Fluorine nuclear magnetic resonance spectra of rabbit carbonmonoxyhemoglobin

19F NMR of trifluoroacetyl-labeled cysteine mutants of myoglobin: structural probes of nitric oxide bound to the H93G cavity mutant

Nitric oxide (NO) binds to the myoglobin (Mb) cavity mutant, H93G, forming either a 5- or 6-coordinate Fe--NO heme complex. The H93G mutation replaces the proximal histidine of Mb with glycine, allowing exogenous ligands to occupy the proximal binding site. In the absence of the covalently attached proximal ligand, NO could bind to H93G from the proximal side of the heme rather than the typical diatomic binding pocket on the distal side when the 5-coordinate complex forms. The question of whether NO binds on the distal or proximal side was addressed by (19)F NMR. Site-directed mutagenesis was used to introduce unique cysteine residues at the protein surface on either the distal (S58C) or proximal (L149C) side, approximately equidistant from and perpendicular to the heme plane of both wild-type and H93G Mb. The cysteine thiols were alkylated with 3-bromo-1,1,1-trifluoroacetone to attach a trifluoroacetyl group at the mutation site. (19)F NMR spectra of 5-coordinate, NO bound S58C/H93G and L149C/H93G double mutants depict peaks with line widths of 100 and 23 Hz, respectively. As fluorine peaks broaden with increasing proximity to paramagnetic centers, such as 5-coordinate Fe--NO, the (19)F NMR data are consistent with NO binding in the distal heme pocket of H93G, even in the absence of a sixth axial ligand. Additionally, (19)F NMR spectra are reported for deoxy, oxy, CO, met CN, and met H(2)O forms of the labeled cysteine mutants. These results demonstrate that the fluorine probes are sensitive to subtle conformational changes in the protein structure due to ligation and oxidation state changes of the heme iron in Mb.

Assignment and analysis of fluorine nuclear magnetic resonance spectra of 4-fluorotryptophan myoglobins and hemoglobins

We have obtained the 470 MHz 19F NMR spectra of wild type [4-F]Trp-labeled myoglobins (MbCO, MbO2, deoxyMb, metMb, and MbCN) and hemoglobins (HbCO, HbO2, and deoxyHb), as well as those of several mutants (W7F Mb, betaW15F Hb, betaW37S Hb, and betaY130F Hb, all as the carbonmonoxy adducts), prepared via site-directed mutagenesis. The maximum observed chemical shift range induced by folding is 6.4 ppm. Using a multipole shielding polarizability-local reaction field approach, we have computed the electrostatic field contributions to the fluorine shielding. For residues which do not have F atoms in contact with neighboring groups, we find an approximately 1 ppm mean square deviation in shift from experiment, with the R2-like structure of HbCOA being in very close accord with experiment.

Fluorine-19 nuclear magnetic resonance spectroscopic study of fluorophenylalanine- and fluorotryptophan-labeled avian egg white lysozymes

We report the 470-MHz (11.7 T) 19F solution nuclear magnetic resonance (NMR) spectra of 2-, 3-, and 4-fluorophenylalanine incorporated into the egg white lysozymes (EC 3.2.1.17) of chicken, pheasant, and duck, as well as spectra of 4-fluorotryptophan incorporated into chicken, California valley quail, and Bob White quail and 5- and 6-fluorotryptophan-labeled chicken lysozyme. The 19F solution NMR spectrum of [4-F]Phe hen egg white lysozyme (HEWL) consists of three sharp resonances, which span a total chemical shift range of 4.8 ppm (at p2H = 6.1). For [3-F]Phe HEWL, the chemical shift range is much smaller, 1.0 ppm (at p2H = 5.9), due presumably to the occurrence of fast phenyl ring flips about the C beta-C gamma bond axis. For [2-F]Phe HEWL, six resonances are observed, spanning a chemical shift range of 7.4 ppm (at p2H = 5.8), due to slow C beta-C gamma ring flips, i.e., both ring-flip isomers appear to be "frozen in" because of steric hindrance. Rotation of the [2-F]Phe residues remains slow up to 55 degrees C (p2H = 4.7). With the [F]Trp-labeled proteins, we find a maximal 14.6-ppm shielding range for [4-F]Trp HEWL but only a 2.8- and 2.4-ppm range for [5- and 6-F]Trp HEWL, respectively, due presumably to increased solvent exposure in the latter cases. Guanidinium chloride denaturation causes loss of essentially all chemical shift nonequivalence, as does thermal denaturation. Spectra recorded as a function of pH show relatively small chemical shift changes (< 1.4 ppm) over the pH range of approximately 1.2-7.8.(ABSTRACT TRUNCATED AT 250 WORDS)

Prediction of fluorine chemical shifts in proteins

Molecular dynamics calculations have been used in an effort to estimate the change in fluorine nmr shielding when a fluorine nucleus enters the tertiary structure of a protein. Considerations of the possible interactions that can define the shift parameter change suggest that van der Waals interactions are the leading determinant of fluorine shifts in proteins, although aromatic ring currents, other magnetic anisotropies, and electrostatic field effects could result in shift distinctions of 1 ppm or smaller. Results of our studies of a fluorine-containing analogue of the ribonuclease A S-protein/S-peptide complex indicate that static structures such as those implied by crystallographic data lead to overestimates of the magnitude of the van der Waals shielding term; molecular dynamics simulations provide indications of the effects of conformational averaging in defining this term. The treatment used predicts the correct direction of the shift change when the fluorine enters this protein environment from aqueous solution and, with an experimentally supported choice of adjustable parameters, gives agreement with the magnitude of the shift.