26 AAs - Class - acid base and net charge

26 AAs - Class - acid base and net charge - Amino Acids and...

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Amino Acids and Proteins: Classifications Acid/Base Properties and Net Charge HNFE 3025 Fall 2009
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Classified in several ways Net electrical charge At physiologic pH, is it 0, + or – Structure The side chain (R group) makes each amino acid unique Polarity The polarity of the R group determines the degree of interaction with water Carbon skeleton fate Gluconeogenic or ketogenic Essentiality Dietary requirement Amino Acid Classification 2
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All amino acids have: a central ( α ) carbon – at least 1 amino group (-NH 2 ) at least 1 carboxyl group (-COOH) a side chain (R group) Makes AA unique Amino Acid Structure 3 Carboxyl carbon is carbon # 1. The central carbon is carbon # 2 No amino acid looks like this in vivo impossible because of acid-base properties H | H 2 N – C – COOH | R 1 2
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Depending on the pH of the environment, the amino group may accept or the carboxyl group may donate a proton (H + ) Effect of pH 4 Fig. 6-5, p. 186
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An acid is a proton (H + ) donor A base is a proton acceptor The pH scale is used to represent how acidic or basic a solution is (0 – 14) pH = – log [H + ] pH = 7 is neutral pH < 7 is acidic (the lower the pH, the more acidic the solution) pH >7 is alkaline or basic (the higher the pH, the more basic the solution) Acid-Base Properties 5
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If HA is a relatively strong acid, there will be little undissociated HA (the acid) and lots of A - (the conjugate base) and protons in solution. But pH is a measure of proton concentration in a solution. Therefore if HA is a relatively strong acid: It will dissociate more There will be a higher concentration of protons And the pH will be lower Acids and Bases 6 HA A - + H + WEAK ACID CONJUGATE BASE PROTON
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[A - ] [H + ] = K a [HA] • The larger the value of the K a , the stronger the acid because more HA is dissociated Acid Dissociation Constant (K a ) 7 HA A - + H +
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26 AAs - Class - acid base and net charge - Amino Acids and...

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