A carbon-13 spin lattice relaxation study of alumichrome at 25.1 MHz and 90.5 MHz

Abstract

Listen

Translate article

The effect of metal binding on the structure of ferrichrome in [ 2H 6]Me 2SO solutions at 45 °C, was studied by comparing the 13C spin-lattice relaxation of deferriferrichrome and alumichrome at 25.1 MHz. As has been found for other flexible cyclic oligopeptides, the metal-free siderophore exhibits consistent NT 1 values for α-carbons with a marked trend towards smaller relaxation rates for sidechain carbons. The rotational correlation time, τ r, calculated on the basis of the average NT 1 for the cyclohexapeptide backbone, < NT 1> = 93 ms, and under the assumption of isotropic motion is ≈ 0.56 ns. After binding Al 3+ the overall carbon skeleton exhibits rather uniform N ∗T 1 values, with a mean value <N ∗T 1> = 125 ms which yields an effective τ r ≈ 0.40 ns. Compared with the results for deferriferrichrome, these data indicate a more rigid compact structure with a faster rotational motion for the coordination compound. The 13C T 1 values were also measured at 90.5 MHz. It was found that agreement between the low and high field data can be obtained by assuming the C-H bond to be somewhat longer than 1.09 Å. A consistent value for the alumichrome isotropic rotational correlation time, τ r = 0.58 ns, was thus derived under the assumption that the effective <r CH > = 1.15 A ̊ . Nuclear Overhauser enhancements as well as the T 1 data indicate that at 25.1 MHz the 1H- 13C dipolar interaction is the dominant relaxation mechanism for the various peptide carbonyls. The observations agree well with expectations from σ i r −6 CHI calculations based on the crystallographic coordinates for the isomorphic analogue ferrichrome A.