As an expert in the field of molecular biology and biochemistry, I have extensive experience in handling and analyzing nucleic acids and proteins. When it comes to the question of what absorbs at 260 nm, it's important to note that this wavelength is particularly relevant in the context of nucleic acid analysis. Here's a detailed explanation:

### Absorption at 260 nm

The most significant absorbers at 260 nm are the nucleic acids, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). The absorption at this wavelength is primarily due to the presence of pyrimidine and purine bases within the nucleic acid structure. These bases contain conjugated double-bond systems that absorb light in the ultraviolet (UV) region, with a peak at around 260 nm.

#### DNA and RNA Structure

- DNA: It consists of two strands that are complementary and antiparallel. The backbone is made of sugar (deoxyribose) and phosphate groups, while the bases are adenine (A), thymine (T), guanine (G), and cytosine (C).
- RNA: It is typically single-stranded and has a backbone of sugar (ribose) and phosphate groups. The bases in RNA are adenine (A), uracil (U), guanine (G), and cytosine (C).

#### Absorption Mechanism

The absorption of UV light by nucleic acids is due to the electronic transitions within the aromatic ring structures of the bases. Specifically, the π-π* transitions in the conjugated systems of the bases are responsible for the absorption at 260 nm.

### Contamination and Ratio Analysis

It's not uncommon for nucleic acid samples to be contaminated with other molecules, such as proteins and organic compounds. To assess the purity and potential contamination of nucleic acid samples, the A260/A280 ratio is often used. This ratio compares the absorbance at 260 nm to the absorbance at 280 nm.

- Proteins, particularly those containing aromatic amino acids like tryptophan, tyrosine, and phenylalanine, absorb light at 280 nm. This is due to the indole and aromatic side chains of these amino acids, which have strong absorption in the UV range.
- Organic compounds may also absorb at 260 nm, but typically to a lesser extent than nucleic acids.

#### Interpreting the A260/A280 Ratio

- A ratio of 1.8 to 2.0 is considered ideal for DNA, indicating minimal protein contamination.
- A ratio around 2.0 to 2.2 is typical for RNA, due to the presence of uracil instead of thymine, which slightly alters the absorption profile.

### Conclusion

In summary, the primary absorbers at 260 nm are nucleic acids, with DNA and RNA being the most significant. The A260/A280 ratio is a valuable tool for assessing the purity and potential contamination of nucleic acid samples with proteins and other organic compounds. Understanding the molecular basis of UV absorption allows for more accurate and reliable analysis in molecular biology and biochemistry research.

read more >>

It is common for nucleic acid samples to be contaminated with other molecules (i.e. proteins, organic compounds, other). ... The ratio of absorbance at 260 nm vs 280 nm is commonly used to assess DNA contamination of protein solutions, since proteins (in particular, the aromatic amino acids) absorb light at 280 nm.read more >>