Radiosynthesis of C-11 Labeled Auxin (3-Indolyl[1-11C]acetic Acid) and Its Derivatives from Gramine

Abstract

3-Indolylacetic acid (IAA) is the major auxin in higher plants and plays a key role in plant growth and development. We report the rapid radiolabeling of the important plant hormone using carbon-11 (half life: 20.4 min) enabling in vivo imaging of its distribution and movement in whole plants. 3-Indolyl[1-11C]acetic acid was synthesized in 2-steps: (1) reaction of gramine with [11C]cyanide to give 3-indolyl[1-11C]acetonitrile in >99% radiochemical purity; (2) hydrolysis of the intermediate in aqueous sodium hydroxide solution to give 3-indolyl[1-11C]acetic acid in >98% radiochemical purity after HPLC purification. The overall nondecay corrected radiochemical yield was 28%, synthesis time was 68 min and specific activity was (0.7 mCi/nmol). Hydrolysis proceeded through the formation of 3-indolyl[1-11C]acetamide and by varying the temperature of this step, either C-11 labeled acid or amide were obtained. This procedure provides unexpectedly high C-11 incorporation in a short time and using a simple and selective hydrolysis without the need of an indole-nitrogen protecting group or a typical leaving group. Since 3-indolylacetonitrile and 3-indolylacetamide are also intermediates in the biosynthesis of IAA, and also function as auxins, this versatile reaction makes all three of these labeled compounds available for imaging studies in whole plants in vivo.