UV–UV hole burning and IR dip spectroscopy of homophenylalanine by laser desorption supersonic jet technique

Neutral Peptides in the Gas Phase: Conformation and Aggregation Issues

Combined IR and UV laser spectroscopic techniques in molecular beams merged with theoretical approaches have proven to be an ideal tool to elucidate intrinsic structural properties on a molecular level. It offers the possibility to analyze structural changes, in a controlled molecular environment, when successively adding aggregation partners. By this, it further makes these techniques a valuable starting point for a bottom-up approach in understanding the forces shaping larger molecular systems. This bottom-up approach was successfully applied to neutral amino acids starting around the 1990s. Ever since, experimental and theoretical methods developed further, and investigations could be extended to larger peptide systems. Against this background, the review gives an introduction to secondary structures and experimental methods as well as a summary on theoretical approaches. Vibrational frequencies being characteristic probes of molecular structure and interactions are especially addressed. Archetypal biologically relevant secondary structures investigated by molecular beam spectroscopy are described, and the influences of specific peptide residues on conformational preferences as well as the competition between secondary structures are discussed. Important influences like microsolvation or aggregation behavior are presented. Beyond the linear α-peptides, the main results of structural analysis on cyclic systems as well as on β- and γ-peptides are summarized. Overall, this contribution addresses current aspects of molecular beam spectroscopy on peptides and related species and provides molecular level insights into manifold issues of chemical and biochemical relevance.

Induced Circular Dichroism of Jet-Cooled Phenol Complexes with (R)-(-)-2-Butanol

The induced circular dichroism (ICD) of phenol complexed with (R)-(-)-2-butanol [PhOH-(-)BOH] in a supersonic jet is investigated using resonant two-photon ionization circular dichroism (R2PICD) spectroscopy. The R2PICD spectrum of PhOH-(-)BOH exhibits nonzero ICD bands near the absorption region of bare PhOH, where (-)BOH is transparent. Two different conformers containing a single hydrogen bond between PhOH and (-)BOH are identified using ultraviolet-ultraviolet hole-burning and infrared ion-dip spectroscopy combined with quantum theoretical calculations. The ICD values of the two conformers are similar to each other. To understand these similar ICD effects of the conformers, the geometrical asymmetry of the PhOH moiety bound to (-)BOH and the coupling strength of the electric transition dipole moments between PhOH and (-)BOH are estimated. Comparing the ICD values of PhOH-(-)BOH with those of PhOH-(-)-l-methyl lactate in the previous report [ Hong , A. ; J. Phys. Chem. Lett. 2018 , 9 , 476 -480 ], we investigate the physical properties that may govern the differences of the ICD values between the two complexes.

Probing the selectivity of Li+ and Na+ cations on noradrenaline at the molecular level

Although several mechanisms concerning the biological function of lithium salts, drugs having tranquilizing abilities, have been proposed so far, the key mechanism for its selectivity and subsequent interaction with neurotransmitters has not been established yet. We report ultraviolet (UV) and infrared (IR) spectra under ultra-cold conditions of Li+ and Na+ complexes of noradrenaline (NAd, norepinephrine), a neurotransmitter responsible for the body's response to stress or danger, in an effort to provide a molecular level understanding of the conformational changes of NAd due to its interactions with these two cations. A detailed analysis of the IR spectra, aided by quantum chemical calculations, reveals that the Li+-noradrenaline (NAd-Li+) complex forms only an extended structure, while the NAd-Na+ and protonated (NAd-H+) complexes form both folded and extended structures. This conformational preference of the NAd-Li+ complex is further explained by considering specific conformational distributions in solution. Our results clearly discern the unique structural motifs that NAd adopts when interacting with Li+ compared with other abundant cations in the human body (Na+) and can form the basis of a molecular level understanding of the selectivity of Li+ in biological systems.

Peptide Bond Ultraviolet Absorption Enables Vibrational Cold-Ion Spectroscopy of Nonaromatic Peptides

Peptide-bond VUV absorption is inherent to all proteins and peptides. Although widely exploited in top-down proteomics for photodissociation, this absorption has never been spectroscopically characterized in the gas phase. We have measured VUV/UV photofragmentation spectrum of a single peptide bond in a cryogenically cold protonated dipeptide. Although the spectrum appears to be very broadband and structureless, vibrational pre-excitation of this and even larger cold peptides significantly increases the UV dissociation yield for some of their photofragments. We use this effect to extend the technique of IR-UV photofragmentation vibrational spectroscopy, developed for aromatic peptides, to nonaromatic ones and demonstrate measurements of conformation-specific and nonspecific IR spectra for di- to hexa-peptides.

Isomer-Specific Induced Circular Dichroism Spectroscopy of Jet-Cooled Phenol Complexes with (-)-Methyl l-Lactate

Induced circular dichroism (ICD) is the CD observed in the absorption of an achiral molecule bound to a transparent chiral molecule through noncovalent interactions. ICD spectroscopy has been used to probe the binding between molecules, such as protein-ligand interactions. However, most ICD spectra have been measured in solution, which only exhibit the averaged CD values of all conformational isomers in solution. Here, we obtained the first isomer-selective ICD spectra by applying resonant two-photon ionization CD spectroscopy to jet-cooled phenol complexes with (-)-methyl l-lactate (PhOH-(-)ML). The well-resolved CD bands in the spectra were assigned to two conformers, which contained different types of hydrogen-bonding interactions between PhOH and (-)ML. The ICD values of the two conformers have different signs and magnitudes, which were explained by differences both in the geometrical asymmetries of PhOH bound to (-)ML and in the electronic coupling strengths between PhOH and (-)ML.

Entropic effects make a more tightly folded conformer of a β-amino acid less stable: UV-UV hole burning and IR dip spectroscopy of l-β3-homotryptophan using a laser desorption supersonic jet technique

Gas-phase spectroscopy of homotryptophan was done to understand conformations of β-peptides.

Understanding the conformational behaviour of Ac-Ala-NHMe in different media. A joint NMR and DFT study

The conformational behaviour of Ac-Ala-NHMe was studied in the gas-phase and in solution by theoretical calculations (B3LYP-D3/aug-cc-pVDZ level) and experimental (1)H NMR. The conformational preferences of this compound were shown to result from a complex interplay between the strengths of possible intramolecular hydrogen bonds, steric interactions, hyperconjugation, entropy effects and the overall dipole moments. The Ac-Ala-N(Me)2 derivative was studied in addition, to design a system akin to Ac-Ala-NHMe, but with disrupted intramolecular hydrogen bonds involving the -NHMe group, mimicking the effect of polar protic solvents.