Acidic Properties of Carboxyl Groups

Amino Acids

• Carboxyl group is deprotonated, making it acidic, and receives an H+ to become basic
• Amino acids are soluble in polar solvents due to their ionic character, exemplified by the COO- and +NH3 groups
• Solubility is influenced by the structure of the R group

Types of Amino Acids

• Non-Polar Amino Acids: have hydrocarbon chains, are hydrophobic, neutral, and aliphatic
• Polar Amino Acids: dissolve in water, are hydrophilic, neutral, and form H-Bonds
• Aromatic Amino Acids: contain oxygen, are bulky, and have polarity dependent on the R group

Characteristics of Amino Acids

• Some amino acids are found inside the protein, usually in the kinks of the protein

• The R group has a charge, which affects solubility

• Hydrogen bonds are not formed when the R group has no charge### Amino Acids

• There are 20 standard amino acids, each with a unique R-group

• Glycine (Gly) and Alanine (Ala) have small R-groups

• Serine (Ser) and Threonine (Thr) have R-groups with a hydroxyl (-OH) group

• Cysteine (Cys) has an R-group with a sulfur atom, which can form a disulfide bridge with another Cysteine

• Tyrosine (Tyr) and Tryptophan (Trp) have large, rigid R-groups

• Proline (Pro) has a unique, rigid structure that can cause kinks in protein chains

• Valine (Val) and Isoleucine (Ile) have non-polar, hydrophobic R-groups

• Asparagine (Asn) and Glutamine (Gln) have R-groups with an amide group

• Methionine (Met) has a sulfide group in its R-group, making it easily soluble in water

• Leucine (Leu) has a non-polar, hydrophobic R-group

Polarity of Amino Acids

• Amino acids can be classified as polar or non-polar based on their R-group
• Polar amino acids have R-groups with a hydroxyl (-OH) or amide group
• Non-polar amino acids have R-groups with hydrocarbon chains or sulfur atoms
• Glycine (Gly) and Serine (Ser) are polar amino acids
• Alanine (Ala) and Valine (Val) are non-polar amino acids
• Proline (Pro) is a non-polar amino acid, but its rigid structure makes it unique

Protein Structure

• Disulfide bridges can form between two Cysteine (Cys) residues, stabilizing protein structure

• Hydrogen bonds and weak interactions between amino acids help stabilize protein structure

• Proline (Pro) can cause kinks in protein chains due to its rigid structure### pH Scale and Acid-Base Properties

• pH is a measure of the concentration of hydrogen ions (H+) in a solution, calculated as pH = -log[H+].

• pH scale ranges from 0 to 14, with a pH below 7 being acidic, pH 7 being neutral, and pH above 7 being basic.

Weak Acids and Bases

• A weak acid is a molecule that partly dissociates in water, releasing H+ ions.
• Weak acids have a higher pKa (acid dissociation constant) than strong acids, indicating a lower tendency to donate a proton (H+).
• A weak base is a molecule that accepts H+ ions to form a conjugate acid.

Acid-Base Properties of Amino Acids

• Amino acids have both an acidic carboxyl (-COOH) and a basic amino (-NH2) group.
• The carboxyl group is protonated (acidic) at low pH and deprotonated (basic) at high pH.
• The amino group is deprotonated (basic) at low pH and protonated (acidic) at high pH.

Titration of Glycine

• Glycine, an amino acid, has two pKa values: pK1 (COOH) = 2.34 and pK2 (NH3+) = 9.6.
• At pH < 2.34, glycine is fully protonated (acidic).
• At pH > 9.6, glycine is fully deprotonated (basic).
• The zwitterion form of glycine, where the carboxyl group is deprotonated and the amino group is protonated, is most prevalent at pH around 6-7.