Methionine-90-spin-labeled bovine .alpha.-lactalbumin: electron spin resonance and NMR distance measurements

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Abstract
The unique methionine residue of bovine .alpha.-lactalbumin was modified by irreversible alkylation with the bromoacetamido nitroxide spin-label 4-(2-bromoacetamido)-2,2,6,6-tetramethylpiperidine-N-oxyl. The line shape of the electron spin resonance (ESR) spectrum was indicative of a fairly mobile spin-label and was sensitive to the calcium-induced conformational change. Paramagnetic broadening of the spin-label ESR lines by a Gd(III) ion substituted at the high-affinity calcium site of the protein yielded a distance between the spin-label and the metal-binding site of 8.0 .+-. 1.0 .ANG.. The extent of the paramagnetic line broadening by the covalently attached nitroxide spin-label on the proton resonances of several amino acid residues of the protein at 500 MHz allowed estimation of intramolecular distances between the methionine-90 residue and several resolvable protons.