title: 2D-IR study of a photoswitchable isotope-labeled alpha-helix.
abstract: A series of photoswitchable, alpha-helical peptides were studied using two-dimensional infrared spectroscopy (2D-IR). Single-isotope labeling with (13)C(18)O at various positions in the sequence was employed to spectrally isolate particular backbone positions. We show that a single (13)C(18)O label can give rise to two bands along the diagonal of the 2D-IR spectrum, one of which is from an amide group that is hydrogen-bonded internally, or to a solvent molecule, and the other from a non-hydrogen-bonded amide group. The photoswitch enabled examination of both the folded and unfolded state of the helix. For most sites, unfolding of the peptide caused a shift of intensity from the hydrogen-bonded peak to the non-hydrogen-bonded peak. The relative intensity of the two diagonal peaks gives an indication of the fraction of molecules hydrogen-bonded at a certain location along the sequence. As this fraction varies quite substantially along the helix, we conclude that the helix is not uniformly folded. Furthermore, the shift in hydrogen bonding is much smaller than the change of helicity measured by CD spectroscopy, indicating that non-native hydrogen-bonded or mis-folded loops are formed in the unfolded ensemble.
authors.affiliation: FOM Institute for Atomic and Molecular Physics, Science Park 104, 1098 XG Amsterdam, The Netherlands.
authors.forename: Ellen H G
authors.initials: EH
authors.lastname: Backus
authors.forename: Elin
authors.initials: E
authors.lastname: Bloem
authors.forename: Paul M
authors.initials: PM
authors.lastname: Donaldson
authors.forename: Janne A
authors.initials: JA
authors.lastname: Ihalainen
authors.forename: Rolf
authors.initials: R
authors.lastname: Pfister
authors.forename: Beatrice
authors.initials: B
authors.lastname: Paoli
authors.forename: Amedeo
authors.initials: A
authors.lastname: Caflisch
authors.forename: Peter
authors.initials: P
authors.lastname: Hamm
mesh_terms: D000595:Amino Acid Sequence; D002247:Carbon Isotopes; D002942:Circular Dichroism; D006860:Hydrogen Bonding; D007553:Isotope Labeling; D010103:Oxygen Isotopes; D010455:Peptides / Q000737:chemistry*; D017510:Protein Folding; D017433:Protein Structure, Secondary; D013055:Spectrophotometry, Infrared
chemical_list: D002247:Carbon Isotopes; D010103:Oxygen Isotopes; D010455:Peptides