## 3 Results and Discussion  
### The Complexes of Cadmium and Amino Acids  
#### 3.1.1 The Complexes of Gly, Leu, and Ala with Cd\({}^{2+}\)  
Most amino acids exist as zwitterions in which the N-terminus is protonated and the C-terminus is deprotonated [29]. According to the study of [PERSON], four structures of calculated PheCs\({}^{2+}\) complexes were considered starting points for geometry and vibrational frequency calculations [30; 31]. The structures of metal-Gly complexes calculated in previous works were used as starting points for geometry and vibrational frequency calculations for fatty amino acids [32]. The structures of ThrCd\({}^{2+}\) complexes were considered according to the study of [PERSON] and [PERSON] [30; 32]. For GluCd\({}^{2+}\), the structures of GluBa\({}^{2+}\) and GluLi\({}^{+}\) complexes proposed by [PERSON] were considered [33]. Gly, Leu, and Ala were classified as fatty amino acids with similar side chains. The possible isomer structures of Gly-Cd\({}^{2+}\) complexes after calculation are shown in Figure 1. A series of complex structures were found through calculation: [N] structure, where Cd is bound with the backbone amino nitrogen; [CO] structure, where Cd is bound with the backbone carbonyl; [N, OH] structure, where Cd is bound with the amino nitrogen and hydroxyl group; and [CO\({}_{c}\), CO\({}_{c}\)] structure, where Cd is bound with both the oxygens of the carboxylic acid and ammonia, forming a strong H-bond with the carboxylic group of the protonated Gly. \begin{table}
\begin{tabular}{c c} \hline
**Soil Components** & **Values** \\ \hline Organic matter & 4.88\% \\ Clay (!0.002 mm) & 16.92\% \\ Sand (2–0.02 mm) & 68.32\% \\ Silt (0.02–0.002 mm) & 14.76\% \\ pH & 6.7 \\ CEC (meq NH\({}_{4}\)\({}^{+}\)/100 g soil) & 21.41 \\ Total Cd & 20.33 mg/kg \\ \hline \end{tabular}
\end{table} 
Table 2: Basic properties of the studied soils collected from Changing. Figure 1: _Cont._  
In the experimental FTIR spectrum of GlyCd\({}^{2+}\) (Figure 1a), the main characteristic peaks were observed at 1630, 1590, 1480, 1450, 1410, 1325, 1110, 950, and 720 cm\({}^{-1}\). For the calculated FTIR spectra of GlyCd\({}^{2+}\), the most intense peaks in the [CO\({}_{\rm c}\) CO\({}_{\rm c}\)] structure of GlyCd\({}^{2+}\) were relatively well correlated with the experimental spectrum. Briefly, the bands observed at 1640 cm\({}^{-1}\) (rotation vibration of N-H), 1580 cm\({}^{-1}\) (bending vibration of N-H), 1500 cm\({}^{-1}\), 1410 cm\({}^{-1}\) (stretching vibration of C=O), 1325 cm\({}^{-1}\) (stretching vibration of C-O), 1100 cm\({}^{-1}\) (stretching vibration of C-N), and 1040 cm\({}^{-1}\) were consistent with those of the calculated [CO\({}_{\rm c}\), CO\({}_{\rm c}\)] structure. However, the bands at 1660 cm\({}^{-1}\) and 1600 cm\({}^{-1}\) of the [CO\({}_{\rm c}\), CO\({}_{\rm c}\)] structure were redshifted by 20 cm\({}^{-1}\) compared with the experimental spectrum. Moreover, the bands near 1410 cm\({}^{-1}\) and 1350 cm\({}^{-1}\) were redshifted by 25 cm\({}^{-1}\) and blueshifted by 30 cm\({}^{-1}\), respectively. The details of complexes are depicted in Table 3. The lengths of Cd-O of [CO\({}_{\rm c}\), CO\({}_{\rm c}\)] were 2.18 and 2.13 A, which were shorter than those of other structures, and the bond angle of O-Cd-O was 59\({}^{\circ}\). Due to their similar structures, the results of Ala and Leu were similar to those of Gly. As depicted in Figure 1b,c, the isomer structure of AlaCd\({}^{2+}\) and LeuCd\({}^{2+}\) was also [CO\({}_{\rm c}\), CO\({}_{\rm c}\)]. As presented in Table 3, the length of the O-Cd bonds was calculated to be 2.23 and 2.28 A. The O-Cd bond length was longer than that of N-Cd, while a previous study found that the strength of O-Cd was much stronger than that of N-Cd, indicating that the [CO, CO] complex structure was more stable than others [34]. Thus, GlyCd\({}^{2+}\), AlaCd\({}^{2+}\), and LeuCd\({}^{2+}\) mainly exist in the [CO\({}_{\rm c}\), CO\({}_{\rm c}\)] complex structure.