Site-Specific Photochemistry along a Protonated Peptide Scaffold.

We present a detailed study of the time-dependent photophysics and photochemistry of a known conformation of the two protonated pentapeptides Leu-enkephalin (Tyrosine-Glycine-Glycine-Phenylalanine-Leucine, YGGFL) and its chromophore-swapped analogue FGGYL, carried out under cryo-cooled conditions in the gas phase. Using ultraviolet-infrared (UV-IR) double resonance, we record excited state IR spectra as a function of time delay between UV and IR pulses. We identify unique Tyr OH stretch transitions due to the S state and the vibrationally excited triplet state(s) formed by intersystem crossing, T(v).

Capturing CO in Quadrupolar Binding Pockets: Broadband Microwave Spectroscopy of Pyrimidine-(CO), = 1,2.

Pyrimidine has two in-plane CH(δ+)/(δ-)/CH(δ+) binding sites that are complementary to the (δ-/2δ+/δ-) quadrupole moment of CO. We recorded broadband microwave spectra over the 7.5-17.

Molecular Cage Reports on Its Contents: Spectroscopic Signatures of Cryo-Cooled K- and Ba-Benzocryptand Complexes.

UV photofragment spectroscopy and IR-UV double resonance methods are used to determine the structure and spectroscopic responses of a three-dimensional [2.2.2]-benzocryptand cage to the incorporation of a single K or Ba imbedded inside it (labeled as K-BzCrypt, Ba-BzCrypt).

Jet-Cooled Phosphorescence Excitation Spectrum of the T(n,π*) ← S Transition of 4-Pyran-4-one.

The 4-pyran-4-one (4PN) molecule is a cyclic conjugated enone with spectroscopically accessible singlet and triplet (n,π*)excited states. Vibronic spectra of 4PN provide a stringent test of electronic-structure calculations, through comparison of predicted vs measured vibrational frequencies in the excited state. We report here the T(n,π*) ← S phosphorescence excitation spectrum of 4PN, recorded under the cooling conditions of a supersonic free-jet expansion.

Control of the Structural Charge Distribution and Hydration State upon Intercalation of CO into Expansive Clay Interlayers.

Numerous experimental investigations indicated that expansive clays such as montmorillonite can intercalate CO preferentially into their interlayers and therefore potentially act as a material for CO separation, capture, and storage. However, an understanding of the energy-structure relationship during the intercalation of CO into clay interlayers remains elusive. Here, we use metadynamics molecular dynamics simulations to elucidate the energy landscape associated with CO intercalation.

Correction: Single-conformation spectroscopy of cold, protonated PG-containing peptides: switching β-turn types and formation of a sequential type II/II' double β-turn.

Correction for 'Single-conformation spectroscopy of cold, protonated PG-containing peptides: switching β-turn types and formation of a sequential type II/II' double β-turn' by John T. Lawler , , 2022, , 2095-2109, https://doi.org/10.

Trapping Ca inside a molecular cavity: computational study of the potential energy surfaces for Ca-[]cycloparaphenylene, = 5-12.

Ion trap quantum computing utilizes electronic states of atomic ions such as Ca to encode information on to a qubit. To explore the fundamental properties of Ca inside molecular cavities, we describe here a computational study of Ca bound inside neutral []-cycloparaphenylenes ( = 5-12), often referred to as "nanohoops". This study characterizes optimized structures, harmonic vibrational frequencies, potential energy surfaces, and ion molecular orbital distortion as functions of increasing nanohoop size.

Conformer-Specific Spectroscopy and IR-Induced Isomerization of a Model γ-Peptide: Ac-γ-Phe-NHMe.

Single-conformation IR and UV spectroscopy of the prototypical capped γ-peptide Ac-γ-Phe-NHMe (γF) was carried out under jet-cooled conditions in the gas phase in order to understand its innate conformational preferences in the absence of a solvent. We obtained conformer-specific IR and UV spectra and compared the results with calculations to make assignments and explore the differences between the γ- and γ-substituted molecules. We found four conformers of γF in our experiment.

Bond Length Alternation and Internal Dynamics in Model Aromatic Substituents of Lignin.

Broadband microwave spectra were recorded over the 2-18 GHz frequency range for a series of four model aromatic components of lignin; namely, guaiacol (ortho-methoxy phenol, G), syringol (2,6-dimethoxy phenol, S), 4-methyl guaiacol (MG), and 4-vinyl guaiacol (VG), under jet-cooled conditions in the gas phase. Using a combination of C isotopic data and electronic structure calculations, distortions of the phenyl ring by the substituents on the ring are identified. In all four molecules, the r bond between the two substituted C-atoms lengthens, leading to clear bond alternation that reflects an increase in the phenyl ring resonance structure with double bonds at r , r and r .

Single-conformation spectroscopy of cold, protonated PG-containing peptides: switching β-turn types and formation of a sequential type II/II' double β-turn.

D-Proline (Pro, P) is widely utilized to form β-hairpin loops in engineered peptides that would otherwise be unstructured, most often as part of a PG sub-unit that forms a β-turn. To observe whether PG facilitated this effect in short protonated peptides, conformation specific IR-UV double resonance photofragment spectra of the cold (∼10 K) protonated P and P diastereomers of the pentapeptide YAPGA was carried out in the hydride stretch (2800-3700 cm) and amide I/II (1400-1800 cm) regions. A model localized Hamiltonian was developed to better describe the 1600-1800 cm region commonly associated with the amide I vibrations.

Two-Color IRMPD Applied to Conformationally Complex Ions: Probing Cold Ion Structure and Hot Ion Unfolding.

Two-color infrared multiphoton dissociation (2C-IRMPD) spectroscopy is a technique that mitigates spectral distortions due to nonlinear absorption that is inherent to one-color IRMPD. We use a 2C-IRMPD scheme that incorporates two independently tunable IR sources, providing considerable control over the internal energy content and type of spectrum obtained by varying the trap temperature, the time delays and fluences of the two infrared lasers, and whether the first or second laser wavelength is scanned. In this work, we describe the application of this variant of 2C-IRMPD to conformationally complex peptide ions.

Spectroscopic Manifestations of Indirect Vibrational State Mixing: Novel Anharmonic Effects on a Prereactive H Atom Transfer Surface.

The NH stretch region of the IR spectrum of methyl anthranilate is modeled in the S state to understand the connection between the absence of this fundamental in the fluorescence-dip infrared spectra of Blodgett et al. [ , , 14077] and its relevance to the H atom dislocation that occurs upon electronic excitation. A set of coordinates are chosen that highlight the role of certain low-frequency modes.

Gas-phase pyrolysis of trans 3-pentenenitrile: competition between direct and isomerization-mediated dissociation.

The flash pyrolysis of trans 3-pentenenitrile (3-PN, CH-CH[double bond, length as m-dash]CH-CH-CN) was studied by combining the results of VUV photoionization mass spectra with broadband microwave spectra recorded as a function of the temperature of the pyrolysis tube. The two separated functional groups (vinyl and nitrile) open up isomerization as an initial step in competition with unimolecular dissociation. Primary products were detected by keeping the 3-PN concentration low and limiting reaction times to the traversal time of the gas in the pyrolysis tube (∼100 μs).

Insights into the photoprotection mechanism of the UV filter homosalate.

Homosalate (HMS) is a salicylate molecule that is commonly included within commercial sunscreen formulations to provide protection from the adverse effects of ultraviolet (UV) radiation exposure. In the present work, the mechanisms by which HMS provides UV photoprotection are unravelled, using a multi-pronged approach involving a combination of time-resolved ultrafast laser spectroscopy in the gas-phase and in solution, laser-induced fluorescence, steady-state absorption spectroscopy, and computational methods. The unique combination of these techniques allow us to show that the enol tautomer of HMS undergoes ultrafast excited state intramolecular proton transfer (ESIPT) upon photoexcitation in the UVB (290-320 nm) region; once in the keto tautomer, the excess energy is predominantly dissipated non-radiatively.

The missing NH stretch fundamental in S methyl anthranilate: IR-UV double resonance experiments and local mode theory.

The infrared spectra of jet-cooled methyl anthranilate (MA) and the MA-HO complex are reported in both S and S states, recorded using fluorescence-dip infrared (FDIR) spectroscopy under jet-cooled conditions. Using a combination of local mode CH stretch modeling and scaled harmonic vibrational character, a near-complete assignment of the infrared spectra is possible over the 1400-3700 cm region. While the NH stretch fundamentals are easily observed in the S spectrum, in the S state, the hydrogen bonded NH stretch shift is not readily apparent.

Coexistence of Left- and Right-Handed 12/10-Mixed Helices in Cyclically Constrained β-Peptides and Directed Formation of Single-Handed Helices upon Site-Specific Methylation.

The inherent conformational preferences of the neutral β-peptide foldamer series, Ac-(ACHC)-NHBn, = 2-4, are studied in the gas phase using conformation-specific IR-UV double resonance methods. The cyclically constrained chiral β-amino acid -2-aminocyclohexane carboxylic acid (ACHC) is designed to bring both right- and left-handed helices into close energetic proximity. Comparison of the infrared spectra in the NH stretch and amide I/II regions with the predictions of DFT calculations lead to the unambiguous assignment of four out of the six observed conformations of the molecules in this series, while corroborating computational and spectral evidence, affords tentative assignments of the remaining two conformers for which IR data were not recorded.

Local and global approaches to treat the torsional barriers of 4-methylacetophenone using microwave spectroscopy.

The Fourier transform microwave spectrum of 4-methylacetophenone recorded from 8 GHz to 18 GHz under jet-cooled conditions has revealed large tunneling splittings arising from a low barrier to internal rotation of the ring methyl group and small splittings from a high torsional barrier of the acetyl methyl group. The large splittings are especially challenging to model, while the small splittings are difficult to analyze due to the resolution limit of 120 kHz. The combination of two methyl groups undergoing internal rotations caused each rotational transition to split into five torsional species, which were resolved and fitted using a modified version of the XIAM code and the newly developed ntop code to a root-mean-square deviation close to measurement accuracy, providing an estimate of the V potential barriers of about 22 cm and 584-588 cm for the ring and the acetyl methyl groups, respectively.

The unusual symmetry of hexafluoro-o-xylene-A microwave spectroscopy and computational study.

The rotational constants and quartic centrifugal distortion coefficients of hexafluoro-o-xylene were precisely derived from the 8 GHz to 18 GHz gas phase microwave spectrum. In addition, the rotational constants of all singly substituted C isotopologues were determined. Instead of the intuitively expected symmetry of C, as in o-xylene, calculations with a variety of methods (B3LYP, CAM-B3LYP, ωB97XD, MP2, and coupled-cluster singles, doubles, and perturbative triples) predict a C symmetry structure in which the two CF groups rotate in opposite directions by about 16°.

Broadband rotational spectroscopy of trans 3-pentenenitrile and 4-pentenenitrile.

Titan, a moon of Saturn, has a nitrogen- and methane-rich atmosphere that is similar to prebiotic earth, and is replete with organic nitriles. Pentenenitriles have not yet been detected in Titan's atmosphere or in molecular clouds, but are potential precursors to hetero-aromatic compounds such as pyridine. We performed broadband microwave studies in the 8-18 GHz range on the trans isomer of 3-pentenenitrile (3-PN) and 4-pentenenitrile (4-PN) under jet-cooled conditions.

Vibronic spectroscopy of methyl anthranilate and its water complex: hydrogen atom dislocation in the excited state.

Laser-induced fluorescence (LIF) excitation, dispersed fluorescence (DFL), UV-UV-hole burning, and UV-depletion spectra have been collected on methyl anthranilate (MA, methyl 2-aminobenzoate) and its water-containing complex (MA-HO), under jet-cooled conditions in the gas phase. As a close structural analog of a sunscreen agent, MA has a strong absorption due to the S-S transition that begins in the UV-A region, with the electronic origin at 28 852 cm (346.6 nm).

The effects of site asymmetry on near-degenerate state-to-state vibronic mixing in flexible bichromophores.

Laser-induced fluorescence excitation and dispersed fluorescence spectra of a model flexible bichromophore, 1,1-diphenylethane (DPE), have been recorded under jet-cooled conditions in the gas phase in the region near the first pair of near-degenerate excited states (S and S). The S and S origin transitions have been identified at 37 397 and 37 510 cm, a splitting of 113 cm. This splitting is four times smaller than the excitonic splitting calculated by ab initio methods at the EOM-CCSD/cc-pVDZ level of theory (410 cm), which necessarily relies on the Born-Oppenheimer approximation.

Propagating molecular rotational coherences through single-frequency pulses in the strong field regime.

In the weak-field limit in which microwave spectroscopy is typically carried out, an application of a single-frequency pulse that is resonant with a molecular transition will create a coherence between the pair of states involved in the rotational transition, producing a free-induction decay (FID) that, after Fourier transform, produces a molecular signal at that same resonance frequency. With the advent of chirped-pulse Fourier transform microwave methods, the high-powered amplifiers needed to produce broadband microwave spectra also open up other experiments that probe the molecular response in the high-field regime. This paper describes a series of experiments involving resonant frequency pulses interrogating jet-cooled molecules under conditions of sufficient power to Rabi oscillate the two-state system through many Rabi cycles.

Strong-field coherence breaking as a tool for identifying methyl rotor states in microwave spectra: 2-hexanone.

The rotational spectrum of 2-hexanone was recorded over the 8-18 GHz region using a chirped pulse Fourier transform microwave spectrometer. Strong field coherence breaking (SFCB) was utilized to selectively modulate the intensities of rotational transitions belonging to the two lowest energy conformers of 2-hexanone, aiding the assignment. In addition, the SFCB method was applied for the first time to selectively identify rotational transitions built off the two lowest energy hindered methyl rotor states of each conformer, 0a and 1e.