Real Chemistry: Side Chains on Amino Acids

Introduction

• Amino acids are the building blocks of proteins.
• Focus of lecture: Identifying amino acids as polar, nonpolar, or charged based on their side chains.
• Importance: Side chains determine interactions when amino acids form proteins.

Side Chains

• Definition: The part of an amino acid that varies among different amino acids.
• Influence whether an amino acid is polar, nonpolar, or charged.

Identifying Side Chains

Polar Side Chains

• General Rule: Contain oxygen.
  • Exceptions: Cysteine contains sulfur and hydrogen and is still polar.
• Examples: Most polar side chains have oxygen.

Nonpolar Side Chains

• General Rule: Contain only carbon and hydrogen.
  • Exceptions:
    • Methionine: Contains sulfur in the middle of the carbon chain, making it nonpolar.
    • Tryptophan: Contains nitrogen but is nonpolar.
• Examples: 7 out of 9 nonpolar amino acids fit this rule.

Charged Side Chains

• Identification: Look for explicit charges in the side chain.
  • Dominant feature for interaction over being polar.
• Examples: Can contain oxygen but categorized as charged.

Practice Problem

• Amino Acid A: Contains oxygen & explicit charge -> Charged.
• Amino Acid B: Contains oxygen & no charge -> Polar.
• Amino Acid C: Explicit charge shown -> Charged.
• Amino Acid D: Contains only carbon and hydrogen -> Nonpolar.

Conclusion

• Understanding side chain properties is crucial for predicting amino acid interactions.
• Future discussions will explore side chain interactions in proteins.

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• Recommendation: Memorize tricky exceptions (methionine, tryptophan, cysteine) and review charged amino acids regularly.
• Resources: Utilize amino acid lists showing charges for easier identification.