This Letter describes an unprecedentedly large and photoreversible localized surface plasmon resonance (LSPR) wavelength shift caused by photoisomerization of azobenzenes attached to gold nanoprisms that act as nanoantennas. The blue light-induced cis to trans azobenzene conformational change occurs in the solid state and controls the optical properties of the nanoprisms shifting their LSPR peak up to 21 nm toward longer wavelengths. This shift is consistent with the increase in thickness of the local dielectric environment (0.6 nm) surrounding the nanoprism and perhaps a contribution from plasmonic energy transfer between the nanoprism and azobenzenes. The effects of the azobenzene conformational change and its photoreversibility were also probed through surface-enhanced Raman spectroscopy (SERS) showing that the electronic interaction between the nanoprisms and bound azobenzenes in their cis conformation significantly enhances the intensity of the Raman bands of the azobenzenes. The SERS data suggests that the isomerization is controlled by first-order kinetics with a rate constant of 1.0 × 10(-4) s(-1). Our demonstration of light-induced photoreversibility of this type of molecular machine is the first-step toward removing present limitations on detection of molecular motion in solid-state devices using LSPR spectroscopy with nanoprisms. Modulating the LSPR peak position and controlling energy transfer across the nanostructure-organic molecule interface are very important for the fabrication of plasmonic-based nanoscale devices.

The infrared and ultraviolet spectra of a series of capped asparagine-containing peptides, Ac-Asn-NHBn, Ac-Ala-Asn-NHBn, and Ac-Asn-Asn-NHBn, have been recorded under jet-cooled conditions in the gas phase in order to probe the influence of the Asn residue, with its −CH 2 −C(O)−NH 2 side chain, on the local conformational preferences of a peptide backbone. The double-resonance methods of resonant ion-dip infrared (RIDIR) spectroscopy and infrared−ultraviolet holeburning (IR−UV HB) spectroscopy were used to record singleconformation spectra in the infrared and ultraviolet, respectively, free from interference from other conformations present in the molecular beam. Ac-Asn-NHBn spreads its population over two conformations, both of which are stabilized by a pair of Hbonds that form a bridge between the Asn carboxamide group and the NH and CO groups on the peptide backbone. In one the peptide backbone engages in a 7-membered H-bonded ring (labeled C7 eq ), thereby forming an inverse γ-turn, stabilized by a C6/C7 Asn bridge. In the other the Asn carboxamide group forms a C8/C7 H-bonded bridge with the carboxamide group facing in the opposite direction across an extended peptide backbone involving a C5 interaction. Both Ac-Ala-Asn-NHBn and Ac-Asn-Asn-NHBn are found exclusively in a single conformation in which the peptide backbone engages in a type I β-turn with its C10 H-bond. The Asn residue(s) stabilize this β-turn via C6 H-bond(s) between the carboxamide CO group and the same residue's amide NH. These structures are closely analogous to the corresponding structures in Gln-containing peptides studied previously [

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.

Gas-phase single-conformation spectroscopy is used to study Ac-Gln-Gln-NHBn in order to probe the interplay between sidechain hydrogen bonding and backbone conformational preferences. This small, amide-rich peptide offers many possibilities for backbone-backbone, sidechain-backbone, and sidechain-sidechain interactions. The major conformer observed experimentally features a type-I β-turn with a canonical 10-membered ring C=O-H-N hydrogen bond between backbone amide groups. In addition, the C=O group of each Gln sidechain participates in a seven-membered ring hydrogen bond with the backbone NH of the same residue. Thus, sidechain hydrogen-bonding potential is satisfied in a manner that is consistent with and stabilizes the β-turn secondary structure. This turn-forming propensity may be relevant to pathogenic amyloid formation by polyglutamine segments in human proteins.

Hydrogen atom dislocation in S₁ methyl anthranilate is characterized with infrared spectroscopy, and a novel explanation for the missing S₁ NH stretch fundamental is presented.