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The chemical difference between DNA and RNA nucleosides is their 2′-hydrogen versus 2′-hydroxyl substituents. Modification of the ribosyl moiety at the 2′-position and 2′-O-methylation in particular, is common among natural post-transcriptional modifications of RNA. 2′-Modification may alter the electronic properties and hydrogen-bonding characteristics of the nucleoside and thus may lead to enhanced stabilization or malfunction. The structures and relative glycosidic bond stabilities of the protonated forms of the 2′-O-methylated purine nucleosides, 2′-O-methyladenosine (Adom) and 2′-O-methylguanosine (Guom), were examined using two complementary tandem mass spectrometry approaches, infrared multiple photon dissociation action spectroscopy and energy-resolved collision-induced dissociation. Theoretical calculations were also performed to predict the structures and relative stabilities of stable low-energy conformations of the protonated forms of the 2′-O-methylated purine nucleosides and their infrared spectra in the gas phase. Low-energy conformations highly parallel to those found for the protonated forms of the canonical DNA and RNA purine nucleosides are also found for the protonated 2′-O-methylated purine nucleosides. Importantly, the preferred site of protonation, nucleobase orientation, and sugar puckering are preserved among the DNA, RNA, and 2′-O-methylated variants of the protonated purine nucleosides. The 2′-substituent does however influence hydrogen-bond stabilization as the 2′-O-methyl and 2′-hydroxyl substituents enable a hydrogen-bonding interaction between the 2′- and 3′-substituents, whereas a 2′-hydrogen atom does not. Further, 2′-O-methylation reduces the number of stable low-energy hydrogen-bonded conformations possible and importantly inverts the preferred polarity of this interaction versus that of the RNA analogues. Trends in the CID50%values extracted from survival yield analyses of the 2′-O-methylated and canonical DNA and RNA forms of the protonated purine nucleosides are employed to elucidate their relative glycosidic bond stabilities. The glycosidic bond stability of Adom is found to exceed that of its DNA and RNA analogues. The glycosidic bond stability of Guom is also found to exceed that of its DNA analogue; however, this modification weakens this bond relative to its RNA counterpart. The glycosidic bond stability of the protonated purine nucleosides appears to be correlated with the hydrogen-bond stabilization of the sugar moiety.

We present a simple upgrade of a time-resolved attosecond extreme ultraviolet and infrared (XUV-IR) setup designed to perform time-resolved or spectrally resolved studies of a target under similar experimental conditions. A flat XUV grating inserted in the path of an attosecond pulse train obtained via high order harmonic generation in gases is used either in the zeroth order of diffraction to follow attosecond dynamics or in the first order of diffraction to study the target spectral response with a temporally stretched single harmonic. Electron momentum measurement is performed with a velocity map imaging spectrometer, and the 10 W femtosecond laser system operating at 5 kHz provides an XUV photon flux compatible with rapid acquisition in both the monochromatic and broadband configurations. The change of experimental configuration between broadband and monochromatic sources is rapid and performed in situ. We present the experimental implementation applied to krypton atoms and detail the capabilities and limits of this approach when an XUV grating with constant groove density is used with a converging XUV beam.

Using a combination of deflection experiments and atomistic modeling, we investigate the structural and electrostatic properties of lithium and sodium clusters on a C60molecule. The electric susceptibilities are measured for LinC60and NanC60clusters containing up to n20 alkali atoms. The electric dipole moments are inferred from these data through the use of the Langevin formula. We also parametrize a many-body force field for this problem, extending a previous work by Roques and co-workers (Phys. Rev. B2003,68, 205412) and investigate the most stable structures and finite temperature effects on the experimental observables. The two alkalis show similar behavior and coat the fullerene at low coverage, then form a clear segregated droplet above eight atoms. However, while the sodium droplet grows monotonically above this cross-over size, we find evidence that a second structural transition takes place in lithium-coated C60, possibly indicating a momentary resurgence of homogeneous coating.

Abstract:: The electric dipole moments of the isolated amino acid tryptophan and small glycine-based peptides (WGn, n = 1-5, W = tryptophan, G = glycine) have been measured by deflection of a molecular beam in an inhomogeneous electric field. The measurements are compared to the results of ab initio calculations and Monte-Carlo simulations. The conformation and the flexibility of the peptides, at different temperatures, are discussed. The WGn peptides are much more floppy than an isolated tryptophan, even a single glycine is enough to make the peptide floppy on the timescale of the electric deflection measurements.

Abstract:: Gas phase Ti-C60 clusters are studied by molecular beam electric deflection. The permanent dipole moment of the TiC60 molecule is determined. It is equal to 8.1±1.5 D. This dipole is due to a transfer of electron from the transition metal atom to the C60 cage. No dipole is observed for Ti(C60)2 molecules. This is in agreement with the symmetrical dumbbell-like structure that has been previously proposed.