12332A stock solution comprising a compound of formula (I) wherein R together with the carbonyl group to which it is attached is a residue of a carboxylic acid and NRR in which R is H or together with R and the N atom to which they are attached a 5 membered ring is a residue of an amino acid in particular a proteinogenic amino acid ornithine gamma aminobutyric acid or beta alanine or a 1 amino cycloalkyl carboxylic acid.