Identification of Serine Conformers by Matrix-Isolation IR Spectroscopy Aided by Near-Infrared Laser-Induced Conformational Change, 2D Correlation Analysis, and Quantum Mechanical Anharmonic Computations

Abstract

The conformers of α-serine were investigated by matrix-isolation IR spectroscopy combined with NIR laser irradiation. This method, aided by 2D correlation analysis, enabled unambiguously grouping the spectral lines to individual conformers. On the basis of comparison of at least nine experimentally observed vibrational transitions of each conformer with empirically scaled (SQM) and anharmonic (GVPT2) computed IR spectra, six conformers were identified. In addition, the presence of at least one more conformer in Ar matrix was proved, and a short-lived conformer with a half-life of (3.7 ± 0.5) × 10(3) s in N2 matrix was generated by NIR irradiation. The analysis of the NIR laser-induced conversions revealed that the excitation of the stretching overtone of both the side chain and the carboxylic OH groups can effectively promote conformational changes, but remarkably different paths were observed for the two kinds of excitations.

Conformers of Gaseous Serine

“…Based on at least 6 band assignments for each species, Tarczay et al 52 identified six serine conformers (I, II, III, IV, VI, VIII) using matrix-isolation IR spectroscopy assisted by NIR laser irradiation and comparison with anharmonic GVPT2 vibrational computations on a hybrid B2PLYP-D3/B3LYP-D3 force field (Table S6) respectively. The theoretical frequencies in Table 4 for which an experimental assignment exists are italicized.…”

“…Italicized entries indicate frequencies for which experimental assignments exist. 52 b Computed using a reduced Fermi resonance interaction threshold ϕ > 16 cm −1 . !…”

“…48-51 Also, the conformational behavior of neutral serine was investigated in low-temperature argon matrices. 18,30,52 The combination of B3LYP/6-31++G** and anharmonic (GVPT2) computed IR spectra with experimental matrix FT-IR results demonstrated the presence of six Equilibrium structures and accurate relative energies, obtained within the focalpoint analysis (FPA) approach, [57][58][59][60][61][62] are available for the amino acids glycine, 20,22,63 alanine, 21,63 threonine, 64 proline, 65,66 and cysteine. 19 In this study, we perform an exhaustive search of serine conformers and use a correlated wave function method with a high-quality basis set (MP2/cc-pVTZ) to locate 85 unique conformers.…”

A Combined Systematic-Stochastic Algorithm for the Conformational Search in Flexible Acyclic Molecules

“…For n-alcohols the set of the { eq j } conformers was re-optimized employing the MPWB1K functional (Zhao et al, 2004) in combination with the 6-31+G(d,p) basis set (Hehre et al, 1972). In the case of serine, B3LYP/6-31++G * * was the method of choice with the objective of establishing a direct comparison with previous calculations (Najbauer et al, 2015). Geometry optimizations and frequency calculations were carried out with the Gaussian 09 package (Frisch et al, 2004).…”

“…To the best of our knowledge this is the first work dealing with the conformational flexibility of n-heptanol. Najbauer et al (2015) reported the 14 conformers of L-serine with the lowest Gibbs free energies at 0 K, G o 0K , calculated at the B3LYP/6-31++G * * level. Our algorithm found a total of 72 LL conformers, number that was reduced to 60 (listed in the Supplementary Material) after the HL reoptimizations, also at the B3LYP/6-31++G * * level.…”