Role of apically bound water molecule on aliphatic–aliphatic side-chain interactions in bis[ N, N-dimethyl- l-α-isoleucinato]copper(II) studied by EPR spectroscopy

Abstract

In order to show the importance of conformation of the amino acid side-chains on water binding or water release to the copper(II) in complexes with long aliphatic side-chains, the temperature dependence of Brownian motion of the bis[ N, N-dimethyl- l-α-alaninato]copper(II) complex compared with the bis[ N, N-dimethyl- l-α-isoleucinato]copper(II) complex dissolved in deuterated methanol is analyzed. The hypothesis is that binding or release of the water molecule from copper(II) in the complex with N, N-dimethyl- l-α-isoleucine may be related to the conformation change of the amino acid side-chains. In the case of bis[ N, N-dimethyl- l-α-alaninato]copper(II) the conformation change of the amino acid side-chains (CH 3) is not possible. If the copper(II) complex retains one conformation in the examined temperature interval, the reorientation correlation time τ 2, describing the Brownian motion of the complex will be a linear function of η/ T, as is obtained for bis[ N, N-dimethyl- l-α-alaninato]copper(II). In the whole temperature interval measured (276–332 K), the electron paramagnetic resonance (EPR) spectra are fitted well with characteristic parameters for the complex with bound water. For bis[ N, N-dimethyl- l-α-isoleucinato]copper(II) more complex temperature dependence of τ 2 on η/ T is obtained. Above 300 K, the EPR spectra could not be fitted with the parameters of the complex with bound water, but are fitted well with the parameters characteristic for a copper(II) complex that does not contain an apical water molecule.