Chemical Synthesis of Albumin Tag to Aptamers

4-(p-Iodophenyl) butyric acid (Sigma, St. Louis, MO, USA) was converted into N-hydroxysuccinimide (NHS) ester using standard procedures. 4-(p-Iodophenyl) butyric acid NHS ester was used for the conjugation with 3′-amino or 5′-amino-labeled oligonucleotides. One micromole of oligonucleotide was dissolved in 750 μL of water, and 400 μL of DMSO was added, followed by 150 μL of 1.0 M phosphate buffer (pH 7.5). A solution of 5 equiv of 4-(p-Iodophenyl) butyric acid NHS ester in 100 μL of DMSO was added. The reaction mixture was vigorously stirred at room temperature. Progress of the reaction was monitored using high-pressure liquid chromatography (HPLC) on a PRP1 column (Hamilton, Reno, NV, USA) in triethylammonium acetate (TEAA) buffer. Buffer A was 50 mM TEAA in water; buffer B was 50 mM TEAA in acetonitrile-water, 9:1. After 1 h, the reaction was completed. To remove the DMSO, the reaction mixture was precipitated into isopropanol, kept at −20°C overnight, and centrifuged at 4°C for 20 min. The supernatant was discarded, and the resulting pellet was washed with an 8:2 solution of cold ethanol/water.