Patent ID: 8030279

Claim:
A compound of Formula I: or a pharmaceutically acceptable salt thereof, wherein R 1 and R 2 are independently H or C 1-4 alkyl, or R 1 and R 2 together form the alkyl ring of a proline or homoproline residue; R 3 is selected from the group consisting of a side chain of an amino acid and a first fluorophore; R 4 is H or CH 3 ; R 5 is H, an amine protecting group, an amino acid residue, a polypeptide, a peptide which contains a second fluorophore, a chemical moiety bound to a solid support, or a moiety containing from about 1 to about 50 non-hydrogen atoms; R 6 is an isoleucine side chain or a valine side chain; W is O or NH; X is O; and Y is H or a hydroxyl protecting group; Z is C(O) or C(O)—CH(CH 3 )—C(O); provided that when R 1 and R 2 together form the alkyl ring of a proline residue, then R 4 is H.