title: High accuracy of Karplus equations for relating three-bond J couplings to protein backbone torsion angles.
abstract: (3) JC'C' and (3) JHNHα couplings are related to the intervening backbone torsion angle ${\varphi }$ by standard Karplus equations. Although these couplings are known to be affected by parameters other than ${\varphi }$, including H-bonding, valence angles and residue type, experimental results and quantum calculations indicate that the impact of these latter parameters is typically very small. The solution NMR structure of protein GB3, newly refined by using extensive sets of residual dipolar couplings, yields 50-60 % better Karplus equation agreement between ${\varphi }$ angles and experimental (3) JC'C' and (3) JHNHα values than does the high-resolution X-ray structure. In intrinsically disordered proteins, (3) JC'C' and (3) JHNHα couplings can be measured at even higher accuracy, and the impact of factors other than the intervening torsion angle on (3) J will be smaller than in folded proteins, making these couplings exceptionally valuable reporters on the ensemble of ${\varphi }$ angles sampled by each residue.
authors.affiliation: Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892 (USA).
authors.forename: Fang
authors.initials: F
authors.lastname: Li
authors.forename: Jung Ho
authors.initials: JH
authors.lastname: Lee
authors.forename: Alexander
authors.initials: A
authors.lastname: Grishaev
authors.forename: Jinfa
authors.initials: J
authors.lastname: Ying
authors.forename: Ad
authors.initials: A
authors.lastname: Bax
mesh_terms: D000465:Algorithms; D015295:Antigens, Tumor-Associated, Carbohydrate / Q000737:chemistry* / Q000378:metabolism; D006860:Hydrogen Bonding; D019906:Nuclear Magnetic Resonance, Biomolecular; D011789:Quantum Theory
chemical_list: D015295:Antigens, Tumor-Associated, Carbohydrate; C472766:Gb3 antigen
keywords: DFT calculations; Karplus equation; NMR spectroscopy; backbone torsion angles; protein structure