[enigma1122]

hi can anyone have a look at this question in the pdf about amino acids and their charges, and perhaps write a few notes explaining how oto go about it please? appreciate much <3 qustion 49 to 51 at the bottom of the first page! Its about finding the average charge on a peptide at ifferent ph's, even if you tell me how to do it for a single amino acid, that will be helpful

PLEASE OPEN THE ATTACHMENT I CANT COPY THE WHOLE QUESTION IN FOR SOME REASON

On lines 49 – 51 use the letters A – M shown in the table below to select the likely average charge on this
peptide at each of the following values of pH (there is only 1 correct answer for each of lines 49 – 51):
49. pH 1
50. pH 7
51. pH 10
A B C D E F G H I J K L M
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0 +0.5 +1.0 +1.5 +2.0 +2.5 +3.0

[enigma1122]

bump

[ash92:)]

Once you understand this, it's actually really easy.

At a low (acidic) pH, the substances that makes the environment acidic does so by releasing protons (aka H+). The amino acid takes up the excess protons, becoming PROTONATED, ie has protons bound to it.

In acidic conditions (eg pH2):



You'd just add up the charges, in this case the R group has no charge, so the net charge is +1.


in alkali conditions:



Same again - adding up the charges gives -1.


If the R group has an amine group, you just treat it the same way as the amine group attached to the alpha carbon (see diagrams) i.e it is positively charged in high pH. Vice versa for an R group with negative charge i.e. with an COOH group.

Hope it helped
(and sorry about the scruffy diagrams, the ones on the net were too overcomplicated so I improvised)

[enigma1122]

(Original post by ash92:))
Once you understand this, it's actually really easy.

At a low (acidic) pH, the substances that makes the environment acidic does so by relaesing protons (aka H+). it does this by the dissociation of the carboxyl (COOH) group to give COO- and H+.

In acidic conditions (eg pH2):

(see later, currently editing post)

heyhey,

Thank you ill have a look back here later thanks for your help...If it saves you time the mRNA sequence translates to the peptide sequence as

Glu-Asp-Met-Gly-Trp-Thr-Pro-Asn

.... confused as they don give pKa, and from the internet, it looks like i'd need the pka of each amino acid to calculate this, and then there is different pka's for different side groups, and with Trp im not sure if the NH in the ring can lose/gain H's... been a long time since i did A level chemistry lol

[Maryam A]

which subject and level is this?

[ash92:)]

(Original post by enigma1122)
heyhey,

Thank you ill have a look back here later thanks for your help...If it saves you time the mRNA sequence translates to the peptide sequence as

Glu-Asp-Met-Gly-Trp-Thr-Pro-Asn

.... confused as they don give pKa, and from the internet, it looks like i'd need the pka of each amino acid to calculate this, and then there is different pka's for different side groups, and with Trp im not sure if the NH in the ring can lose/gain H's... been a long time since i did A level chemistry lol

yes, you would need pKa but if it isn't given in the question, just generalise. eg if they ask you about pH2, just assume it is below pKa. And no, the NH group of Trp isn't involved in this because the N-C bonding is much more stable than the N-H bonding due to electronegativity, etc.

[ash92:)]

(Original post by Maryam A)
which subject and level is this?

Doesn't matter tbh, after school it's all one and the same. But to answer your question: biology/chemistry

[enigma1122]

(Original post by Maryam A)
which subject and level is this?

medicine lol

[Maryam A]

(Original post by enigma1122)
medicine lol

oh right! i'm hoping to study medicine too! what's it like?

[enigma1122]

(Original post by ash92:))
Once you understand this, it's actually really easy.

At a low (acidic) pH, the substances that makes the environment acidic does so by releasing protons (aka H+). The amino acid takes up the excess protons, becoming PROTONATED, ie has protons bound to it.

In acidic conditions (eg pH2):



You'd just add up the charges, in this case the R group has no charge, so the net charge is +1.


in alkali conditions:



Same again - adding up the charges gives -1.


If the R group has an amine group, you just treat it the same way as the amine group attached to the alpha carbon (see diagrams) i.e it is positively charged in high pH. Vice versa for an R group with negative charge i.e. with an COOH group.

Hope it helped
(and sorry about the scruffy diagrams, the ones on the net were too overcomplicated so I improvised)

Okay i think i get the idea now, thank you you're a , so once i've found the charge on each amino acid in the peptide, i add them all up an divide by the number of amino acids in the peptide? to get one of the options a to m?

[enigma1122]

(Original post by Maryam A)
oh right! i'm hoping to study medicine too! what's it like?

pain in the bum lol. don't do it. haha im joking. I love it, the actual material isn't difficult, but its the sheer amount you have to know, stuff you have to memorize etc etc.. If you aren't afraid of hard work then you'll be fine

[Maryam A]

(Original post by enigma1122)
pain in the bum lol. don't do it. haha im joking. I love it, the actual material isn't difficult, but its the sheer amount you have to know, stuff you have to memorize etc etc.. If you aren't afraid of hard work then you'll be fine

No, i'm alright when it comes to hard work. From what i have seen, i think it pays off in the end. I only want to do medicine because i think it's the most rewarding career out there! Well good luck with medical school! hope everything goes well! sure u'll be a great doctor!

[ash92:)]

(Original post by enigma1122)
Okay i think i get the idea now, thank you you're a , so once i've found the charge on each amino acid in the peptide, i add them all up an divide by the number of amino acids in the peptide? to get one of the options a to m?

aha, thanks

erm...not sure what you're asking there Are you asking how to find the net charge of a peptide? If so, then no - you're overcomplicating it! You just add up all the charges of the amino acids (to simply for eg, if the peptide has 5+ and 7- charges in total, the net charge of the peptide is 2-). No need for any division or anything