D-amino acids are never found in proteins, although they exist in nature. D-amino acids are often found in polypetide antibiotics. The aromatic R-groups in amino acids absorb ultraviolet light with an absorbance maximum in the range of 280nm. The ability of proteins to absorb ultraviolet light is predominantly due to the presence of the tryptophan which strongly absorbs ultraviolet light. The Peptide Bond Peptide bond formation is a condensation reaction leading to the polymerization of amino acids into peptides and proteins.
Peptides are small consisting of few amino acids.
Proteins are polypeptides of greatly divergent length. The simplest peptide, a dipeptide , contains a single peptide bond formed by the condensation of the carboxyl group of one amino acid with the amino group of the second with the concomitant elimination of water. The peptide bond is slightly shorter than a standard single bond, reflecting the partial delocalization of pi electrons from the carbonyl group into orbitals shared with the lone pair electrons of the amide nitrogen. A polypeptide chain may be considered as a series of planes with twoangles of rotation between each plane. View a polypetide fragment with rotable Phi and Psi, in a kinemage and give a look to the tutorial prepared at Newcastle University.