Some amino acids are neutral because the acidic carboxyl group and the basic amino group neutralize each other, and the R-group (side chain) does not contribute a significant positive or negative charge at physiological pH.
Here's a more detailed breakdown:
-
The Balancing Act: Every amino acid has a central carbon atom bonded to:
- An amino group (-NH2), which is basic.
- A carboxyl group (-COOH), which is acidic.
- A hydrogen atom (-H).
- An R-group (also called a side chain), which is unique to each amino acid and determines its specific properties.
The amino and carboxyl groups can react with each other in solution, leading to the formation of a zwitterion. A zwitterion is a molecule with both positive and negative electrical charges, but the net charge is zero. This internal neutralization is a key factor in the neutrality of some amino acids.
-
The R-Group's Role: The crucial factor determining whether an amino acid is neutral, acidic, or basic is the nature of its R-group.
-
Neutral R-Groups: If the R-group is nonpolar or contains polar but uncharged groups, it doesn't significantly alter the overall charge of the amino acid at physiological pH. These amino acids are considered neutral. Examples include:
- Glycine: The R-group is simply a hydrogen atom (-H).
- Alanine: The R-group is a methyl group (-CH3).
- Valine, Leucine, Isoleucine: These have larger, nonpolar hydrocarbon R-groups.
- Serine, Threonine: These have polar, uncharged -OH groups.
- Asparagine, Glutamine: These have polar, uncharged amide groups.
-
Acidic R-Groups: Amino acids like aspartic acid and glutamic acid have R-groups that contain carboxyl groups (-COOH). These R-groups can donate a proton (H+) and become negatively charged at physiological pH, making the amino acid acidic.
-
Basic R-Groups: Amino acids like lysine, arginine, and histidine have R-groups that contain amino groups (-NH2) or similar nitrogen-containing groups. These R-groups can accept a proton (H+) and become positively charged at physiological pH, making the amino acid basic.
-
-
Physiological pH: The term "neutral" in this context usually refers to the charge at physiological pH (around 7.4). The precise charge can vary slightly depending on the specific pH of the environment.
In summary, amino acids with neutral R-groups are considered neutral because the inherent positive charge of the amino group is balanced by the negative charge of the carboxyl group, and the R-group does not contribute a significant charge at physiological pH.
