The conformational preferences of a series of capped peptides containing the helicogenic amino acid aminoisobutyric acid (Aib) (Z-Aib-OH, Z-(Aib)-OMe, and Z-(Aib)-OMe) are studied in the gas phase under expansion-cooled conditions. Aib oligomers are known to form 3-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)-OMe are both observed to have minor conformations that exhibit dihedral angles consistent with the 3-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 3-helix. For Z-(Aib)-OMe three conformers are observed in the gas phase. Single-conformation infrared spectroscopy in both the NH stretch (Amide A) and C[double bond, length as m-dash]O stretch (Amide I) regions identifies the main conformer as an incipient 3-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 3-helices, and a dipole moment that shrinks to 5.4 D. These results show Aib to be a 3-helix former in the gas phase at the earliest stages of oligomer growth.

Ultrafast 2DIR reveals rotational relaxation rates, critical slowing effects, and co-existence of free rotor and liquid populations in supercritical fluids.

Role of substrate type, gold or silver, and surface roughness on the parity odd–even effect in n-alkanethiolate (n = 10–16) self-assembled monolayers (SAMs), materials of potential importance to molecular scale electronics, is studied using vibrational sum-frequency generation (SFG) spectroscopy. An inverted odd–even effect is observed for SAMs on Ag substrates relative to SAMs on Au. The metal-specific SFG spectra in the methyl and methylene stretching regions provide a sensitive probe of the in situ cant angle and molecular twist dependence of the interfacial group orientation. Within the precision of these measurements, disorder due to gauche defects is not evident in this spectral analysis. SFG methyl vibrational frequencies and line widths show parity and substrate dependence. Metal substrate roughness, an under-reported experimental parameter, affects the extent of odd–even methyl orientation anisotropy. Parity-dependent methyl orientation effects are seen on Ag beyond the roughness limit established by hydrophobicity data on Au. The SFG analysis predicts an ∼2.8 nm rms roughness limit for odd–even effects on Ag substrates. These SFG results further confirm the role competing pairwise short-range dispersive (∝1/r 6) and longer range polar (∝1/r to 1/r 4) interactions play in controlling the observed odd–even wetting behavior of SAMs on metal substrates as a function of surface roughness.