Role of hydrogen bonding to bound dioxygen in soybean leghemoglobin

Electron spin echo envelope modulation (ESEEM) spectroscopy was applied to oxy cobaltous soybean leghemoglobin (oxyCoLb) in D2O at various pH values to investigate electron nuclear superhyperfine coupling to N epsilon of the proximal histidyl imidazole and to exchangeable deuterons. Two spectroscopically distinct forms of oxyCoLb, acid and neutral, were identified. In the acid form, a 0.82-MHz hyperfine coupling to 2H was found, indicating the presence of a hydrogen bond to bound O2. No hyperfine-coupled 2H was found in the neutral form. Nuclear hyperfine and nuclear quadrupole couplings to the proximal histidyl N epsilon in the acid form are smaller than those in the neutral form: Aiso = 2.22 MHz and e2qQ = 1.98 MHz for the acid form; Aiso = 2.90 MHz and e2qQ = 2.22 MHz for the neutral form. The differences are believed to result from the presence of a hydrogen bond to bound O2 in the acid form. A discussion of the contribution of this hydrogen bond to the pH-dependent O2 affinity of leghemoglobin is presented.