Paramagnetic ion binding to amino acids, structural and dynamical information on a Mn(II) complex of L-proline from 13C relaxation data

Abstract

The 13C relaxation times (T 1 and T 2) and isotropic contact shift (Δω) of 1.28 molar aqueous solutions of L-Proline at pH = 11 (or pD = 11.4) containing 10 −4 - 10 −5 M manganese perchlorate are measured at 62.86 MHz over a temperature range of 28–80°C. Under these conditions, the Mn 2+ cation is bound to three L-Proline molecules in their dibasic form, and a fast exchange is occurring between bound and bulk L-Proline molecules. The longitudinal relaxation of carbons α, β, γ, δ of L-Proline molecules in this complex is shown to be purely dipolar, and is controlled by the rotational reorientation of the complex. The transverse relaxation of bound L-Proline molecules is mainly scalar and is controlled by the electronic relaxation. Overall relaxation rates and paramagnetic shifts also depend on the ligand exchange rate k M (from bound to free sites) at lower temperatures. The measurement of these quantities allow us to determine (i) the structure of the complex: the Mn(II) cation may be positioned with respect to each proline ligand, the sites of coordination are the unchanged nitrogen and one carboxylic atom, the distance to the Mn 2+ cation are respectively 2.08 and 1.97 Å; (ii) Hyperfine coupling constants: A= + 0.16; 0.08; 0.25 and 0.22 MHz for carbons α, β, γ, δ, respectively. (iii) Electronic relaxation parameters: assuming that T 1e ( = 2.18 x 10 −8 s at 25°C) is controlled by the modulation of the quadratic crystalline zero-field splitting interaction allows us to estimate the trace of the corresponding tensor: Δ = 0.0305 cm −1, and a correlation time τν(25°C) = 1.32 ps for the impact of solvent molecules against the Mn 2+-L-Proline complex (iv) Kinetic parameters for ligand exchange: k M(25°C) = 7.41 x 10 4s −1; ΔH ‡ = 15.6 kcal.mol. −1; ΔS ‡ = 16.1 e.u.