Abstract
The broadband rotational spectrum of jet-cooled laser-ablated thioproline was recorded. Two conformers of this system were observed and identified with the help of DFT and ab initio computations by comparison of the observed and calculated rotational constants and 14N quadrupole coupling constants as well as the predicted energies compared to the observed relative populations. These conformers showed a mixed bent/twisted arrangement of the five-membered ring similar to that of the related compound thiazolidine with the N–H bond in axial configuration. The most stable form had the COOH group in an equatorial position on the same side of the ring as N-H. The arrangement of the C=O group close to the N-H bond led to a weak interaction between them (classified as type I) characterized by a noncovalent interaction analysis. The second form had a trans-COOH arrangement showing a type II O–H···N hydrogen bond. In thioproline, the stability of conformers of type I and type II was reversed with respect to proline. We show how the conformation of the ring depends on the function associated with the endocyclic N atom when comparing the structures of isolated thioproline with its zwitterion observed in condensed phases and with peptide forms.
Highlights
Protein activity is a consequence of the three-dimensional arrangement of their different functional groups
We have observed for the first time the rotational spectrum of laserablated Scheme 1. The thioproline (SPro)
The arrangement of the C=O group with the N–H bond gives rise to a weak interaction which has been characterized from an Non-covalent interaction (NCI) analysis
Summary
Protein activity is a consequence of the three-dimensional arrangement of their different functional groups. The activity of folded proteins relies on factors as the secondary structure formation, the hydrophobic effect, and additional noncovalent interactions to organize functional groups to allow molecular recognition of specific substrate partners and catalysis. Proline residues are unique among the canonical amino acids, due to the conformational restraint of backbone cyclization and the presence of a tertiary amide bond [4]. These structural characteristics limit the available conformations for its residues, which enables proline to be preferentially observed in specific structural contexts, such as secondary structure termination, loops, turns, and helices [5]. The importance of proline derivatives can be exemplified by the role of
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