For determining the crystal structure, a short sequence of histidine residues is sometimes attached to the N- or C-terminus of the protein. What is the purpose of this?

The structure of an alpha helix orients the oxygens of the carbonyl group of the peptide bond towards the C-terminal end of the helix while the hydrogens of the NH groups orient toward the N-terminal end, thus imparting a dipole along the length of the helix.  As a result, which kinds of amino acids would favor the C-terminal end, and which would favor the N-terminal end? negatively charged, positively charged positively charged, negatively charged negatively charged, hydrophobic positively charged, hydrophobic hydrophobic, negatively charged hydrophobic, positively charged

Proline is one of the amino acids in the "special" groups of sidechains. One reason for this is the dramatic impact it can have on secondary structure.  a) draw the lewis structure for a tripeptide with the sequence Ala-Pro-Ala b) Proline residues are rarely found in alpha helices - in fact, they are often referred to as helix-disrupting amino acids. Thinking about intermolecular interactions, provide an explanation for why proline might disrupt alpha-helices. c) Proline also has a similar effect on beta-sheets, and is rarely found in the middle of beta sheets. Thinking about noncovalent interactions, provide an explanation for why proline might disrupt beta-sheets.

The folding of intrinsically disordered proteins is prevented by the presence of Ser, Lys, and Glu residues. What do these amino acids have in common? Why do they disrupt IDPs?