Magnetic Properties of Ferromagnetic Quasi-1D Copper-Peptide Compounds:  Exchange Interactions and Very Low Temperature Phase Transitions

Abstract

The magnetic properties of the Cu(II)-peptide compounds (L-tyrosyl-L-leucinato)Cu(II) and (L-tryptophyl-glycinato)Cu(II), to be identified as Cu(II)Tyr-Leu and Cu(II)Trp-Gly, respectively, have been investigated by specific heat (0.08 < T < 28 K), dc magnetization (2 < T < 80 K, with B(0) = mu(o)H < or = 9 T), and ac magnetic susceptibility (with B(0) = 0 for 0.03 < T < 3 K and B(0) up to 9 T for 2 < T < 80 K) measurements. Above approximately 1 K, the specific heat and magnetization of both compounds display a ferromagnetic (FM) spin chain behavior sustained by syn-anti carboxylate bridges connecting equatorially Cu(II) ions at about 5 A. To model this behavior, we calculated the eigenvalues of Heisenberg chains with up to 20 spins 1/2 and used the method of Bonner and Fisher. A global fit of the model to the specific heat and magnetization data gives 2J(0)/k(B) = 3.60(5) K and 2.59(5) K for the intrachain exchange interactions in Cu(II)Tyr-Leu and Cu(II)Trp-Gly, respectively (H(ex)(i,j) = -2J(0) S(i).S(j)). These values of 2J(0) are discussed in terms of structural properties of the carboxylate bridges in the two compounds. Using the parameters obtained from the global fit, we calculated isothermal susceptibilities in agreement with the ac susceptibilities measured with small applied dc magnetic fields. However, the ac susceptibility measured with applied dc fields larger than 1 T lie between the values calculated for the isothermal and adiabatic susceptibilities. At 0.16 K for Cu(II)Tyr-Leu and 0.53 K for Cu(II)Trp-Gly, the observed specific heat and magnetic susceptibility display peaks associated to three-dimensional magnetic phase transitions. The interchain exchange couplings 2J(1) producing the 3D magnetic order are ferromagnetic and have magnitudes 2J(1)/k(B) approximately 0.015 and 0.073 K for Cu(II)Tyr-Leu and Cu(II)Trp-Gly, respectively.

EPR measurements of weak exchange interactions coupling unpaired spins in model compounds

“…However, they may be the leading factors in the magnetic properties (see, for example, refs. 45,47,50,55,63,67,68). In other cases the thermodynamic measurements allow measuring interactions with magnitudes larger than those that can be observed by EPR [50,63].…”

“…), and structural [64] measurements of the exchange coupling between Cu ions at 0.973 nm, connected by a chemical path containing 12 diamagnetic atoms including one H-bond were reported for the copper compound of the peptide Tyr-Leu. At temperatures above 1 K this compound displays specific heat and magnetic susceptibility characteristic of ferromagnetic spin chains with intrachain coupling `/= 2.50 cm -~ [63]. At 0.16 K the magnetic properties show a discontinuity characteristic of a transition to a magnetically ordered phase.…”

“…The specific heat and the susceptibility data above 1 K display a linear chain behavior with J = 1.80 cm -~ sustained by the carboxylate bridges. At 0.073 K, the specific heat and susceptibility data show a transition to a 3-D ordered magnetic phase [63]. Conside¡ the temperature of the transition and using a spin wave model for the magnetic excitations it was estimated JP = 0.051 cm -1 for the interchain interaction.…”

“…These findings indicate that it is valid to estimate values for the exchange interactions between redox centers in proteins transmitted along long chemical paths containing sigma and H bonds, from data obtained in model systems, and emphasize the relevance of measuring exchange interactions in biologically relevant model systems. Thermodynamic [63], EPR (N. M. C. Casado et al, Universidad Nacional del Litoral, Santa Fe, Argentina, unpubl. ), and structural [64] measurements of the exchange coupling between Cu ions at 0.973 nm, connected by a chemical path containing 12 diamagnetic atoms including one H-bond were reported for the copper compound of the peptide Tyr-Leu.…”

“…Thermodynamic, EPR and structural measurements [50,63,65] have been reported for the copper complex of the peptide Trp-Gly. The strucmre of the compound displays spin chains with Cu ions connected by equatorial-equatorial syn-anti carboxylate bridges similar to those described for Cu(II)Tyr-Leu.…”