Final answer:
The correct answer is arginine. At pH 7.4, arginine (a basic amino acid with basic side chains that ionize) carries a net positive charge, unlike aspartic acid, cysteine, and asparagine, which do not.
Step-by-step explanation:
The student asked which amino acid has a net positive charge at pH 7.4. From the options provided, arginine is the amino acid that has a net positive charge at this pH level. This is because arginine is a basic amino acid with basic side chains that are hydrophilic and contain a primary or secondary amine group that ionizes to cation -NH3 or -NRH₂.
The side chains of aspartate and glutamate are negatively charged at pH 7.4, therefore aspartic acid, an acidic amino acid, has a negative charge at this pH. Cysteine, a sulphur containing amino acid, and asparagine, which has an amide functional group, have charges that are generally neutral at physiological pH. Hence, the correct answer is arginine, which is known to engage in hydrogen bonding due to its positive charge.