TY - JOUR
T1 - Proton magnetic relaxation and anion effect in solutions of acid ferricytochrome c
AU - Lanir, Amos
AU - Aviram, Irit
PY - 1975/2
Y1 - 1975/2
N2 - Measurements of the longitudinal relaxation rates of water protons in aqueous solutions of ferricytochrome c and their temperature dependence, were used for the elucidation of the heme iron ligands at acid pH. The relaxation rates increased with a decrease in pH and pK values of 2.5 and 4.48 were evaluated for the aqueous and 6 m urea solutions, respectively. The results at acid pH are compatible with a structure in which two water molecules exchange rapidly between the coordination sphere of high spin heme iron and the bulk. They suggest that concomitantly with the low-high spin transition the histidine-18 and methionine-80 iron bonds break simultaneously. Addition of various anions, including methanesulfonate at pH 1.95 caused a 85% decrease in the net longitudinal relaxation rate. However, neither the chemical shift nor the width of the methyl proton nmr line of methanesulfonate in solution of acid ferricytochrome c were affected indicating that the effect of anions is not due to a direct binding to the heme iron. The relaxation mechanism of the water molecules in the first coordination sphere of the ferric ion in acid cytochrome c is discussed. It appears that the longitudial relaxation rate is modulated by the electronic correlation time of the ferric ion which was calculated to be τs = 6 × 10-11 sec at 60 MHz.
AB - Measurements of the longitudinal relaxation rates of water protons in aqueous solutions of ferricytochrome c and their temperature dependence, were used for the elucidation of the heme iron ligands at acid pH. The relaxation rates increased with a decrease in pH and pK values of 2.5 and 4.48 were evaluated for the aqueous and 6 m urea solutions, respectively. The results at acid pH are compatible with a structure in which two water molecules exchange rapidly between the coordination sphere of high spin heme iron and the bulk. They suggest that concomitantly with the low-high spin transition the histidine-18 and methionine-80 iron bonds break simultaneously. Addition of various anions, including methanesulfonate at pH 1.95 caused a 85% decrease in the net longitudinal relaxation rate. However, neither the chemical shift nor the width of the methyl proton nmr line of methanesulfonate in solution of acid ferricytochrome c were affected indicating that the effect of anions is not due to a direct binding to the heme iron. The relaxation mechanism of the water molecules in the first coordination sphere of the ferric ion in acid cytochrome c is discussed. It appears that the longitudial relaxation rate is modulated by the electronic correlation time of the ferric ion which was calculated to be τs = 6 × 10-11 sec at 60 MHz.
UR - http://www.scopus.com/inward/record.url?scp=0016608239&partnerID=8YFLogxK
U2 - 10.1016/0003-9861(75)90407-5
DO - 10.1016/0003-9861(75)90407-5
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AN - SCOPUS:0016608239
SN - 0003-9861
VL - 166
SP - 439
EP - 445
JO - Archives of Biochemistry and Biophysics
JF - Archives of Biochemistry and Biophysics
IS - 2
ER -