Zwitterions are the most stable form for neutral arginylglycine in gas phase: Clear theoretical evidence

Identification of the smallest peptide with a zwitterion as the global minimum: a first-principles study on arginine-containing peptides

Zwitterions are believed to play an important role in determining the structures, properties and functions of peptides and proteins. However, the smallest peptide with a zwitterionic structure as the global minimum in the gas phase is still not yet identified. In this study, an effective step-by-step strategy has been used to characterize the stable conformers of arginine-containing peptides arginylalanine (ArgAla) and arginylserine (ArgSer). Energy calculations at the DSD-PBEP86-D3BJ/aug-cc-pVTZ level and further extrapolation to the complete basis set (CBS) limit have confirmed, for the first time, that ArgSer appears to be a promising candidate as the smallest peptide with a zwitterionic global minimum structure. First-principles simulations have been performed for near-edge X-ray absorption fine-structure (NEXAFS) spectra and X-ray photoelectron spectra (XPS) at C, N and O K-edges, as well as for infrared (IR) spectra of these arginine-containing peptides. Notable spectral differences were found which enable the unambiguous identification of different neutral forms in future experiments. Our study thus provides valuable insights into the structural stability of zwitterions with the increase of molecular size and illustrates the competition between the canonical and zwitterionic isomers.

Systematic Study on Hydrated Arginine: Clear Theoretical Evidence for the Canonical-to-Zwitterionic Structure Transition

Extensive ab initio investigations have been performed to characterize the stable conformers of hydrated arginine (Arg-H₂O). Many new low-energy canonical Arg-H₂O conformers were identified and they are more stable than previous results. The large energy differences (more than 5.00 kcal mol^-1) between the canonical and zwitterionic Arg-H₂O isomers calculated by the composite CBS-QB3 method confirmed the dominance of the zwitterions. The micro effects of corrections of the zero-point energy and the basis set superposition error on the stability of hydrated isomers were carefully examined for the first time. The infrared (IR) spectra were simulated at a recommended temperature and the notable spectral differences enable the unambiguous identification of the different hydrated forms. Further transition state calculations revealed that the canonical Arg-H₂O can be transformed to the zwitterions using the amino group as a bridge. Our study thus shows valuable insights into the hydration of large flexible molecules and provides solid theoretical evidence for the canonical-to-zwitterionic structure transition of hydrated arginine.

Theoretical spectroscopic studies on chemical and electronic structures of arginylglycine

The global minimum of the dipeptide ArgGly is found to be in the canonical form, rather than the zwitterionic form.