Amino-acid synthesis in aqueous media under ultrasonic irradiation

Abstract

The now-famous "Miller-Urey experiment" used a reduced mixture of gases to form basic organic molecules, such as amino acids. Since then, several methods have tried to reproduce the synthesis of amino acids under unusual conditions. All authors agree that the formation mechanisms of amino acids under such conditions might be of radical origin. If so, sonochemistry, known to enhance or to promote radical pathways, should be a powerful tool to access the synthesis of these compounds. We explored the feasibility of amino acid synthesis under ultrasonic irradiation in aqueous medium under a reductive atmosphere. The effects of several experimental parameters such as incident ultrasound frequency, temperature, nature and concentration of initial reactants, reaction time, and acoustic energy were explored.In 1953, Miller studied the formation of amino acids by the action of electric discharges on a mixture of methane, nitrogen, and water with traces of ammonia [1]. This synthetic reaction, known as the strecker reaction, provides one of the most efficient methods for the synthesis of D-amino nitriles, which are hydrolyzed to give amino acids [2][3][4]. These moieties are not only very versatile intermediates for the synthesis of D-amino acids [5] but also for 1,2-diamines [6], amides [7], and various nitrogen-containing heterocycles such as thiadiazoles and imidazoles [8].Ultrasound is known to enhance some chemical processes through a physical phenomenon called cavitation, which is the formation, growth, and collapse of bubbles in an elastic medium. By imploding, these bubbles create local high pressures (up to 1000 bars) and temperatures (up to 5000 K), which leads to high-energy radical mechanisms with some physical effects such as micro-mixing, mass transport, or reduction of particles size [9][10][11]. It is well known that many organic reactions driven by a radical pathway can be accelerated or promoted when submitted to ultrasound [11][12][13][14][15]. Compared with traditional methods, this technique is more convenient taking green chemistry concepts into account.Our hypothesis is based on the sonolysis of water, which leads to the formation of radical intermediates OH with a strong dependence of concentration on the working frequency [16]. If the production rate of OH is at a reasonable level, it should react with a very simple organic molecule to afford basic amino acids.The water-formamide system is of prominent interest; it is one of the simplest molecules usually chosen as a model for studying biological systems exhibiting the peptide type of bonding and DNA structures [17][18][19][20][21][22][23]. The nature of the hydrogen bonding between formamide and water can explain the hydrogen-bonding mechanism expected in the hydration of proteins [24][25][26]. The formamide-water (HCONH 2 -H 2 O) system is an important model system for investigating hydrophilic and hydrophobic interactions. The endothermal effects observed in water-formamide mixtures indicate that water loses its structure as a result of specific...

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