Investigating the Secondary Structure of Membrane Peptides Utilizing Multiple ²H‑Labeled Hydrophobic Amino Acids via Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy

An electron spin echo envelope modulation (ESEEM) approach was used to probe local secondary structures of membrane proteins and peptides. This ESEEM method detects dipolar couplings between ²H-labeled nuclei on the side chains of an amino acid (Leu or Val) and a strategically placed nitroxide spin-label in the proximity up to 8 Å. ESEEM spectra patterns for different samples correlate directly to the periodic structural feature of different secondary structures. Since this pattern can be affected by the side chain length and flexibility of the ²H-labeled amino acid used in the experiment, it is important to examine several different hydrophobic amino acids (d₃ Ala, d₈ Val, d₈ Phe) utilizing this ESEEM approach. In this work, a series of ESEEM data were collected on the AChR M2δ membrane peptide to build a reference for the future application of this approach for various biological systems. The results indicate that, despite the relative intensity and signal-to-noise level, all amino acids share a similar ESEEM modulation pattern for α-helical structures. Thus, all commercially available ²H-labeled hydrophobic amino acids can be utilized as probes for the further application of this ESEEM approach. Also, the ESEEM signal intensities increase as the side chain length gets longer or less rigid. In addition, longer side chain amino acids had a larger ²H ESEEM FT peak centered at the ²H Larmor frequency for the i ± 4 sample when compared to the corresponding i ± 3 sample. For shorter side chain amino acids, the ²H ESEEM FT peak intensity ratio between i ± 4 and i ± 3 was not well-defined.