Conformation-specific spectroscopy of capped, gas-phase Aib oligomers: tests of the Aib residue as a 310-helix former. Issue 36 (2nd September 2016)

• Record Type: Journal Article
• Authors: Gord, Joseph R.; Hewett, Daniel M.; Hernandez-Castillo, Alicia O.; Blodgett, Karl N.; Rotondaro, Matthew C.; Varuolo, Adalgisa; Kubasik, Matthew A.; Zwier, Timothy S.
• Is Part Of: Physical chemistry chemical physics. Volume 18:Issue 36(2016)
• Page Start: 25512
• Page End: 25527
• Publication Date: 2016-09-02
• Subjects: Chemistry, Physical and theoretical -- Periodicals; 541.3
• DOI: 10.1039/c6cp04909e
• Languages: English
• ISSNs: 1463-9076

Abstract

Abstract : Single-conformation spectroscopy is used to probe the preference for helical structural in Aib-homopeptides. Abstract : The conformational preferences of a series of capped peptides containing the helicogenic amino acid aminoisobutyric acid (Aib) (Z-Aib-OH, Z-(Aib)2 -OMe, and Z-(Aib)4 -OMe) are studied in the gas phase under expansion-cooled conditions. Aib oligomers are known to form 310 -helical secondary structures in solution and in the solid phase. However, in the gas phase, accumulation of a macrodipole as the helix grows could inhibit helix stabilization. Implementing single-conformation IR spectroscopy in the NH stretch region, Z-Aib-OH and Z-(Aib)2 -OMe are both observed to have minor conformations that exhibit dihedral angles consistent with the 310 -helical portion of the Ramachandran map ( ϕ, ψ = −57°, −30°), even though they lack sufficient backbone length to form 10-membered rings which are a hallmark of the developed 310 -helix. For Z-(Aib)4 -OMe three conformers are observed in the gas phase. Single-conformation infrared spectroscopy in both the NH stretch (Amide A) and CO stretch (Amide I) regions identifies the main conformer as an incipient 310 -helix, having two free NH groups and two C10 H-bonded NH groups, labeled an F-F-10-10 structure, with a calculated dipole moment of 13.7 D. A second minor conformer has an infrared spectrum characteristic of an F-F-10-7 structure in which the third and fourth Aib residues have ϕ, ψ = 75°, −74° and −52°, 143°, Ramachandran angles which fall outside of the typical range for 310 -helices, and a dipole moment that shrinks to 5.4 D. These results show Aib to be a 310 -helix former in the gas phase at the earliest stages of oligomer growth.