Electron spin resonance of TOAC labeled peptides: Folding transitions and high frequency spectroscopy

• Published in: Biopolymers 2000, Vol.55 (6), p.479-485
• Main Authors: McNulty, Joseph C., Silapie, Jennifer L., Carnevali, Maia, Farrar, Christian T., Griffin, Robert G., Formaggio, Fernando, Crisma, Marco, Toniolo, Claudio, Millhauser, Glenn L.
• Format: Article
• Language: English
• Subjects: Cyclic N-Oxides - chemistry; Cyclic N-Oxides - metabolism; electron spin resonance; Electron Spin Resonance Spectroscopy; folding transitions; Guanidine - chemistry; high frequency spectroscopy; Molecular Structure; Peptides - chemical synthesis; Peptides - chemistry; Protein Conformation; Protein Denaturation; Protein Folding; Spin Labels; Temperature; TOAC labeled peptides
• ISSN: 0006-3525; 1097-0282
• DOI: 10.1002/1097-0282(2000)55:6<479::AID-BIP1023>3.0.CO;2-F

The unnatural, conformationally constrained nitroxide amino acid TOAC (2,2,6,6‐tetramethylpiperidine‐1‐oxyl‐4‐amino‐4‐carboxylic acid) stabilizes helical structure and provides a means for studying rigidly spin labeled peptides by electron spin resonance (ESR). Two new directions in TOAC research are described. The first investigates intermediates formed during α‐helix unfolding. Double TOAC labeled α‐helical peptides were unfolded at low temperature in aqueous solution with increasing concentrations of guanidine hydrochloride. Comparison of ESR spectra from two doubly labeled peptides suggests that 310‐helix emerges as an intermediate. The second research direction involves the use of high frequency ESR (140 GHz) at low temperature to assess dipolar couplings and, hence, distances between TOAC pairs in a series of 310‐helical peptides. Preliminary simulations suggest that high frequency ESR is able to extract correct distances between 6 and 11 Å. In addition, the spectra appear to be very sensitive to the relative orientation of the TOAC labels. © 2001 John Wiley & Sons, Inc. Biopolymers (Pept Sci) 55: 479–485, 2000