## 1 Introduction  
\"Itai-Itai disease\", which occurred in Japan during the 1950s and aroused worldwide concern, was attributed to the prolonged oral intake of cadmium-contaminated rice [1]. Unlike organic contaminants, cadmium cannot be removed via microbial or chemical degradation [2] and poses a threat to human health and ecosystems. Thus, the contamination of soils and sediments by cadmium has been recognized as an urgent global problem [3]. A large number of approaches have been developed to remediate Cd-contaminated soils, such as chemical fixation, chemical leaching, electrokinetics, encapsulation, and biosparging [4]. Among them, ex situ soil washing has been successfully applied for decades due to its high efficiency and economic feasibility. Metal bioavailability mainly depends on the chemical speciation of metals and the activity of free Cd ions in soil solution [5]. The ligands that exist in soils are able to form stable complexes with metal ions and affect the mobility and toxicity of heavy metals. The soil washing method usingorganic ligands has been widely used to remove Cd from contaminated soil [6]. In soil washing processes, powerful synthetic Cd chelating agents such as ethylene diamine taraectic acid (EDTA), and diethylene triamine pentaacetic acid (DTPA) have been widely used. However, residual nonbiodegradable synthetic chelators in soils introduce ecological damage to the soil structure through the aggressive extraction of calcium, causing harm to plants [7]. Thus, the use of natural organic ligands to remedy contaminated soil has attracted increasing attention [8; 9; 10]. Recently, plant root exudates, a kind of widely existing natural organic ligand, have had a great impact on the bioavailability and mobility of heavy metals in the soil [11; 12; 13; 14]. They display lower extraction efficiency than EDTA and DTPA toward heavy metals; nevertheless, they possess the advantages of a soil-friendly ability and biodegradability [15]. Among these exudates, natural amino acids with functional groups have significant effects on the mobility and bioavailability of heavy metals by forming stable complexes with heavy metals [16; 17; 18]. The study of [PERSON] et al. (2013) indicated that amino acids could produce stable complexes with heavy metals and affect the mobility of Cd. Thus, natural amino acids might have great potential for the remediation of Cd-contaminated soil. According to [PERSON] et al [19], the common amino acids in soil are glutamic acid (Glu), alanine (Ala), glycine (Gly), threonine (Thr), leucine (Leu), and phenylalanine (Phe). Gly has been applied for the extraction of Cu, Pb, and Zn from soils, and it was found to be competitive with synthetic chelators [20]. In the amino groups, the carboxyl group is an electron-donating group capable of complexing metal ions, and the amino group is an amphoteric group capable of donating and obtaining electrons [21]. According to [PERSON] and [PERSON], despite the proton affinities of amino acids, the structure is a critical factor in determining the stability of metal-amino acid complexes [22]. For example, the nonzwitte-rionic structures of lithiated arginine are more stable than the corresponding zwitterionic structures since the heteroatoms in side chains play a critical role in the complexity [23]. Thus, the molecular fragments of Cd-amino acid complexes are worth investigating. Nevertheless, to the best of our knowledge, the complexation mechanisms of the molecular fragments of amino groups with heavy metals have rarely been illustrated in detail. Herein, six typical amino acids in soil, Gly, Leu, Ala, Glu, Thr, and Phe, were chosen as model organic chelators, and their complexation mechanisms with Cd\({}^{2+}\) were fully investigated via theoretical calculations and Fourier transform infrared spectrometry (FTIR) tests. Theoretical calculation is a widely used method to probe the affinity of amino acids toward metals and to investigate the structures of the obtained complexes [24].