Measurement of 15N longitudinal relaxation rates in 15NH4+ spin systems to characterise rotational correlation times and chemical exchange

Abstract

Many chemical and biological processes rely on the movement of monovalent cations and an understanding of such processes can therefore only be achieved by characterising the dynamics of the involved ions. It has recently been shown that ¹⁵N-ammonium can be used as a proxy for potassium to probe potassium binding in bio-molecules such as DNA quadruplexes and enzymes. Moreover, equations have been derived to describe the time-evolution of ¹⁵N-based spin density operator elements of ¹⁵NH₄⁺ spin systems. Herein NMR pulse sequences are derived to select specific spin density matrix elements of the ¹⁵NH₄⁺ spin system and to measure their longitudinal relaxation in order to characterise the rotational correlation time of the ¹⁵NH₄⁺ ion as well as report on chemical exchange events of the ¹⁵NH₄⁺ ion. Applications to ¹⁵NH₄⁺ in acidic aqueous solutions are used to cross-validate the developed pulse sequence while measurements of spin-relaxation rates of ¹⁵NH₄⁺ bound to a 41kDa domain of the bacterial Hsp70 homologue DnaK are presented to show the general applicability of the derived pulse sequence. The rotational correlation time obtained for ¹⁵N-ammonium bound to DnaK is similar to the correlation time that describes the rotation about the threefold axis of a methyl group. The methodology presented here provides, together with the previous theoretical framework, an important step towards characterising the motional properties of cations in macromolecular systems.

Keywords: AX(4) spin systems; Ammonium; Chemical exchange; NMR; Nuclear spin relaxation.