Question: What is one of the amino acids used in the biosynthesis of proteins, that is in a synthetic peptide consisting of eight amino acids?
Context: Protein structure is the three-dimensional arrangement of atoms in a protein molecule.  Proteins are polymers — specifically polypeptides — formed from sequences of amino acids, the monomers of the polymer.  A single amino acid monomer may also be called a residue indicating a repeating unit of a polymer.  Proteins form by amino acids undergoing condensation reactions, in which the amino acids lose one water molecule per reaction in order to attach to one another with a peptide bond.  By convention, a chain under 30 amino acids is often identified as a peptide, rather than a protein.  To be able to perform their biological function, proteins fold into one or more specific spatial conformations driven by a number of non-covalent interactions such as hydrogen bonding, ionic interactions, Van der Waals forces, and hydrophobic packing.  To understand the functions of proteins at a molecular level, it is often necessary to determine their three-dimensional structure.  This is the topic of the scientific field of structural biology, which employs techniques such as X-ray crystallography, NMR spectroscopy, and dual polarisation interferometry to determine the structure of proteins. Peptide amphiphiles are peptide-based molecules that self-assemble into structures including high aspect ratio nanofibers.  A peptide amphiphile typically comprises a hydrophilic peptide sequence with an attached lipid chains, in this case being a lipopeptide.  They were first described by the group of Matthew Tirrell in 1995.  These molecules are often composed of multiple domains which allow self-assembly into various supramolecular structures.  The individual domains associate with each other and result in growth of larger assemblies.  Work by Hartgerink et al. in the early 2000s demonstrated a peptide amphiphile which has three regions: a hydrophobic tail, a region of beta-sheet forming amino acids, and a peptide epitope designed to allow solubility of the molecule in water, perform a biological function by interacting with living systems, or both.  Self-assembly occurs by the combination of hydrogen-bonding between beta-sheet forming amino acids and hydrophobic collapse of the tails to yield the formation of cylindrical micelles that present the peptide epitope at extremely high density at the nanofiber surface.  In a newer system developed by Hartgerink in the mid 2000s, multidomain peptides consisting nominally of alternating hydrophobic and hydrophilic amino acids, with terminal lysines self-assemble into parallel or anti-parallel beta sheets.  This self-assembly of molecules into fibers occurs spontaneously in all solutions of peptide amphiphiles, which have a vanishingly small critical micelle concentration.  By changing pH or adding counterions to screen the charged surfaces of fibers, gels can be formed.  It has been shown that injection of peptide amphiphile solutions in vivo leads to in situ gel formation due to the presence of counterions in physiological solutions.  This, along with the complete biodegradability of the materials, suggests numerous applications in in vitro and in vivo therapies. Silk amino acid (SAAs) also known as Sericin is a natural water-soluble glycoprotein extracted from raw silk.  It is used as an additive in skin and hair care products due to its high levels of serine which has excellent moisture preservation characteristics.  As a water-based additive it is used to provide a protective barrier and silky feel to lotions, soaps, personal lubricants, hair and skincare products.  Silk amino acids are produced by hydrolyzing (or breaking apart) silk proteins into smaller peptide chains, typically 18 to 19 amino acids in length.  Silk amino acids have a lower molecular weight than silk protein powders and are moisturizing to skin and hair. Transamination, a chemical reaction that transfers an amino group to a ketoacid to form new amino acids.  This pathway is responsible for the deamination of most amino acids.  This is one of the major degradation pathways which convert essential amino acids to nonessential amino acids (amino acids that can be synthesized de novo by the organism). Histidine (abbreviated as His or H; encoded by the codons CAU and CAC) is an α-amino acid that is used in the biosynthesis of proteins.  It contains an α-amino group (which is in the protonated –NH form under biological conditions), a carboxylic acid group (which is in the deprotonated –COO form under biological conditions), and an imidazole side chain (which is partially protonated), classifying it as a positively charged amino acid at physiological pH. Initially thought essential only for infants, longer-term studies have shown it is essential for adults also. The Strep-tag® system is a method which allows the purification and detection of proteins by affinity chromatography.  The Strep-tag II is a synthetic peptide consisting of eight amino acids (Trp-Ser-His-Pro-Gln-Phe-Glu-Lys).  This peptide sequence exhibits intrinsic affinity towards Strep-Tactin®, a specifically engineered streptavidin and can be N- or C- terminally fused to recombinant proteins.  By exploiting the highly specific interaction, "Strep"-tagged proteins can be isolated in one step from crude cell lysates.  Because the "Strep"-tag elutes under gentle, physiological conditions it is especially suited for generation of functional proteins. Amino acids are organic compounds containing amine (-NH) and carboxyl (-COOH) functional groups, along with a side chain (R group) specific to each amino acid.  The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen, although other elements are found in the side chains of certain amino acids.  About 500 amino acids are known (though only 20 appear in the genetic code) and can be classified in many ways.  They can be classified according to the core structural functional groups' locations as alpha- (α-), beta- (β-), gamma- (γ-) or delta- (δ-) amino acids; other categories relate to polarity, pH level, and side chain group type (aliphatic, acyclic, aromatic, containing hydroxyl or sulfur, etc.).  In the form of proteins, amino acid residues form the second-largest component (water is the largest) of human muscles and other tissues.  Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis. A polyproline helix is a type of protein secondary structure which occurs in proteins comprising repeating proline residues.  A left-handed polyproline II helix (PPII, poly-Pro II) is formed when sequential residues all adopt (φ,ψ) backbone dihedral angles of roughly (-75°, 150°) and have "trans" isomers of their peptide bonds.  This PPII conformation is also common in proteins and polypeptides with other amino acids apart from proline.  Similarly, a more compact right-handed polyproline I helix (PPI, poly-Pro I) is formed when sequential residues all adopt (φ,ψ) backbone dihedral angles of roughly (-75°, 160°) and have "cis" isomers of their peptide bonds. Of the twenty common naturally occurring amino acids, only proline is likely to adopt the "cis" isomer of the peptide bond, specifically the X-Pro peptide bond; steric and electronic factors heavily favor the "trans" isomer in most other peptide bonds.  However, peptide bonds that replace proline with another "N"-substituted amino acid (such as sarcosine) are also likely to adopt the "cis" isomer. A tripeptide is a peptide consisting of three amino acids joined by peptide bonds.  The exact nature and function of proteins is determined by the amino acids present and the Nisin is a polycyclic antibacterial peptide produced by the bacterium "Lactococcus lactis" that is used as a food preservative.  It has 34 amino acid residues, including the uncommon amino acids lanthionine (Lan), methyllanthionine (MeLan), didehydroalanine (Dha), and didehydroaminobutyric acid (Dhb).  These unusual amino acids are introduced by posttranslational modification of the precursor peptide.  In these reactions a ribosomally synthesized 57-mer is converted to the final peptide.  The unsaturated amino acids originate from serine and threonine, and the enzyme-catalysed addition of cysteine residues to the didehydro amino acids result in the multiple (5) thioether bridges.
Answer: Histidine