Newly identified C-H⋯O hydrogen bond in histidine.

New Cδ-H⋯O histidine hydrogen bonding interactions in various proteins are identified by neutron diffraction and computationally characterized. Neutron diffraction data shows several H-bond motifs with the Cδ-H moiety in histidine side chains, including interactions in β-sheets and with coordinated waters, mostly with histidinium and τ-tautomers. In yellow protein, an active site histidine H-bonds Cδ-H to a main chain carbonyl while the Cε-H bond coordinates a water molecule.

Protein Solvent Shell Structure Provides Rapid Analysis of Hydration Dynamics.

The solvation layer surrounding a protein is clearly an intrinsic part of protein structure-dynamics-function, and our understanding of how the hydration dynamics influences protein function is emerging. We have recently reported simulations indicating a correlation between regional hydration dynamics and the structure of the solvation layer around different regions of the enzyme Candida antarctica lipase B, wherein the radial distribution function (RDF) was used to calculate the pairwise entropy, providing a link between dynamics (diffusion) and thermodynamics (excess entropy) known as Rosenfeld scaling. Regions with higher RDF values/peaks in the hydration layer (the first peak, within 6 Å of the protein surface) have faster diffusion in the hydration layer.

Evaluating electronic structure methods for accurate calculation of F chemical shifts in fluorinated amino acids.

The ability of electronic structure methods (11 density functionals, HF, and MP2 calculations; two basis sets and two solvation models) to accurately calculate the F chemical shifts of 31 structures of fluorinated amino acids and analogues with known experimental F NMR spectra has been evaluated. For this task, BHandHLYP, ωB97X, and Hartree-Fock with scaling factors (provided within) are most accurate. Additionally, the accuracy of methods to calculate relative changes in fluorine shielding across 23 sets of structural variants, such as zwitterionic amino acids versus side chains only, was also determined.

Tautomeric stabilities of 4-fluorohistidine shed new light on mechanistic experiments with labeled ribonuclease A.

Ribonuclease A is the oldest model for studying enzymatic mechanisms, yet questions remain about proton transfer within the active site. Seminal work by Jackson (Science, 1994) labeled Ribonuclease A with 4-fluorohistidine, concluding that active-site histidines act as general acids and bases. Calculations of 4-fluorohistidine indicate that the π-tautomer is predominant in all simulated environments (by ~17 kJ/mol), strongly suggesting that fluoro-labeled ribonuclease A functions with His119 in π-tautomer.

The Biophysical Probes 2-fluorohistidine and 4-fluorohistidine: Spectroscopic Signatures and Molecular Properties.

Fluorinated amino acids serve as valuable biological probes, by reporting on local protein structure and dynamics through F NMR chemical shifts. 2-fluorohistidine and 4-fluorohistidine, studied here with DFT methods, have even more capabilities for biophysical studies, as their altered pK values, relative to histidine, allow for studies of the role of proton transfer and tautomeric state in enzymatic mechanisms. Considering the two tautomeric forms of histidine, it was found that 2-fluorohistidine primarily forms the common (for histidine) τ-tautomer at neutral pH, while 4-fluorohistidine exclusively forms the less common π-tautomer.

Demystifying fluorine chemical shifts: electronic structure calculations address origins of seemingly anomalous (19)F-NMR spectra of fluorohistidine isomers and analogues.

Fluorine NMR spectroscopy is a powerful tool for studying biomolecular structure, dynamics, and ligand binding, yet the origins of (19)F chemical shifts are not well understood. Herein, we use electronic structure calculations to describe the changes in (19)F chemical shifts of 2F- and 4F-histidine/(5-methyl)-imidazole upon acid titration. While the protonation of the 2F species results in a deshielded chemical shift, protonation of the 4F isomer results in an opposite, shielded chemical shift.