Post A level (i think) Buffers and pH

Hey, im 1st year med student and we have this buffers and pH tutorial i need to do and its background work, using the Henderson Hasselbalch equation is simple but these two questions im struggling with. Will rep anyone that can help explain the answers.

1) Write down your prediction of the structure of the predominant ionic species of the dipeptide aspartyl cysteine at each of the following pH values : 1,5,7,9,11
predominant ionic species = no idea!

2)a) What causes the pKa for the side chain of histidine residues in a protein to differ from the value for the simple amino acid?
I put neighbouring amino acid side chains in the haemoglobin molecule?

b) How could the pKa values change, as haemoglobin molecules are oxygenated or deoxygenated?
pKa is the pH at which acid is half dissociated so does oxygen affect that?

c) What is the physiological consequence of this difference in pKa values?
Gives the haemoglobin a wider range for working effectively as a buffer?

Thanks in advance

Reply 1

charco

Study Forum Helper

1. Aminoacids can exist as various ionic forms depending on the pH. Look up Zwitterions.
The NH2 groups can bind to a proton to make NH3+ in acidic solution. Likewise the COOH groups can lose a proton in basic solution.
Each amino or acid group (and hence protein) has characteristic pH values at which these things happen, depending on the rest of the structure.

Reply 2

Wangers

For b and c) look up 2 phenomena called the Bohr shift and the Haldane effect...

ps - sorry to hijack your thread, but since your around Charco - can you tell me where I can get a uni level respected data book - with deltaG values and such - I realise theres wikipedia, but that becomes somewhat irratating.

I am slowly continuing my studies to keep the cogs turning, but alas being a gapper I no longer have access to school resources...cheers.

Reply 3

charco

Study Forum Helper

no hijack - no worries
I use an early edition of the IRC data book ... but it's stupidly large in its recent form. I bought a copy from the 1970's (junk shop) and as data doesn't really change all that much except for occasional trends in units (which you have to get accustomed to converting - thanks google) it's got almost everything barring relatively recent developments such as laser tech and dna data.

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