[moreche28]

Hello, dear colleagues. I'm a high school student and I've got a chemistry problem. Please, can someone give me an easier explanation or a method how to calculate overall charges in hormones - oxytocin and vazopressin - according to the value of pH? I know it is connected to pKa values of each ionizable group, but I'm literally confused by the complicated structures. The pH values are 1,8; 7,4, 10,5 and 13. (I've got a table of values of pKa of all ionizable groups - alpha COOH, alpha NH3+ and the side groups, so RH or RH+...) And another thing is... What would happen to the net charge of both hormones at pH = 10,5 if there weren't the disulfide bonds? I know, presumably, that it would change the structure, but I don't know how it would affect the charges... Thank you so much.

[Babcock_Hall]

Welcome to the forum.  It is a forum rule that you must show your attempt before we can help you.  My first suggestion is that you classify which amino acid side chains are cationic acids and which are neutral acids.  Cationic acids are positively charged when in the form of their conjugate acids.  Neutral acids are negatively charged when in the form of their conjugate bases.  My second suggestion is that you test your understanding of charge on amino acids first.  A peptide is not much harder; there are just more ionizable groups.  For example, assuming that the pK₁, pK₂ and pK_R (the side chain carboxylic acid)d of aspartic acid are 1.9, 9.6 and 3.7, respectively, what is the net charge on aspartate at pH 7?  This can be solved quantitatively, but a qualitative approach is fine as well.

With respect to your second question, if you reduced a disulfide bond, what side chain will now be present, and what is its pK_a?