What is the basis of Kjeldahl's procedure for measuring total protein?

The basis of Kjeldahl's procedure for measuring total protein rests on the principle that the nitrogen content of proteins is relatively constant, typically around 16% by weight. This method involves the digestion of a sample in concentrated sulfuric acid, which converts nitrogen present in organic compounds into ammonium sulfate. By measuring the amount of nitrogen released during this process, one can calculate the total protein content, since nitrogen is a key component of amino acids, the building blocks of proteins.

This approach allows for a relatively accurate estimation of protein concentration in various types of samples, whether they are plant or animal-based, because most proteins share a similar nitrogen content. This is why option C is correct, as it highlights the key factor that underlies the Kjeldahl methodology.

Other options may involve characteristics of proteins but do not directly pertain to the chemical basis of the Kjeldahl method. For instance, the charge of proteins can vary depending on their amino acid composition and the pH of the environment, so it is not a suitable basis for determining total protein content. Similarly, the pKa of proteins is not consistent across different proteins, as it can vary widely due to structural differences. Lastly, while tyrosine and tryptophan are important