| ?-Poly-L-Lysine(?-PL)is a homopolymer(amino acid)composed of several lysine residues connected by ?-COOH and ?-NH2.The molecular mass is 780-4600 Da.Due to the presence of many free amino groups in its main chain,it exhibits multi-cationic properties in acidic to slightly alkaline environments and has an inhibitory effect on a variety of microorganisms,including most Gram-positive and Gram-negative bacteria,yeasts,molds and bacteriophages.?-PL is synthesized by non-ribosomal polypeptide synthesis with L-lysine as precursor under the catalysis of enzyme.Its antibacterial properties affected by the degree of polymerization(DP).When DP < 9,bacteriostatic activity is very low,and bacteriostatic activity is higher when DP > 9.When the DP is 25-35,it has the best antibacterial property.The aim of this paper is to find an efficient separation process,and to improve the content of ?-poly-lysine with high DP(25-35).In this paper,the separation of high degree of polymerization ?-PL by alcohol precipitation,membrane and ion exchange was studied.The main research results are as follows:(1)Separation of ?-PL with high degree of polymerization by method of alcohol precipitation.The relationship between the separation efficiency of ?-PL with high DP and the final concentration of ethanol,the initial material concentration,pH,temperature and the precipitation time was investigated.The results show that the higher the final concentration of ethanol and the higher the initial concentration of material,the higher total recovery,but the worse selectivity ?-PL,and the worse pigment removal effect.At pH=9(pH=p I),the recovery of ?-PL is the highest,and the selectivity is good,but the pigment removal effect is poor.Reducing the temperature can improve the ?-PL recovery and pigment removal,but reduce the selectivity of ?-PL.When the initial material concentration was increased to 200 g·L-1,the volume ratio of solid to liquid ethanol was 1:4,pH=9,and the temperature was 4?for 36 h,the ?-PL recovery was 45.76%,the pigment removal rate was 22%,and the content of ?-PL with high DP was 87.41%,which was 5.41% higher than that of the original product.(2)The effect of membrane separation technology on the separation of ?-PL with high polymerization degree was studied.PES membrane with a molecular weight cut-off of 3000 is most suitable.After dialysis of the feed solution with operating pressure of 0.25 MPa,pH=6and concentration of 20 g·L-1,the content of ?-PL with a degree of polymerization of 25-35 in the circulating fluid increased to 91.22%.The product after dialysis is more conducive to the inhibition of S.aureus and E.coli.A model was built for the separation process of ?-PL with membrane separation.It was explained that although the membrane separation process could not achieve the absolute separation of ?-PL with different degree of polymerization,it could increase the proportion of ?-PL with high degree of polymerization in the product and improve the antibacterial performance of the product against bacteria.(3)Separation effect of ?-PL with high DP by ion exchange.The elution characteristics of ?-PL on ion exchange resin SQD-04 and IRC-50 were investigated.It was found that the low-concentration neutral salt solution can preferentially exchange with the low DP ?-PL on the SQD-04 resin.NaCl is the best,and the elution rate of saturated NaCl solution was 2.76%.Increasing the concentration and pH of saline solution could increase the elution rate,but decrease the selectivity.At pH=12,the elution rate of saturated NaCl solution increased to31.27%,31.08% for ?-PL with high polymerization degree,and 22.78% for ?-PL with low polymerization degree.After elution with 1.8 mol·L-1 NaCl solution and 1.0 mol·L-1 Na OH solution,the total elution rate is 96.47%,and the high degree of polymerization ?-PL content in the collected product is 84.33%,which was 1.49% higher than that of the original product.The antibacterial activity against S.aureus and E.coli was same with that of the original product. |
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