Microsolvation Structures of Protonated Glycine and l-Alanine

Abstract

The IR predissociation spectra of microsolvated glycine and l-alanine, GlyH+(H2O) n and AlaH+(H2O) n , n = 1–6, are presented. The assignments of the solvation structures are aided by H2O/D2O substitution, IR-IR double resonance spectroscopy, and computational efforts. The analysis reveals the water–amino acid as well as the water–water interactions, and the subtle effects of the methyl side chain in l-alanine on the solvation motif are also highlighted. The bare amino acids exhibit an intramolecular hydrogen bond between the protonated amine and carboxyl terminals. In the n = 1–2 clusters, the water molecules preferentially solvate the protonated amine group, and we observed differences in the relative isomer stabilities in the two amino acids due to electron donation from the methyl weakening the intramolecular hydrogen bond. The structures in the n = 3 clusters show a further preference for solvation of the carboxyl group in l-alanine. For n = 4–6 clusters, the solvation structure of the two amino acids is remarkably similar, with one dominant isomer present in each cluster size. The first solvation shell is completed at n = 4, evidenced by a lack of free NH and OH stretches on the amino acid, as well as the first observation of H2O–H2O interactions in the spectra of n = 5. Finally, we note that calculations at the density functional theory (DFT) level show excellent agreement with the experiment for the smaller clusters. However, when water–water interactions compete with water–amino acid interactions in the larger clusters, DFT results show greater disagreement with experiment when compared to MP2 results.

Microhydration of Biomolecules: Revealing the Native Structures by Cold Ion IR Spectroscopy

“…The assignment of the peaks in these spectra was based on the comparison with the computed spectra of the low-energy ionic structures that were calculated for the complexes by the same group. 16 For n = 1, the spectra of the complexes extracted from the solution and the complexes prepared by condensation exhibit all the same transitions, except one extra peak at 3336 cm −1 in the spectrum of the latter. The structure associated with this peak was assigned to the most stable conformer of the singly hydrated GlyH + .…”

“…For comparison, the blue traces in the same figure reproduce the IR spectra measured earlier by the group of Garand for the same complexes (n = 1 to 4), but which were produced in the gas phase by cryogenic condensation. 16 Note that the relative intensities of the transitions in each of the two "action" spectra may differ due to the difference in the used methods of the measurements. The assignment of the peaks in these spectra was based on the comparison with the computed spectra of the low-energy ionic structures that were calculated for the complexes by the same group.…”

“…The peak at 3355 cm −1 , which appears in the spectra of the complexes produced by both methods of hydration, was assigned earlier to the conformer that has no intramolecular H-bond between the NH 3 + and CO groups. 16 Competing with this bond, the first condensed or the last retained water molecule noncovalently binds to the protonated N-terminus, partially solvating its charge. Consistently, this is the only structure detected for the complexes extracted from the solution in our experiments (Figure 1, n = 1, red trace).…”

Probing the conformational landscape and thermochemistry of DNA dinucleotide anions via helium nanodroplet infrared action spectroscopy

“…32 The combination of electrospray ionization and mass spectrometry (ESI-MS) has proven to be the most versatile experimental methodology for the structural interrogation of isolated biomolecular ions. The non-destructive transfer of ions from solution to the gas phase provided by ESI combined with m/z-selective isolation and detection have facilitated the study of an exceptional diversity of species ranging from microsolvated building blocks [33][34][35][36][37][38][39][40][41] to proteins and polynucleotides. [42][43][44][45][46][47][48][49][50][51] Under the appropriate experimental conditions, significant elements of the condensed-phase secondary and tertiary structure can even be preserved upon transfer to vacuum.…”

“…33,35,75 More recently, cryogenic ion spectroscopy, in which ions are cooled to temperatures as low as 4 K prior to spectroscopic characterization, has emerged as a highly effective approach to probe the conformational landscape of biomolecules in vacuum. 37,38,[76][77][78][79][80][81][82][83] Furthermore, the coupling of IMS and IR action spectroscopy has provided a more comprehensive assessment of the structure of biomolecular ions. 52,84,85 Although the outlined experimental techniques have been successful in identifying low-energy conformers of biomolecules, a quantitative measurement of the conformational thermochemistry (e.g., the relative enthalpy and entropy of conformers) remains challenging.…”