Lu Guanghua,  Jiang Shiquan^#,  Zhao Yuanhui
(Department of Environmental Science, Northeast Normal University, ChangChun, 130024; ^#Design Research Institute of Oil Chemical, Jilin Province, Changchun  )

Received Sep. 29, 2000; Supported by the National Natural Science Foundation of China (29877004).

Abstract The heat of formation(DH_f), the energy of the highest occupied molecular orbital(E_HOMO), the molecular weight(MW) of 52 compounds of substituted benzenes were calculated by the quantum chemical method MOPAC6.0-AM1. The quantitative structure-biodegradability relationship studies were performed with the maximum specific removal rates (Q_TOD). Through multiple linear regression analysis, one equation was obtained as follows: Q_TOD=-0.614MW-9.704E_HOMO-0.129DH_f , n=52, R²=0.860, SE=14.89, F=100.21, P=0.000. The QSBR equation was used to predict biodegradability and biodegradation mechanism was discussed.
Keywords structure parameter, biodegradation correlation, QSBR model

1. INTRODUCTION    
The number of synthetic organic chemicals produced is large and increasing every year. The presence of many of these chemicals in the ecosystem is a serious public health problem. Biodegradation is an important mechanism for removing them from natural ecosystems. The high diversity of species and the metabolic efficiency of microorganisms suggest that they play a main role in the ultimate degradation of these chemicals ^[1]. Biodegradation can eliminate hazardous chemicals by biotransforming them into innocuous forms, degrading by mineralization to carbon dioxide and water.
    Information regarding the extent and the rate of biodegradation of organic chemicals is very important in evaluating relative persistence of the chemical in the environment, which is important for regulating its manufacture and use. However, gathering this information is labor intensive, time consuming, and expensive due to the large number of these chemicals. Therefore, it is necessary to develop correlation and predictive techniques in order to estimate their biodegradability ^[2]. At present, the quantitative structure-biodegradability relationships (QSBRs) have been used to predict the fate of organic chemicals and analyse biodegradation mechanism ^[3].

2. SOURCE OF DATA
The maximum specific removal rates of aromatic organic compounds were obtained using an adapted mixed culture (activated sludge). This microbial culture was cultivated semicontinuously on glucose and peptone and subsequently on the compound under study at a mean biomass retention time (MBRT) of approximately 5 days. This MBRT corresponds to biological treatment in the classical activated sludge wastewater treatment process. During the biodegradation experiment the substance tested was the sole source of organic carbon for the microbes of the culture. The initial concentration of the compounds tested corresponded to the oxygen equivalent (TOD-theoretical oxygen demand) of 200 mg/l. The maximum specific removal rates are expressed as mg TOD of the compound removed per gram of the initial solids of the microbial culture per hour (Q_TOD). Q_TOD values of 52 compounds of substituted benzenes were cited from ^[4].

3.CALCULATION METHODS  
MW, DH_f and E_HOMO of the substituted benzenes were calculated by the quantum chemical method MOPAC6.0-AM1 on energy-minimized structures. This method can automatically optimize the bond length, the bond angle and the twist angle, and yield a lot of structure information. Octanol-water partition coefficient (logP) was obtained from Biobyte package. The parameter values of 52 compounds were listed in Table 1. All statistical analyses were carried out using SPSS package.

4.RESULTS AND DISCUSSION         
Biodegradation of a chemical in the aquatic environment is predominantly through microbial attack, and thus, we may presume, via enzymatic processes ^[3]. In general, the factors determining the rate of biodegradation can be divided into two kinds. (1) The uptake rates and transport rates (e.g., the uptake rates by microbial cells or transport rates within the cell to the relevant enzymes). (2) The rates binding to the active site of an enzyme, and/or by the rate at which they undergo enzymatic transformation. In the absence of a specific uptake mechanism, organic compounds are probably transported into bacterial cells by passive diffusion through the lipid membrane. If the diffusion of chemicals in cell membrane and water belong to partition process, the diffusion coefficient should be a direct proportion to logP. Therefore, biodegradation rates should be related macroscopic hydrophobic parameters if diffusion and uptake are rate-limiting step of biodegradation. The enzyme-catalyzed transformation of a compound occurs by its binding to the site of the enzyme through the formation of hydrogen or covalent bonds. The strength of this interaction is influenced by the electronic structure of compound and the steric structure of compound coinciding with the active site of enzyme. So, if bind to enzyme or transformation is rate-limiting step, biodegradation rate of a compounds should be related to the factors influencing the binding or reacting with enzyme (e.g. steric and/or electronic parameters).

Table 1.The experimental and predicted biodegradability and structure parameters of 52 compounds of substituted benzenes

*Pre. is the predicted value calculated from equation (2).

MW, DH_f, E_HOMO and logP are selected as the structure parameters to establish the QSBRs. Through multiple linear regression analyses, two models were obtained as Table 2.

Table 2. The results of regression analyses (including constant or not)

Model 1 and 2 can be expressed as equation (1) and (2) respectively as follows:
Q_TOD=8.549-0.606MW-8.688E_HOMO-0.129DH_f , n=52, R²=0.677, SE=15.03, F=33.48, P=0.000              (1)
Q_TOD=-0.614MW-9.704E_HOMO-0.129DH_f , n=52, R²=0.860, SE=14.89, F=100.21, P=0.000                    (2)
    There, R² is the square of correlation coefficient; SE is the standard error; F is the mean square radio; P is the significant level; and n is the number of compounds.The results in Table 2 show that SE of the constant item of model 1 is very high; through comparing equation (1) and (2), R² and F value of (2) are much higher, and SE of (2) is lower. Therefore, (2) was used to estimate the biodegradability (Table 1).
    The biodegradability of 52 substituted benzenes can be divided into three kinds according to their Q_TOD values in Table 1: non-readily biodegradable (Q_TOD<30mg/g·h), biodegradable (30-60 mg/g·h), and readily biodegradable (Q_TOD>60 mg/g·h). The correct prediction rate of model 2 is up to 80%.
    The obtained QSBR models show that the biodegradation of studied compounds is related mainly to steric parameter MW, and electronic parameter DH_f and E_HOMO. MW is the molecular weight, and it can reflect the size of a molecule. The smaller the molecule, the smaller the steric resistance in microorganisms, the easier the molecule penetrate into the cell through its cell membrane and arrive at the active site of an enzyme ^[5]. Therefore, the smaller the MW, the easier the compound is biodegraded. The heat of formation (DH_f) of a compound is a measure of its stability, and hence it would be expected to reflect the ability of the compound to biodegrade ^[6]. The lower the DH_f value, the easier the chemical is degraded by microbes. E_HOMO is the energy of the highest occupied molecular orbital, and it is related to ionization potential ^[7]. The higher the -E_HOMO, the higher the Q_TOD values, which shows that the enzyme-catalyzed reaction of the compounds is related to electronic transfer.
    Poor correlation between Q_TOD and logP (r is only 0.01) adding above analyses demonstrates that the rate-limiting step within the overall biodegradation process is the rates of binding with the active site of an enzyme and enzymatic reaction instead of the rates of uptake and transport.

REFERENCES            
[1] Alexander M. Science, 1980, 211: 132-138.
[2] Sterier M P.  Environ. Sci. Technol., 1980, 14-28-31.
[3] Dearden J C. SAR and QSAR in Environmental Research, 1996, 5: 17-26.
[4] Petter P, Sykora V.  Biodegradation Prediction, Peijnenburg W J G M and Damborsky J (ed.) Academic Press, 1996,17-26.
[5] Boething R S.  Environ. Toxicol. Chem., 1986, 5: 797-806.
[6] Dearden J C, Cronin M T D.  Biodegradation Prediction, Peijnenburg W J G M and Damborsky J (ed.) Academic Press, 1996, 93-105.
[7] Liu C Q. Quantum Biology and Application, Beijing: Higher Education Press, 1990, 16.

[ Back ] [ Up ] [ Next ] Mirror Site in USA  Europe  China  CSTNet ChinaNet