Biologically Important Chemistry Notes (Biochemistry)

Biologically Important Chemistry Notes (Biochemistry) -...

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Recall: most amino acids are present in nature as the L-isoform, and most carbohydrates are present in nature as the D-isoform. The pH of acetic acid is around 4.7 Carboxyl groups on amino acids are fairly acidic, and amino groups are basic. At a low pH, the amino groups will accept hydrogens, and the carboxylic acids will donate its hydrogen. If the pH is less than the pKa, then protonation wins over dissociation, and a functional group will be protonated. - Similarly if the pH is higher than the pK a , deprotonation wins and the functional group will be fairly negative in charge. At a high pH, there is a low hydrogen ion concentration, and both functional groups on an amino acid may become deprotonated (COO- and NH 2 + this creates a net charge of -1 per glycine molecule . - This establishes equilibrium between the amino acid and the surrounding solution. The pH at which a molecule is uncharged (zwitterionic) is referred to as its isoelectric point (pI) . - The amino acid has both positive and negative charges. - How do we calculate the pI? We average the pK a of the two functional groups . - At a pH greater than an amino acid’s pI, the charge on the amino acid will be negative (Why? the deprotonated form will be favored, giving the amino acid an overall net negative charge) - At a pH lower than the pI, the net charge will be positive Nonpolar – “M-VAGLIP” Polar - Serine, threonine, cysteine, leucine, isoleucine, asparagines Aromatic – phenylalanine, tryptophan, tyrosine (makes epinephrine) Acidic – aspartic acid, glutamic acid Positive – histidine, arginine, lysine
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Biologically Important Chemistry Notes (Biochemistry) -...

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