Conformation and dynamics of flexible molecules from nuclear relaxation induced by Dy3+ and Gd3+

Abstract A general procedure for studying the rotational isomerism and the dynamic behavior of flexible molecules in solution is reported, under conditions where the 13 C relaxation is enhanced by paramagnetic ions. If the electron spin relaxation times are significantly shorter or longer than the time scale of internal motions, the nuclear relaxation rates depend, respectively, on the conformer populations only or on the absolute reorientation rates. This procedure is applied to the conformational study of lysine, norleucine, leucine, and methionine in aqueous solutions, the pH (or pD) being adjusted to bind selectively the Dy 3+ and Gd 3+ ions to the carboxyl group, forming a bidentate adduct. The formation constants of the 1:1 and 2:1 complexes as well as the effective coordination number of the amino acids to the Ln 3+ ions are derived from the concentration dependences of the 13 C paramagnetic shifts. In lysine and norleucine, the gauche α trans isomerizations of the chains about the CC bonds take place at frequencies of the order of 10 10 sec −1 at 300 K, while the rates of internal motions are smaller by a factor 10 to 20 in leucine and methionine, where these rotations are partially restricted. The rotamer populations and interconversion rates are found in agreement with those obtained from the 3 J HH coupling constants and 13 C relaxation in the corresponding La 3+ diamagnetic complexes or in the uncomplexed molecules.