In the absence of the protein translating catalysts of the cell, what would be r

Question

In the absence of the protein translating catalysts of the cell, what would be required to form a peptide bond between two adjacent amino acids such as that found in a proteins primary structure?

If for example that catalyst was available for the formation of life from non-living tissue, how could amino acids be joined without forming peptide bonds with the side chain functional groups of Lysine, Aspartic acid and glutamic acid? How could pH play a role? Is the ionized state of the functional groups important?

Explanation / Answer

Peptide bond formation Two amino acids can undergo a condensation reaction, where the carboxyl group reacts with the amine group. The formation of this peptide bond (see box in figure) produces a dipeptide and a H2O molecule, exhibiting a dipole moment m = 3.7 Debye for the peptide bond (arrow in figure). Note that the dipole moment of the dipeptide is different from the peptide bond dipole moment because of its charged amine and carboxyl groups. Fig. Chemical structure of a dipeptide Adding additional amino acids to the growing peptide chain (always form N-term to C-term direction in living cells) produces a polypeptide chain or protein. The order in which amino acids are linked to the growing chain defines its primary structure or sequence. The geometrical character of the peptide bond is that of a rigid plane between the two flanking a -carbon atoms. The reason for this structural stability (there is no rotation around the C-N bond) can be explained by the electronic resonance character of the O=C-N structure. The double bond character changes between the O-C and C-N bonds. Fig. Electronic resonance character of peptide bond due to the p -like molecular orbital that extends over all 3 atoms (O-C-N) Evidence for this structure comes from X-ray crystallographic studies of simple peptides showing that the N-Ca bond length is 1.46Å as expected for a single bond. The C-N peptide bond is 1.33Å long, only a little longer than the value of 1.27Å for the average C=N bond length in model compounds. Similar X-ray studies show the six atoms Ca NHCOCa very close to being co-planar

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