Cyclen based compound of general formula (I), wherein X is nitrogen and Y, Z are -CH-, or X, Z are -CH- and Y is nitrogen, or X, Y are -CH- and Z is nitrogen; R 1 is independently selected from H; COOH; benzyloxycarbonyl; fluorenylmethyloxycarbonyl; tert- butoxycarbonyl; methylcarbonyl; trifluoromethylcarbonyl; benzyl; triphenylmethyl; tosyl; mesyl; benzyloxymethyl; phenylsulfonyl; ethoxycarbonyl; 2,2,2-trichloroethyloxycarbonyl; methoxycarbonyl; methoxymethyloxycarbonyl; R 2 is selected from H; methylcarbonyl; tert -butyldimethylsilyl; (C1-C4)alkyl, which can be linear or branched, and which can optionally be substituted with CH 3 O-, CH 3 S-; oxacyclohexyl; allyl; tert -butyldiphenylsilyl; tert- butylcarbonyl; phenylcarbonyl; nitrobenzyl; benzyloxymethyl, which can optionally be substituted with CH 3 O-, -NO 2 ; fluorenylmethyloxycarbonyl; trichlorocarbonyl; trifluorocarbonyl; benzyl; tosyl; mesyl; phenylsulfonyl; allylsulphonyl; ethoxycarbonyl; 2,2,2-trichloroethyloxycarbonyl; methoxycarbonyl; methoxymethyloxycarbonyl; R 3 is independently selected from H; (C1-C6)alkyl, which can be linear or branched, and which can optionally be substituted with -CH 3 , -Cl, -F, -CN, tosyl, triisopropylsilyl, CH 3 O-, CH 3 S-; (C5-C6)cycloalkyl, which can optionally be substituted with -CH 3 , -Cl, -F, -CN; (C6-C10)aryl, which can optionally be substituted with -CH 3 , -Cl, -F, -CN; allyl, propargyl; fluorenylmethyl; benzoylmethyl; phenyloxymethyl; oxacyclopentyl; 2-oxo-1,2-diphenylethyl; with the proviso that where R 1 is bound to nitrogen, then R 1 is not COOH; with the proviso that where R 1 is bound to -CH-, then R 1 is independently H or COOH; with the proviso that one R 1 is COOH; and with the proviso that one -CH-R 1 group is -CH 2 -. X is nitrogen and Y, Z are -CH-, or X, Z are -CH- and Y is nitrogen, or X, Y are -CH- and Z is nitrogen; R 1 is independently selected from H; COOH; benzyloxycarbonyl; fluorenylmethyloxycarbonyl; tert- butoxycarbonyl; methylcarbonyl; trifluoromethylcarbonyl; benzyl; triphenylmethyl; tosyl; mesyl; benzyloxymethyl; phenylsulfonyl; ethoxycarbonyl; 2,2,2-trichloroethyloxycarbonyl; methoxycarbonyl; methoxymethyloxycarbonyl; R 2 is selected from H; methylcarbonyl; tert -butyldimethylsilyl; (C1-C4)alkyl, which can be linear or branched, and which can optionally be substituted with CH 3 O-, CH 3 S-; oxacyclohexyl; allyl; tert -butyldiphenylsilyl; tert- butylcarbonyl; phenylcarbonyl; nitrobenzyl; benzyloxymethyl, which can optionally be substituted with CH 3 O-, -NO 2 ; fluorenylmethyloxycarbonyl; trichlorocarbonyl; trifluorocarbonyl; benzyl; tosyl; mesyl; phenylsulfonyl; allylsulphonyl; ethoxycarbonyl; 2,2,2-trichloroethyloxycarbonyl; methoxycarbonyl; methoxymethyloxycarbonyl; R 3 is independently selected from H; (C1-C6)alkyl, which can be linear or branched, and which can optionally be substituted with -CH 3 , -Cl, -F, -CN, tosyl, triisopropylsilyl, CH 3 O-, CH 3 S-; (C5-C6)cycloalkyl, which can optionally be substituted with -CH 3 , -Cl, -F, -CN; (C6-C10)aryl, which can optionally be substituted with -CH 3 , -Cl, -F, -CN; allyl, propargyl; fluorenylmethyl; benzoylmethyl; phenyloxymethyl; oxacyclopentyl; 2-oxo-1,2-diphenylethyl; with the proviso that where R 1 is bound to nitrogen, then R 1 is not COOH; with the proviso that where R 1 is bound to -CH-, then R 1 is independently H or COOH; with the proviso that one R 1 is COOH; and with the proviso that one -CH-R 1 group is -CH 2 -. <EOS>