Wavelength-Dependent Ultraviolet Photodissociation of Protonated Tryptamine

Transition of the coordination modes in sodiated uridine radicals revealed by infrared multiphoton dissociation spectroscopy and theoretical calculations

The stable generation and structural characterization of sodium cationized nucleic acid radicals at the molecular level have always been a difficult problem to solve. Herein, we produced the radical cation of [Urd + Na - H]˙+ through ultraviolet photodissociation (UVPD) of the precursor ion of [I - Urd + Na]+ in the gas phase and further studied its infrared multiphoton dissociation (IRMPD) spectrum in the region of 2750-3850 cm-1. The comparison between the IRMPD spectra of the precursor and radical cations shows their common features at both 3445 and 3705 cm-1 peaks, as well as the difference in the 3628 cm-1 peak that exists only in the case of the latter. By combining with theoretical calculations, it is indicated that the bidentate coordination structure M-B(O2,O2')-1 and the tridentate coordination structure R-T(O2,O',O5')-(C5H-C1')-1 are dominantly populated for the precursor and the radical cations, respectively. After the homo-cleavage of the C-I bond using a UV laser, a multi-step hydrogen transfer process started from the C1' position, followed by a rotation of the intramolecular C-N bond, resulting in the formation of the most stable isomer, characterized by its radical position at C1' and its tridentate coordination mode. This result indicates that the generation of free radicals of metal cationized nucleic acids by UVPD may result in the hydrogen transfer from the sugar ring, as well as the accompanied change of its coordination mode of the attached metal ions.

Simultaneous quantitative chiral analysis of four isomers by ultraviolet photodissociation mass spectrometry and artificial neural network

Although mass spectrometry (MS) has its unique advantages in speed, specificity and sensitivity, its application in quantitative chiral analysis aimed to determine the proportions of multiple chiral isomers is still a challenge. Herein, we present an artificial neural network (ANN) based approach for quantitatively analyzing multiple chiral isomers from their ultraviolet photodissociation mass spectra. Tripeptide of GYG and iodo-L-tyrosine have been applied as chiral references to fulfill the relative quantitative analysis of four chiral isomers of two dipeptides of L/DHisL/DAla and L/DAspL/DPhe, respectively. The results show that the network can be well-trained with limited sets, and have a good performance in testing sets. This study shows the potential of the new method in rapid quantitative chiral analysis aimed at practical applications, with much room for improvement in the near future, including selecting better chiral references and improving machine learning methods.

A novel method for photon unfolding spectroscopy of protein ions in the gas phase

In this study, a new experimental method for photon unfolding spectroscopy of protein ions based on a Fourier transform ion cyclotron resonance (FT ICR) mass spectrometer was developed. The method of short-time Fourier transform has been applied here to obtain decay curves of target ions trapped in the cell of the FT ICR mass spectrometer. Based on the decay constants, the collision cross sections (CCSs) of target ions were calculated using the energetic hard-sphere model. By combining a tunable laser to the FT ICR mass spectrometer, the changes of CCSs of the target ions were recorded as a function of the wavelengths; thus, the photon isomerization spectrum was obtained. As one example, the photon isomerization spectrum of [Cyt c + 13H]¹³⁺ was recorded as the decay constants relative to the applied wavelengths of the laser in the 410-480 nm range. The spectrum shows a maximum at 426 nm, where an unfolded structure induced by a 4 s irradiation can be deduced. The strong peak at 426 nm was also observed for another ion of [Cyt c + 15H]¹⁵⁺, although some difference at 410 nm between the two spectra was found at the same time. This novel method can be expanded to ultraviolet or infrared region, making the experimental study of wavelength-dependent photon-induced structural variation of a variety of organic or biological molecules possible.

Versatile Double-Beam Confocal Laser System Combined with a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer for Photodissociation Mass Spectrometry and Spectroscopy

Both infrared multiphoton dissociation (IRMPD) and ultraviolet photodissociation (UVPD) play important roles in tandem mass spectrometry and the action spectroscopy of organic and biological molecules. A flexible combination of the two methods may provide researchers with more versatile and powerful ion activation/dissociation choices for structural characterization and spectroscopic studies. Here, we report the integration of two tunable lasers with a Fourier transform ion cyclotron resonance mass spectrometer in a confocal mode, which offers multiple capabilities for photon activation/dissociation experiments. The two overlapped beams can be introduced into the cell individually, sequentially, or simultaneously, providing highly flexible and diverse activation schemes. The setup can also measure the UVPD or IRMPD action spectra of fragment ions generated by previous photon dissociation processes. In addition, the multistage tandem-in-time mass spectrometry performance up to MS⁴, including three different activation methods in a single cell, has also been demonstrated.

Fragmentation study of tryptophan-derived metabolites induced by electrospray ionization mass spectrometry for highly sensitive analysis

ESI of tryptophan-derived metabolites produced an intense signal of fragment ion with a spiro[cyclopropane-indolium] backbone. The use of corresponding fragment ions for the precursor of MRM transitions could improve the detection limit.