The present invention provides compounds of the formula where R1-R5 are each, independently, a hydrogen atom or a normal or branched C1-C6-alkyl group; A is a methionyl, phenylalanyl or phenylglycyl residue; n is 0 or 1; R6 is a hydrogen atom; and R7 is a carbocylic group, an aromatic group, a C1-C4-alkyl group, a pyridylalkyl group or a heterocyclic group. In another embodiment, R6 is benzyl or —C(O)OR8, where R8 is a C1-C6-alkyl group, and R7 is a heteroaromatic group, such as a 2-thiazolyl group.