Indicate which of the amino acid residues in the following peptide sequence contains a group that has a negative charge for its most likely charge state at pH 4. Asn-Cys-Gly-Gln-His-Thr-Lys-Ile-Val (If none of the amino acids fit the criterion, select "none".)

Explanation:

In the given sequence there is 9 amino acid in which 2 are positively charged amino acid and 7 are neutral amino acid at physiological pH . In each amino amino acid two oppositely charged amino and carboxyl group is present ,where amino is positively charged and proton acceptor and carboxyl is negatively charged and proton donor at physiological pH . pH below neutral cause addition of proton in amino group and makes amino acid positively charged , while pH above neutral cause addition of hydroxide ion in carboxylic group and makes amino acid negatively charged . At pH-4 , all amino acid in the given sequence become positively charge due to addition of proton in the solution ,these protons in the solution binds to amino acid sequence, hence there is no negative charged amino acid in the sequence .

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.