At \(\mathrm{pH}=4\), glycine exists as: (a) \(\mathrm{H}_{3} \mathrm{~N}-\mathrm{CH}_{2}-\mathrm{COO}^{-}\) (b) \(\mathrm{H}_{3} \mathrm{~N}-\mathrm{CH}_{2}-\mathrm{COOH}\) (c) \(\mathrm{H}_{2} \mathrm{~N}-\mathrm{CH}_{2}-\mathrm{COOH}\) (d) \(\mathrm{H}_{2} \mathrm{~N}-\mathrm{CH}_{2}-\mathrm{COO}\)

Understanding the relationship between pKa (the measure of acidity) and pH (the measure of alkalinity or acidity of a solution) is crucial for biochemistry students preparing for exams like NEET. The pKa is the pH level at which half the molecules of a substance are protonated (they've accepted a proton) and half are deprotonated (they've released a proton).

At a pH below a compound's pKa, the environment is more acidic, favoring a protonated state. Conversely, at a pH above the compound's pKa, the environment is more basic, and the compound is likely to be in a deprotonated state. These concepts are foundational when discussing the behavior of amino acids, such as glycine, in different pH environments.

Amino acids like glycine contain both amine and carboxylic acid functional groups - each with their distinct pKa values. The protonation state of these groups changes depending on the pH of the surrounding solution. In acidic conditions (low pH), the amine group is typically protonated, gaining a hydrogen ion to form NH₃⁺, while the carboxyl group is also protonated, existing as COOH. In basic conditions (high pH), both groups tend to lose protons; the amine group becomes NH₂, and the carboxyl converts to COO^-.

The variations between these states affect how the amino acid behaves and interacts, which is vital for understanding enzyme kinetics, protein structure, and metabolic pathways in biological systems.

Correctly identifying the chemical structure of amino acids at different pH levels is an essential skill for students, especially in biochemistry-related exams such as NEET. The charged state of amino acids influences their structure and function. For instance, glycine at a neutral pH typically exists in its zwitterionic form, where it has both a positive charge (on the amine group) and a negative charge (on the carboxylate group).

Tools such as FTIR spectroscopy, UV-visible spectroscopy, and NMR spectroscopy can help in the identification process by providing valuable data about the molecular vibrations, electronic transitions, and nuclear environments within the amino acids, respectively. Understanding how pH influences the structural form of amino acids can also assist in predicting their behavior in varying biological contexts.

For NEET aspirants, biochemistry forms a crucial part of the syllabus. Concepts like the structure of amino acids at different pH levels, their pKa values, and how this impacts their structural form and reactions are fundamental. Grasping these concepts not only aids in solving textbook problems but also builds a robust understanding that's invaluable for the medical entrance exam. Clinical scenarios, such as estimating blood pH or understanding drug interactions, often revolve around these principles.

To maximize success in NEET, students should focus on mastering these core topics and apply them to various biochemical scenarios, ensuring a comprehensive comprehension of the subject matter.