In certain embodiments, X5 is selected from D leucine, D isoleucine, {TAIL ~ D alanine}, D proline and {HEAD ~ D lysine} 
In certain embodiments , X5 is selected from D leucine , D isoleucine , { TAIL ~ D alanine } , D proline and { HEAD ~ D lysine }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD