Question

Oxytocin is a nonprotein peptide (therefore has less than 40 amino acids). What is the charge on this peptide at pH 6.8?

Answer 1

Ans. AA sequence of oxytocin = CYIQNCPLG

Charge on oxytocin (CYIQNCPLG) at pH 6.8 = -0.09

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Follow the steps-

I. The Charge on a basic group is given by: [10^pKa / (10^pH + 10^pKa)]. This charge is POSITIVE.

II. The sum of positive charges on all basic groups (-NH₂ group at N-ter, side chains of Arg, His and Lys) gives total POSTIVE charge on the peptide.

III. The Charge on an acidic group (Glu, Asp, Cys, Tyr) is given by: [10^pH / (10^pH + 10^pKa)]. This charge is NEGATVE.

IV. The sum of negative charges on all acidic groups (-COOH group at C-ter, side chains of Asp, Glu, Cys, Tyr) gives total NEGATIVE charge on the peptide.

V. The sum of TOTAL POSITIVE and TOTAL NEGATIVE charge gives the net charge on the peptide.

VI. Only acidic and basic groups are accounted for charge calculation.

VII. Use of excel or similar programs is preferred to minimize manual errors and complexity of calculation.