1. The selected polypeptide containing the following amino acid sequence:! X 1 X 2 X 3 X 4 X 5 X 6 X 7 X 8 X 9 X 10 X 11 X 12 X 13 X 14 X 15 X 16 X 17 X 18 X 19 X 20! (SEQ ID NO: 1),! where X1, X7, and X15 are amino acids independently selected from the group consisting of E and D； ! X4, X11, and X18 are amino acids independently selected from the group consisting of E, D, A, and G； ! X2, X6, X9, X10, X12, X13, X16, X17, and X20 are amino acids independently selected from the group consisting of F, L, W, I, V, and A； ! X3, X5 and X19 are amino acids independently selected from the group consisting of R, K and C； ! X14 represents amino acid R, A, E or C； and! X8 represents A, G or V.! 2. The selected polypeptide according to claim 1, where X10, X12, X16 and X17 are independently selected from the group consisting of F, L, I and W.! 3. The selected polypeptide according to claim 2, where X10, X12, X16 and X17 are independently selected from F and L.! 4. The selected polypeptide according to claim 1, where X9 represents L, F or W.! 5. The selected polypeptide according to claim 1, where at least three of the positions X2, X6, X12 and X17 represent L.! 6. The selected polypeptide according to claim 1, where one or more of the provisions of X4, X8 and X11 represents A.! 7. The selected polypeptide according to claim 6, where X8 represents A.! 8. The selected polypeptide according to claim 1, where X4, X11 and X18 are independently selected from the group consisting of D and E.! 9. The selected polypeptide according to claim 1, where C is present in one position selected from the group consisting of X3, X3, X14 and X19. ! 10. The selected polypeptide according to claim 1, where the specified selected polypeptide contains:! X1X2X3X4X5X6X7AX9X10X11X12X13X14X15X16X17X18X19X20! (SEQ ID NO: 27),! where X1, X7, X15 and X18 are independently selected from the group consisting of D and E； ! X2 represents L, I or V； ! X4 and X11 are independently selected from the group consisting
