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What is the value of net charge in a peptide, gwyqr, at ph 10.5? a. -1 b. 0 c. -2 d. +1
What is the value of net charge in a peptide, GWYQR, at pH 10.5? a. -1 b. 0 C. -2 d. +1
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Yogita Basnal Best Answer
There are only two ionizable side chains in this peptide. Side chains of tyrosine and Arginine have pkas 10.07 and 10.48 respectively. At the ends of the peptide we will also calculate charge on alpha amine (om left hand side) and alpha carboxyl group (on right hand side). Also rule says that if peptide is kept at ph higher than pka then H+ will be lost from ionizable group but if ph is lower than pka then H+ will be accepted by ionizable group. Now starting from left we can start calculating charge on each amino acid of the peptide. At extreme left at glycine there is alpha amino group which has pka value = ~10 but the peptide is kept at ph = 10.5 so amino group will loose H+ so charge on glycine will be 0(zero). tryptophan doesn't have ionizable group neither it has alpha amino or carboxyl group so we will skip it. tyrosine has ionizable side chain with pka value = 10.07, again this pka is lower than given ph which is 10.5 so -OH group will loose it's H+ so the charge on tyrosine will be = -1. Glutamine doesn't have any ionizable side so we will skip it. Last amino acid in the peptide is Arginine. The side chain of Arginine has pka = 10.48 which is also less than given ph so charge on side chain will be 0 but Arginine has alpha carboxyl group as well and the pka of this group is ~2.4 which is also less than 10.5 so - COOH group will loose it's H+ so it will have -1 charge. Hence net charge on the peptide will be 0 - 1 + 0 - 1 = -2. So c is correct option. Note: please see images uploaded to understand it better.
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Aditi goud
The right answer here is a) -1... It's simply by the formula most of us use. For pH values between 8-10.9 the formula is (no. of D/E/C/Y - R+1) in negative. here, there's only 1 Y and 1R so according to the formula (1-1+1) = -1.