Proton magnetic relaxation and anion effect in solutions of acid ferricytochrome c

Amos Lanir, Irit Aviram

Research output: Contribution to journalArticlepeer-review

Abstract

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.

Original languageEnglish
Pages (from-to)439-445
Number of pages7
JournalArchives of Biochemistry and Biophysics
Volume166
Issue number2
DOIs
StatePublished - Feb 1975

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