19F and 1, 2H ENDOR Study of Distal-Pocket N(∊)-H···F Hydrogen Bonding in Fluorometmyoglobin

Yang cheng Fann, Jui lin Ong, Judith M. Nocek, Brian M. Hoffman*

*Corresponding author for this work

Research output: Contribution to journalArticle

33 Citations (Scopus)

Abstract

19F and 1, 2H continuous-wave and pulsed electron nuclear double resonance (ENDOR) measurements of fluorometmyoglobin (MbF) frozen solutions are shown to provide probes of subtle structural features at the heme that are not directly accessible through X-ray methods. Although the EPR spectrum of MbF is pH invariant, the 19F data show that MbF exists in low- and high-pH forms that are related by a one-proton equilibrium with pKa = 7.6(1). The 1, 2H ENDOR shows that F- is hydrogen-bonded to the N(∊) H of distal histidine (E7) at all pH values between 5.5 and 11. The pH-dependent interconversion is assigned to deprotonation at N(δ) of the charged (imidazolium) form of His(E7). The data further gives metrical parameters for the H-bond, showing it to be relatively short with r(F··· H) = 1.57(5) Å. Complete determination of the 19F hyperfine tensor discloses a slight tilt of Fe—F-(non-coaxiality of g and hyperfine tensors). The tilt is associated with the H-bond: it is 5.0(5)° in the low-pH form and 3.5(5)° in the high. The 19F tensor components change slightly upon deprotonation: low pH, A||= +127.0(4) MHz, A= +64.0(4) MHz; high pH, A||= +130.0(4) MHz, A= +67.5(4) MHz. Surprisingly, there is a deuterium isotope effect on the 19F coupling in the low-pH form. The signs of the hyperfine components are derived from the pseudonuclear Zeeman effect in the 19F ENDOR. This effect also gives to high precision the axial zero-field splitting parameter, D, whose value is determined by the energies of excited states. D also is pH dependent: low-pH form, D = 6.1(1) cm-1; high-pH form, D = 5.2(1) cm-1.

Original languageEnglish (US)
Pages (from-to)6109-6116
Number of pages8
JournalJournal of the American Chemical Society
Volume117
Issue number22
DOIs
StatePublished - Jun 1995

Fingerprint

Electron Spin Resonance Spectroscopy
Hydrogen Bonding
Tensors
Hydrogen bonds
Deprotonation
Electrons
Hydrogen
Deuterium
Heme
Histidine
Excited states
Isotopes
Paramagnetic resonance
Protons
X rays
metmyoglobin fluoride
X-Rays

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

@article{faf6b35add494180a5909483e4d7624e,
title = "19F and 1, 2H ENDOR Study of Distal-Pocket N(∊)-H···F Hydrogen Bonding in Fluorometmyoglobin",
abstract = "19F and 1, 2H continuous-wave and pulsed electron nuclear double resonance (ENDOR) measurements of fluorometmyoglobin (MbF) frozen solutions are shown to provide probes of subtle structural features at the heme that are not directly accessible through X-ray methods. Although the EPR spectrum of MbF is pH invariant, the 19F data show that MbF exists in low- and high-pH forms that are related by a one-proton equilibrium with pKa = 7.6(1). The 1, 2H ENDOR shows that F- is hydrogen-bonded to the N(∊) H of distal histidine (E7) at all pH values between 5.5 and 11. The pH-dependent interconversion is assigned to deprotonation at N(δ) of the charged (imidazolium) form of His(E7). The data further gives metrical parameters for the H-bond, showing it to be relatively short with r(F··· H) = 1.57(5) {\AA}. Complete determination of the 19F hyperfine tensor discloses a slight tilt of Fe—F-(non-coaxiality of g and hyperfine tensors). The tilt is associated with the H-bond: it is 5.0(5)° in the low-pH form and 3.5(5)° in the high. The 19F tensor components change slightly upon deprotonation: low pH, A||= +127.0(4) MHz, A⊥= +64.0(4) MHz; high pH, A||= +130.0(4) MHz, A⊥= +67.5(4) MHz. Surprisingly, there is a deuterium isotope effect on the 19F coupling in the low-pH form. The signs of the hyperfine components are derived from the pseudonuclear Zeeman effect in the 19F ENDOR. This effect also gives to high precision the axial zero-field splitting parameter, D, whose value is determined by the energies of excited states. D also is pH dependent: low-pH form, D = 6.1(1) cm-1; high-pH form, D = 5.2(1) cm-1.",
author = "Fann, {Yang cheng} and Ong, {Jui lin} and Nocek, {Judith M.} and Hoffman, {Brian M.}",
year = "1995",
month = "6",
doi = "10.1021/ja00127a025",
language = "English (US)",
volume = "117",
pages = "6109--6116",
journal = "Journal of the American Chemical Society",
issn = "0002-7863",
publisher = "American Chemical Society",
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}

19F and 1, 2H ENDOR Study of Distal-Pocket N(∊)-H···F Hydrogen Bonding in Fluorometmyoglobin. / Fann, Yang cheng; Ong, Jui lin; Nocek, Judith M.; Hoffman, Brian M.

In: Journal of the American Chemical Society, Vol. 117, No. 22, 06.1995, p. 6109-6116.

Research output: Contribution to journalArticle

TY - JOUR

T1 - 19F and 1, 2H ENDOR Study of Distal-Pocket N(∊)-H···F Hydrogen Bonding in Fluorometmyoglobin

AU - Fann, Yang cheng

AU - Ong, Jui lin

AU - Nocek, Judith M.

AU - Hoffman, Brian M.

PY - 1995/6

Y1 - 1995/6

N2 - 19F and 1, 2H continuous-wave and pulsed electron nuclear double resonance (ENDOR) measurements of fluorometmyoglobin (MbF) frozen solutions are shown to provide probes of subtle structural features at the heme that are not directly accessible through X-ray methods. Although the EPR spectrum of MbF is pH invariant, the 19F data show that MbF exists in low- and high-pH forms that are related by a one-proton equilibrium with pKa = 7.6(1). The 1, 2H ENDOR shows that F- is hydrogen-bonded to the N(∊) H of distal histidine (E7) at all pH values between 5.5 and 11. The pH-dependent interconversion is assigned to deprotonation at N(δ) of the charged (imidazolium) form of His(E7). The data further gives metrical parameters for the H-bond, showing it to be relatively short with r(F··· H) = 1.57(5) Å. Complete determination of the 19F hyperfine tensor discloses a slight tilt of Fe—F-(non-coaxiality of g and hyperfine tensors). The tilt is associated with the H-bond: it is 5.0(5)° in the low-pH form and 3.5(5)° in the high. The 19F tensor components change slightly upon deprotonation: low pH, A||= +127.0(4) MHz, A⊥= +64.0(4) MHz; high pH, A||= +130.0(4) MHz, A⊥= +67.5(4) MHz. Surprisingly, there is a deuterium isotope effect on the 19F coupling in the low-pH form. The signs of the hyperfine components are derived from the pseudonuclear Zeeman effect in the 19F ENDOR. This effect also gives to high precision the axial zero-field splitting parameter, D, whose value is determined by the energies of excited states. D also is pH dependent: low-pH form, D = 6.1(1) cm-1; high-pH form, D = 5.2(1) cm-1.

AB - 19F and 1, 2H continuous-wave and pulsed electron nuclear double resonance (ENDOR) measurements of fluorometmyoglobin (MbF) frozen solutions are shown to provide probes of subtle structural features at the heme that are not directly accessible through X-ray methods. Although the EPR spectrum of MbF is pH invariant, the 19F data show that MbF exists in low- and high-pH forms that are related by a one-proton equilibrium with pKa = 7.6(1). The 1, 2H ENDOR shows that F- is hydrogen-bonded to the N(∊) H of distal histidine (E7) at all pH values between 5.5 and 11. The pH-dependent interconversion is assigned to deprotonation at N(δ) of the charged (imidazolium) form of His(E7). The data further gives metrical parameters for the H-bond, showing it to be relatively short with r(F··· H) = 1.57(5) Å. Complete determination of the 19F hyperfine tensor discloses a slight tilt of Fe—F-(non-coaxiality of g and hyperfine tensors). The tilt is associated with the H-bond: it is 5.0(5)° in the low-pH form and 3.5(5)° in the high. The 19F tensor components change slightly upon deprotonation: low pH, A||= +127.0(4) MHz, A⊥= +64.0(4) MHz; high pH, A||= +130.0(4) MHz, A⊥= +67.5(4) MHz. Surprisingly, there is a deuterium isotope effect on the 19F coupling in the low-pH form. The signs of the hyperfine components are derived from the pseudonuclear Zeeman effect in the 19F ENDOR. This effect also gives to high precision the axial zero-field splitting parameter, D, whose value is determined by the energies of excited states. D also is pH dependent: low-pH form, D = 6.1(1) cm-1; high-pH form, D = 5.2(1) cm-1.

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