. . "u, Pro, or Ser; most preferably, X8 is Pro; X10 is an amino acid other than Cys and having an aliphatic uncharged polar or aliphatic nonpolar (hydrophobic) side chain (i.e., X10 is Leu, Ile, Met, Asn, Gln, Ser, Thr, or Val); preferably X10 is Leu or Thr; X11 is an aromatic hydrophobic amino acid (i.e., X11 is Phe, Trp, or Tyr); X12 is Cys or another amino acid capable of forming a covalent cross-l" . .