Origin of observed changes in ¹⁴ N hyperfine interaction accompanying R → T transition in nitrosylhemoglobin

Abstract

Theoretical investigations of electronic distributions in eight different structural forms of nitrosylhemoglobin were carried out to study the changes in 14N hyperfine interaction observed with the transition from R to T structures under the influence of inositol hexaphosphate or changing pH. Four of the eight forms studied consisted of protonated and deprotonated N ros in the proximal imidazole ligand with linear and bent Fe-N-0 structures. Two other forms had a straight Fe-N-O structure and Fe-Im bond stretched by 0.5 and 1.0 .A. The other two systems we have studied are five-liganded NO-heme with bent and straight Fe-N-O structures. Our investigations show that arrangements of energy levels did not differ significantly among all the structures, the unpaired electron always occupying an antibonding orbital with d.2 symmetry. The protonated and deprotonated systems with either linear or bent Fe-N-O structure showed substantial hyperfine interaction of the 14N nuclei of the NO group and the Nf atom of the proximal imidazole, indicating that a 9-line electron spin resonance hyperfine pattern (R structure) would be expected in all four cases. On the other hand, the extensions of the Fe-Im bond produce a sizeable decrease in the 14Nf hyperfine interaction, indicating that an extension beyond 1.0 A\ would provide a 3-line hyperfine pattern close to that found for the five-liganded NO-heme system. Our results thus provide quantitative support for the model of severe extension or cleavage of the Fe-N, bond proposed in the literature for explaining the R-to-T transition of the a-chain of nitrosylhemoglobin.The study of the properties of nitrosylhemoglobin (NO-Hb) has assumed major importance (1-4) in recent years for two main reasons. First, it is a six-liganded hemoglobin derivative that is paramagnetic with spin 1/2, which makes it amenable for electron paramagnetic resonance (EPR) studies (2-10). Such studies allow one to follow the changes in the electron distribution over the molecule associated with transitions between R and T states, considered to be of crucial importance in the explanation of cooperativity (11) in the oxygen-binding properties of hemoglobin. The second reason is that, in contrast to oxyhemoglobin and carbonmonoxyhemoglobin, NO-Hb shows a pronounced R-to-T transition (12,13) in the presence of inositol hexaphosphate, this transition being also rather sensitive to changes (5, 9, 10) in pH of the surrounding environment. One of the properties of NO-Hb that is a sensitive indicator of changes of structure under the influence of inositol hexaphosphate or changes in pH is the splitting produced (3-10) in EPR spectra by 14N hyperfine interactions. Thus, in NO-Hb A, in the presence of inositol hexaphosphate or at low pH one finds for the aY chain a 3-line hyperfine pattern associated with the hyperfine interaction of the '4N nucleus of NO. On increasing the pH beyond 8.0, even in the presence of inositol hexaphosphate one sees a change (5) from the 3-line to a 9-line pattern, indicating that th...

Electron-nuclear coupling in nitrosyl heme proteins and in nitrosyl ferrous and oxy cobaltous tetraphenylporphyrin complexes

“…Couplings of this magnitude do not give rise to envelope modulations in our spin-echo experiments (Mims & Peisach, 1978). The ESEEM results are also consistent with a very small hyperfine interaction at the remote or amino nitrogen nucleus, predicted theoretically to be less than 1 G (3 MHz) in hexacoordinate NO-iron porphyrin complexes (Mun et al, 1979). Hyperfine interaction at the amino l4N is observed for Cun(imid) complexes, however, when the directly coordinated 14N coupling is approximately twice as large as in the porphyrin complexes examined here (Mims & Peisach, 1978;Van Camp et al, 1981) .…”

“…The featureless ESEEM spectrum suggests that the pyrrole nitrogen atoms of the porphyrin are too weakly coupled to the unpaired spin in the complex to contribute to the spin echo envelope. Calculations (Mun et al, 1979) predict that pyrrole nitrogen couplings should be less than 3 G (9 MHz) and could therefore be in a range expected to give rise to ESEEM. The data in Figure 1A suggest that the Fermi contact coupling to the pyrrole nitrogens is considerably smaller and, on the basis or [1'N]pyridine]; (C) NO-Fen(TPP)(imidazole).…”

Characterization of Bimodal Coordination Structure in Nitrosyl Heme Complexes through Hyperfine Couplings with Pyrrole and Protein Nitrogens

“…The 1 H( 2 D)-ENDOR investigations of NO−heme systems have suggested the presence of a hydrogen bond between the exchangeable N ε proton of His E7 and oxygen of the NO−ligand . Unpaired electron spin density is known to significantly spread over the Fe−NO fragment with nearly 30% residing on the NO ligand . It, therefore, seems reasonable to expect a delocalization of noticeable unpaired electron spin density to the N ε nitrogen of His E7 (ρ(2s) = 0.20−0.35% corresponds to a iso = 3−5 MHz) .…”

Glutaraldehyde effect on hemoglobin: evidence for an ion environment modification based on electron paramagnetic resonance and Mossbauer spectroscopies

“…2) characterizes the hyperfine interaction between the unpaired electron and the nitrogen atom ( 1 4~) nucleus of NO (Kon 1968;Kon and Katoaka 1969;Mun et al 1979). Another hyperfine interaction can superimpose itself to the preceding one and arise from the histidine F8 trans to the heme plane.…”