Chemical aspects of synthetic biology.

Synthetic biology as a broad and novel field has also a chemical branch: whereas synthetic biology generally has to do with bioengineering of new forms of life (generally bacteria) which do not exist in nature, 'chemical synthetic biology' is concerned with the synthesis of chemical structures such as proteins, nucleic acids, vesicular forms, and other which do not exist in nature. Three examples of this 'chemical synthetic biology' approach are given in this article. The first example deals with the synthesis of proteins that do not exist in nature, and dubbed as 'the never born proteins' (NBPs). This research is related to the question why and how the protein structures existing in our world have been selected out, with the underlying question whether they have something very particular from the structural or thermodynamic point of view (for example, the folding). The NBPs are produced in the laboratory by the modern molecular biology technique, the phage display, so as to produce a very large library of proteins having no homology with known proteins. The second example of chemical synthetic biology has also to do with the laboratory synthesis of proteins, but, this time, adopting a prebiotic synthetic procedure, the fragment condensation of short peptides, where short means that they have a length that can be obtained by prebiotic methods; for example, from the condensation of N-carboxy anhydrides. The scheme is illustrated and discussed, being based on the fragment condensation catalyzed by peptides endowed with proteolitic activity. Selection during chain growth is determined by solubility under the contingent environmental conditions, i.e., the peptides which result insoluble are eliminated from further growth. The scheme is tested preliminarily with a synthetic chemical fragment-condensation method and brings to the synthesis of a 44-residues-long protein, which has no homology with known proteins, and which has a stable tertiary folding. Finally, the third example, dubbed as 'the minimal cell project'. Here, the aim is to synthesize a cell model having the minimal and sufficient number of components to be defined as living. For this purpose, liposomes are used as shell membranes, and attempts are made to introduce in the interior a minimal genome. Several groups all around the world are active in this field, and significant results have been obtained, which are reviewed in this article. For example, protein expression has been obtained inside liposomes, generally with the green fluorescent protein, GFP. Our last attempts are with a minimal genome consisting of 37 enzymes, a set which is able to express proteins using the ribosomal machinery. These minimal cells are not yet capable of self-reproduction, and this and other shortcomings within the project are critically reviewed.