Question

1- Briefly explain why the absorptivities of DNAs and RNAs are remarkably constant while the absorptivities of proteins can show wide variation

Answer 1

Absorption of nucleic acid are 260nm while of protein can be within 240 - 300, this region is called the near-UV region.

Nucleic acids have an absorbance wavelength of 260 nm because of the nucleobases that they are made of. It originates from the -* transitions of the pyrimidine and purine ring systems of the nucleobases. The bases can be protonated and therefore the spectra of DNA and RNA are sensitive to pH. At neutral pH the absorption maxima range from 253 nm (for guanosine) to 271 nm (for cytidine), and, as a consequence, polymeric DNA and RNA show a broad and strong absorbance near 260 nm.

DNA or RNA purity can also be determined by measuring the absorption ratios of 260 to 280 nm.  This is because the nucleic acids that make up DNA and RNA absorb strongly at 260 nm. A ratio of about 2.0 is considered "pure" for RNA while a ratio of about 1.8 is considered "pure" for DNA. Lower ratios indicate the presence of protein.  

On the other hand, proteins absorbance depened on the aromatic amino acids. There are 3 aromatic amino acids, tryptophan (trp), tyrosine (tyr) and phenylalanine (phe), trp have absorbance at 280nm, tyr show absorbption at 270nm. Phe (260nm) and disulphide bond. (255) also contribute in absorbance but since it is relatively insignificant, it can only be observed in the absence of both tryptophan and tyrosine. Different absorbtion range for different protein depend upon the relative concentrations of each of these amino acids. Therefore, different proteins can have different absorption coefficients and even the wavelength of the maximum absorption may differ.