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Conformation-specific infrared and ultraviolet spectroscopy of tyrosine-based protonated dipeptides
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Image of FIG. 1.
FIG. 1.

(Color online) Ultraviolet photofragmentation spectra of recorded by fragmentation into (a) mass channel 136 and (b) mass channel 146. The transitions are labeled by conformation, as determined by infrared-ultraviolet double resonance spectroscopy. The same transitions are present with different intensities in both spectra, but are only labeled in one for clarity. The mass of the parent molecule is .

Image of FIG. 2.
FIG. 2.

Fragmentation mass spectra of recorded at (a) and (b) , with the major fragment peaks labeled. The parent mass is .

Image of FIG. 3.
FIG. 3.

(Color online) Infrared spectra of the NH stretch region of , recorded with the UV laser set to 35 174.8 and , together with calculated spectra of four conformational families.

Image of FIG. 4.
FIG. 4.

(Color online) Structures and zero-point-corrected energies in kJ/mol of calculated at the level of theory. The four lowest energy structures (family I) are all characterized by a interaction and a -hydrogen bond of the ammonium group and a interaction of the amide NH. The structures are named according to the orientation of the COOH with respect to the ring (anti or gauche) and the location of the phenol oxygen lone pair with respect to the ammonium group (anti or syn). Three representative structures of higher energy families (II–IV) are also shown.

Image of FIG. 5.
FIG. 5.

(Color online) Infrared spectra of the four band origins of , located at (A), (B), (C), and (D). The calculated stick spectra are shown for the four conformers of family I, showing conformers A and C to be anti conformers and B and D to be gauche conformers. The right panel shows the free NH stretch region expanded. Conformer D is distinguished from conformer B by the smaller splitting of the free NH stretches, while the free ammonium NH stretch of conformer A is slightly but reproducibly lower in frequency than that of conformer C. The dashed lines are through the center of the peaks of conformers A and B to highlight the differences. The assignment of the absolute orientation of the phenolic OH group is uncertain, given the small differences in the spectra.

Image of FIG. 6.
FIG. 6.

Ultraviolet photofragmentation spectrum of , recorded using the fragment at 182.

Image of FIG. 7.
FIG. 7.

Fragmentation mass spectrum of , recorded at the electronic origin at . The parent ion mass is .

Image of FIG. 8.
FIG. 8.

(Color online) Families of structures calculated using . Energies are given in kJ/mol and are zero-point corrected.

Image of FIG. 9.
FIG. 9.

(Color online) Infrared-ultraviolet depletion spectrum of recorded at the electronic origin at , with the peaks assigned by comparison to the calculated spectra of five conformer families.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Conformation-specific infrared and ultraviolet spectroscopy of tyrosine-based protonated dipeptides