frequently employed constitutional units in constructing the backbone of nanoscopic molecules with specific shape and mechanical or electronic function. How well these properties are defined depends on the flexibility of the backbone, which can be characterized via the end-to-end distance distribution. This distribution is accessible by pulse electron paramagnetic resonance (EPR) distance measurements between spin labels that are attached at the backbone. Four sets of oligomers with different sequences of p-phenylene and ethynylene units and different spin labels were prepared using polar tagging as a tool for simple isolation of the targeted compounds. By variation of backbone length, of the sequence of p-phenylene and ethynylene units, and of the spin labels a consistent coarse-grained model for backbone flexibility of oligo(p-phenyleneethynylene)s and oligo(p-phenylenebutadiynylene)s is obtained. The relation of this harmonic segmented chain model to the worm-like chain model for shape-persistent polymers and to atomistic molecular dynamics simulations is discussed. Oligo(p-phenylenebutadiynylene)s are found to be more flexible than oligo(p-phenyleneethynylene)s, but only slightly so. The end-to-end distance distribution measured in a glassy state of the solvent at a temperature of 50 K is found to depend on the glass transition temperature of the solvent. In the range between 91 and 373 K this dependence is in quantitative agreement with expectations for flexibility due to harmonic bending. For the persistence lengths at 298 K our data predict values of (13.8 +/- 1.5) nm for poly(p-phenyleneethynylene)s and of (11.8 +/- 1.5) nm for poly(p-phenylenebutadiynylene)s.

Distance measurements in model bis-Gd(III) complexes with flexible "bridge". Emulation of biological molecules having flexible structure with Gd(III) labels attached

In this work, we continue to explore Gd(III) as a possible spin label for high-field Double Electron-Electron Resonance (DEER) based distance measurements in biological molecules with flexible geometry. For this purpose, a bis-Gd(III) complex with a flexible "bridge" was used as a model. The distances in the model were expected to be distributed in the range of 5-26 A, allowing us to probe the shortest limits of accessible distances which were found to be as small as 13 A. The upper distance limit for these labels was also evaluated and was found to be about 60 A. Various pulse duration setups can result in apparent differences in the distribution function derived from DEER kinetics due to short distance limit variations. The advantages, such as the ability to perform measurements at cryogenic temperatures and high repetition rates simultaneously, the use of very short pumping and observation pulses without mutual interference, the lack of orientational selectivity, as well as the shortcomings, such as the limited mw operational frequency range and intrinsically smaller amplitude of oscillation related to dipolar interaction as compared with nitroxide spin labels are discussed. Most probably the use of nitroxide and Gd-based labels for distance measurements will be complementary depending on the particulars of the problem and the availability of instrumentation.