Amino acids are the building blocks of proteins. Which of the following general structural formulas correctly represents the basic structure of an alpha amino acid?

Introduction / Context:
Proteins are made of amino acids linked together by peptide bonds. Most biologically important amino acids are alpha amino acids, meaning that both the amino group and the carboxyl group are attached to the same central carbon atom. This question asks you to recognise the general structural formula of an alpha amino acid, using R to represent the variable side chain.

Given Data / Assumptions:

- The question uses R to indicate the side chain that differs between amino acids. - The formulas show different positions for the NH2 group and the carboxyl group. - We assume standard organic chemistry notation for alpha amino acids. - The task is to identify which formula has both amino and carboxyl groups on the same alpha carbon.

Concept / Approach:
The general structure of an alpha amino acid consists of a central alpha carbon atom (C alpha) bonded to four different groups: an amino group (NH2), a carboxyl group (COOH), a hydrogen atom, and a variable side chain (R). In structural shorthand, this is written as R-CH(NH2)-COOH, where CH indicates the alpha carbon bearing both the NH2 and the hydrogen, with the COOH attached to the same carbon. Any formula where NH2 is not attached to the alpha carbon or where the functional groups are otherwise misarranged does not correctly represent an alpha amino acid.

Step-by-Step Solution:
1. Recall that in an alpha amino acid, the amino group and carboxyl group are attached to the same central (alpha) carbon. 2. The side chain R is also attached to this alpha carbon, and a hydrogen atom completes the four single bonds of the carbon. 3. The shorthand R-CH(NH2)-COOH indicates that the CH group (the alpha carbon with one hydrogen) carries the NH2 group and is directly connected to COOH. 4. In option B, R-CH(NH2)-COOH, the notation correctly shows the amino group attached to the same carbon that links to the carboxyl group, fitting the alpha amino acid definition. 5. In option A, R-CH3(NH2)-COOH, the arrangement is ambiguous and suggests an extra substituent structure that is not standard. 6. In option C, R-CH2(NH2)-COOH, the CH2 group suggests the amino group is on a carbon that is not clearly the same alpha carbon shared with COOH in the standard representation. 7. In option D, R-CH3(NH2)-COH, the functional group COH is not a carboxyl group; a proper carboxyl group must be COOH. 8. Therefore, option B best represents the general structure of an alpha amino acid.

Verification / Alternative check:
Consider a specific amino acid such as alanine. Its structure is CH3-CH(NH2)-COOH, where CH3 is the R group and CH(NH2)-COOH is the alpha amino acid backbone. This matches the general formula R-CH(NH2)-COOH with R equal to CH3. Other amino acids like glycine (R = H) and valine (R = CH(CH3)2) can also be written in this pattern, confirming that R-CH(NH2)-COOH is the correct general formula.

Why Other Options Are Wrong:
- R-CH3(NH2)-COOH: This notation is unclear and does not follow the standard representation of the alpha carbon. It suggests a substituted methyl group rather than a single alpha carbon bearing both NH2 and COOH. - R-CH2(NH2)-COOH: The CH2 group notation suggests a different bonding pattern and does not clearly represent the alpha carbon carrying both the amino and carboxyl groups in the conventional way. - R-CH3(NH2)-COH: This contains COH, which is not a full carboxyl group. A proper carboxyl group is COOH, so this formula does not represent an amino acid.

Common Pitfalls:
Students sometimes focus only on the presence of NH2 and COOH somewhere in the formula and overlook their exact attachment to the same alpha carbon. It is also easy to misread condensed structural formulas. A helpful strategy is to rewrite the formulas in a more expanded form, checking whether the amino and carboxyl groups share the same central carbon. Remember that alpha amino acids follow the R-CH(NH2)-COOH pattern, with R determining the specific amino acid identity.

Final Answer:
The general structural formula of an alpha amino acid is correctly represented by R-CH(NH2)-COOH.