Which amino acid is characterized as positively-charged?

Arginine is characterized as positively charged due to its side chain, which contains a guanidinium group. At physiological pH (around 7.4), this group's nitrogen atoms can accept protons, giving the entire side chain a positive charge. This positive charge plays a critical role in various biochemical interactions, including enzyme-active sites and binding with negatively charged molecules.

Cysteine, while important, contains a sulfur atom and is primarily known for forming disulfide bonds, not for having a positive charge. Histidine can exhibit either a positive charge or be neutral depending on the pH, but it is generally considered a polar amino acid with weaker basicity compared to arginine. Aspartate carries a negative charge at physiological pH due to its carboxylic acid side chain. Thus, arginine stands out as the amino acid that is definitively characterized by a positive charge in physiological conditions.