Fluorine in protein environments: a QM and MD study.

Noncanonical amino acids with newly designed side-chain functionalities represent powerful tools to improve structural, biological, and pharmacological properties of peptides and proteins. In this context, fluorinated amino acids have increasingly gained importance. Despite the current wide use of fluorination in protein engineering, the basic properties of fluorine in protein environments are still not completely understood. Our aim has been to characterize the physicochemical properties of fluorinated amino acids by using quantum mechanics (QM) and molecular dynamics (MD) approaches. We have analyzed geometry, charges, and hydrogen bonding abilities of several ethane fluorinated derivatives at different QM theory levels and have used them as simplified models for fluorinated amino acid side chains. We have parametrized four fluorinated L-amino acids for the AMBER ff94/99 force field: 4-monofluoroethylglycine (MfeGly), 4,4-difluoroethylglycine (DfeGly), 4,4,4-trifluoroethylglycine (TfeGly), and 4,4-difluoropropylglycine (DfpGly). We have characterized them in terms of molecular volumes, conformational preferences, and hydration properties. The obtained results illustrate that fluorine and hydrogen atoms of fluoromethyl groups could be potential acceptors or donors of weak hydrogen bonds in protein environments. Hydration of the studied fluorinated amino acids was found to be more favorable than for their nonfluorinated analogues, and hydrophobicity was observed to increase with the number of fluorine atoms, which is in accordance with the experimental retention times we obtain for these amino acids. This study broadens our understanding of the properties of fluorine within protein environments, which is important to exploit the full potential of fluorine's unique properties for applications in the field of protein engineering.