wherein: Xaa at position 3 is any naturally occurring amino acid except Cys and Lys; Xaa at position 27 is either Thr or absent; Xaa at position 28 is selected from the group consisting of Pro, Leu, Val, Ala, Lys, and Asp; Xaa at position 29 is selected from the group consisting of Pro and Lys; Xaa at position 30 is absent or any naturally occurring amino acid except Cys or Lys; further wherein po