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Lecture 2

Biochemistry 2280A Lecture 2: 2 - Amino Acids

5 Pages
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Department
Biochemistry
Course Code
Biochemistry 2280A
Professor
Eric Ball

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** only these 20 are used to make Friday, September 11, 2015 proteins but others can be found due to modifications after the protein is Amino Acids made ** Meakin Lecture 2 By the end of today: - be able to calculate the charge of amino acid groups - understand logarithms and the pH scale - understand Ka and pKa - be able to calculate pH and pKa values - understand peptide bonds Net Charge - to predict the properties of a protein in solution we need to calculate the protein’s net charge - we’ll start by calculating the net charge of a single amino acid in solution - three parts of an amino acid can carry a charge in solution • amino group • carboxylic acid group side-chain (some amino acids) • - smallest you can add is H - disulfide bonds hold stuff in place Proteins Move Continuously in Aqueous Solutions - calculating net charge of an amino acid - need to know two things: • the strength of the group to attract a proton (pKa) • the number of protons available in solution (pH) The pH Scale - pH is the symbol for the logarithm of the reciprocal of the hydrogen ion concentration in gram atoms per litre - logarithm: the exponential that indicates the power to which a base number is raised to produce a given number (eg) 10^2 = 100 - pH = -log[H+] What is pKa? - pKa can be used to describe the strength of acids - lowering pH (increasing H+) - acidic & basic amino acids ionize (become charged) when placed in water will drive equilibrium to left - fraction of molecules that are - the chemical reaction is known as dissociation ionized decreases - when this reaction (—>) is in equilibrium, Ka = dissociation constant - net charge on group - Ka is a quantitative measure of the strength of an acid, or a “personal scale of acidity” decreases - ** for basic groups, fraction
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