Re: How to find net charge of this peptide at pH 9?
Sequence is NH2-Thr-Ile-Met-Glu-Leu-Tyr-Arg-Ala-Ile-Asp-Ile-Asn-Gly-Pro-Ala-Arg-Thr-Tyr-COOH
Among above sequence Thr,Ile,Met,Leu,Ala,Asn,Gly,Pro not having any charge at any pH because of their side chain (i.e. this amino acid give charge only if they have a N terminal or Carboxyl terminal position in peptide)
Glu,Asp having No charge (Approx) below pH 4 (as pKa 4) and above pH 4 each contribute -1 charge
Arg,Lys,His having +1 charge below their pKa value and If pH is above their pKa value they having 0 Charge (i.e. No charge)
Cys ,tyr having Zero charge below their pKa value while contribute -1 charge at pH above their pKa value.
N terminal amino of Thr having pKa value 9.1 so it upmost uncharge and contribute Zero Charge to Peptide, While COOh of tyrosine having -1 Charge at pH 9 So
(0) Amino of Thr + (-1) Glu + (-1) Asp + (+1) Arg + (+1) Arg + (-1) Tyr = -1 Net Charge