Probing Membrane Hydration at the Interface of Self-Assembled Peptide Amphiphiles Using Electron Paramagnetic Resonance

Abstract

The relative hydrophilicity at the interface of a nanoparticle was measured utilizing electron paramagnetic resonance (EPR) spectroscopy. The supramolecular structure was assembled from spin-labeled peptide amphiphiles (PA) derived from N-carboxy anhydrides (NCA). Cyanuric chloride, or 2,4,6-trichloro-1,3,5-triazine (TCT), was used as a modular platform to synthesize the spin-labeled, lipidmimetic macroinitiator used for the ring-opening polymerization of γ-benzyl-L-glutamic acid NCA to produce polyglutamate-b-dodecanethiol 2 . Through static and dynamic light scattering, as well as transmission electron microscopy, PAs with DP of 50 and 17 were shown to assemble into stable nanoparticles with an average hydrodynamic radius of 117 and 84 nm, respectively. Continuous wave EPR spectroscopy revealed that the mobility parameter (h −1 /h 0 ) and 2A iso of the nitroxide radical increased with increasing pH, in concert with the deprotonation of the PE side chains and associated helix−coil transition. These results are consistent with an increase in the relative hydration and polarity at the nanoparticle interface, which would be dependent on the secondary structure of the polypeptide. This research suggests that a pH stimulus could be used to facilitate water diffusion through the membrane.

A metabolically engineered spin-labeling approach for studying glycans on cells

“…2 and SI-3 †), can be used to compare the mobility among spin-labeled sites. [62][63][64][65][66][67][68] Line shape parameters such as the intensities and widths of each of the three transitions can also be utilized to determine an effective correlation time, as was shown for earlier methods where sialic acid residues were selectively modied with a nitroxide label. 69 For a nitroxide such as 3 dissolved in solvent ( Fig.…”

Self-Assembly of Oligo- and Polypeptide-Based Amphiphiles: Recent Advances and Future Possibilities

“…One key aspect in the future of PAs is the use of molecular probes to determine local structure and properties and how this may translate into control over structure. In a recent study, Smith and co-workers produced PAs containing a poly­(glutamic acid) headgroup (with varying molecular weight,) two lipophilic tails, and a spin-label placed at the interface . They found that these PAs assembled into vesicles, and as the solution pH increased, the spin-label mobility and local polarity at the interface increased.…”

“…In a recent study, Smith and co-workers produced PAs containing a poly(glutamic acid) headgroup (with varying molecular weight,) two lipophilic tails, and a spin-label placed at the interface. 55 They found that these PAs assembled into vesicles, and as the solution pH increased, the spin-label mobility and local polarity at the interface increased. This proof-of-concept study is significant because it shows the ability to probe local structure using small probes (i.e., not large fluorescent tags, so as to not disrupt morphology) using techniques common to structural biology.…”

Mapping protein–polymer conformations in bioconjugates with atomic precision

“… 24 Other isolated coarse-grained experiments probing polymer behavior have been published, such as a 15A resolution Cryo-EM structure 25 of a PEG-bottlebrush-modified virus-like particle (a privileged system with high symmetry), and motional dynamics probed by electron paramagnetic resonance (EPR) on the surface of polymer-modified self-assembling peptide amphiphiles. 26 …”

“…24 Other isolated coarse-grained experiments probing polymer behavior have been published, such as a 15A resolution Cryo-EM structure 25 of a PEG-bottlebrush-modied virus-like particle (a privileged system with high symmetry), and motional dynamics probed by electron paramagnetic resonance (EPR) on the surface of polymer-modied selfassembling peptide amphiphiles. 26 Finally, most structural information has been on PEGconjugates, which constitute a subset of protein-polymer bioconjugates. Thus, experiments that report on conformations and proximities within protein-polymer bioconjugates are greatly needed.…”