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

¹⁹F and ^{1, 2}H 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 pK_a = 7.6(1). The ^{1, 2}H 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.