The protonated and sodiated dimers of proline studied by IRMPD spectroscopy in the N–H and O–H stretching region and computational methods

Abstract

IRMPD spectroscopy and computational chemistry techniques have been used to determine that the proton- and sodium-bound dimers of proline exist as a mixture of a number of different structures. Simulated annealing computations were found to be helpful in determining the unique structures of the protonated and sodiated dimers, augmenting chemical intuition. The experimental and computational results are consistent with the proton-bound dimer of N-protonated proline bound to zwitterionic proline. There was no spectroscopic evidence in the 3200-3800 cm(-1) region for a canonical structure which is predicted to have a weak N-H stretch at about 3440 cm(-1). A well resolved band at 1733 cm(-1) from a previous spectroscopic study (DOI: 10.1021/ja068715a ) was reassigned from a high energy canonical isomer to the C=O stretch of a lower energy zwitterionic structure. This band is a free carboxylate C=O stretch where protonated proline is hydrogen bonded to the other carboxylate oxygen which is also involved in an intramolecular hydrogen bond. Fifteen structures of the sodium bound proline dimer were computed to be within 10 kJ mol(-1) of Gibbs energy and eight structures were within 5 kJ mol(-1). None of these structures can be ruled out based on the experimental IRMPD spectrum. They all have an N-H stretching band predicted in a position that agrees with the experimental spectrum. However, only structures where one of the proline monomers is in the canonical form and having a free O-H bond can produce the band at ∼3600 cm(-1).

The Effects of Drying Temperature on Nitrogen Concentration Detection in Calcium Soil Studied by NIR Spectroscopy

“…Compared with the O-H bonds in water, the N-H bonds exist mostly in the form of multiple frequency or combination frequency. Thus, the N-H bonds are relatively weak for extracting soil nitrogen information in soil [40]. Second, from the aspects of the preprocessing methods with PLS, the ranking of prediction effects is RAW > S-G > DT > 1st-Der > SNV > MSC.…”

“…Compared with the full spectrum model by PLS, the prediction of the characteristic bands by CARS-BIPLS-PLS is obviously better than that in the condition when the soil was detected at a 25 • C placement. The reason might be that the vibration of N-H was covered by little soil water content in the full spectrum [40]. However, the most optimal characteristic variables were selected with combined CARS and BIPLS and the noise information was also removed [30][31][32].…”

Evidence for cooperative Na+ and Cl− binding by strongly hydrated l-proline

“…Cation binding was found to be negligible for the compounds of that study. However, quantum-chemical calculations 66 and gas-phase vibrational spectra 67 suggest that for Pro the situation might be different.…”

Side-chain effects on the structures of protonated amino acid dimers: A gas-phase infrared spectroscopy study

“…Previously, several protonated AA dimers have been studied, both experimentally as well as by theory. [22][23][24][25][26][27][28][29][30][31][32][33][34][35] O-H and N-H stretching vibrations in Gly 2 H + , 22,25 Ser 2 H + , 23 Thr 2 H + , 32 Lys 2 H + , 22,29 Pro 2 H + , 31 and Trp 2 H + 33 were investigated in the 2700-3600 cm -1 range and Gly 2 H + , 24,26 Ala 2 H + , 26 Val 2 H + , 26 Cys 2 H + , 35 and Pro 2 H + 24 were studied in the 1000-2000 cm -1 region, in which C=O stretching and N-H bending vibrations can be monitored. Heterodimers, such as Ser•PheH + , 28,30 Ala•GlyH + , 26 Gly•LysH + , 22,25 and Gly•PheH + , 34 were also investigated in both, the 2700-3700 cm -1 and 800-2000 cm -1 ranges.…”