[NH2:1][CH2:2][CH2:3][CH2:4][CH2:5][CH2:6][CH2:7][CH2:8][CH2:9][CH2:10][CH2:11][C:12](O)=O.C(N(CC)CC)C.C1C(SSC2C=CC([N+]([O-])=O)=C(C(O)=O)C=2)=CC(C(O)=O)=C([N+]([O-])=O)C=1.CN(C=O)C>O1CCCC1>[NH2:1][CH2:2][CH2:3][CH2:4][CH2:5][CH2:6][CH2:7][CH2:8][CH2:9][CH2:10][CH2:11][CH3:12]In this example, the spacer unit is 11-amino-undecanoic acid. The spacer (411 mg, 2.04 mmol, 4 eq) is suspended in a dry organic solvent, for example tetrahydrofuran (THF). Depending on its solubility and the stability of the Raman marker unit, the suspension temperature can be varied between −78° C. and reflux. First, an auxiliary base (2.1 ml triethylamine) is added. Then, the activated marker unit in a suitable organic solvent, for example DTNB-NHS (300 mg, 0.51 mmol in 10 ml THF and 5 ml DMF), is added dropwise. Depending on the reactivity, the reaction mixture is stirred 10 to 48 h, in this case 16 h. The solvent is removed by vacuum, the remaining solid is washed with NaHSO4 solution and a suitable organic solvent. Further purification can be achieved by column chromatography, preferably on reversed phase silica (yield: 260 mg, 0.34 mmol, 67%).
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