Q26 Which one of the following molecules (~ 1mg/mL) do NOT absorb at 280 nm
in an aqueous solution of pH 7.00 at room temperature?
(A) Poly deoxy–Guanylate (poly dG)
(B) Adenosine triphosphate
(C) Phenylalanine
(D) Tyrosine

Adenosine triphosphate (ATP) does not significantly absorb at 280 nm in aqueous solution at pH 7 and room temperature, unlike the other options.

UV absorbance at 280 nm arises mainly from aromatic rings in proteins (tryptophan, tyrosine) and to lesser extents in nucleic acid bases, but purine/pyrimidine nucleotides like ATP peak near 260 nm with minimal tailing to 280 nm. At ~1 mg/mL (~1.5 mM for ATP), its low extinction coefficient at 280 nm yields negligible absorbance compared to protein standards.

Option Analysis

Poly deoxy-Guanylate (poly dG):
This homopolymer contains stacked guanine bases with strong π-π* transitions, showing absorbance peaks near 260 nm and a shoulder/tailing into 280 nm due to hypochromicity and base interactions. Guanine’s aromatic structure ensures measurable 280 nm absorption.

Adenosine triphosphate (ATP):
ATP’s adenine base absorbs maximally at 259 nm; at 280 nm, absorbance is low (shoulder, not peak), as confirmed in spectra and protein assays where ATP interferes minimally unless at high concentrations. No significant chromophore matches 280 nm protein-like absorption.

Phenylalanine:
Its benzene ring provides weak but detectable absorbance at 280 nm (ε ≈ 200 M⁻¹cm⁻¹), contributing to protein A280 readings alongside stronger Trp/Tyr signals.

Tyrosine:
The phenolic ring yields strong absorbance peaking near 275 nm with extension to 280 nm (ε ≈ 1400 M⁻¹cm⁻¹), a key protein chromophore at pH 7.

Introduction to 280 nm UV Absorbance

Molecules that do not absorb at 280 nm lack strong aromatic chromophores tuned to this wavelength, critical for protein quantification in biochemistry. In aqueous solutions at pH 7 (~1 mg/mL, room temperature), nucleic acids peak at 260 nm while amino acids like phenylalanine and tyrosine contribute to 280 nm via π→π* transitions. This MCQ tests CSIR NET-level understanding of extinction coefficients and spectral properties.

Why ATP Fails to Absorb at 280 nm

Adenosine triphosphate peaks at 259 nm due to adenine’s purine ring, with negligible 280 nm absorbance (low ε, no tailing overlap). Unlike proteins, ATP’s phosphate chain and ribose do not shift absorption; assays confirm minimal interference. Correct answer: (B).

UV Spectra of Other Options

• Poly dG: Guanine-rich polymer absorbs at 252-260 nm with 280 nm shoulder from base stacking.

• Phenylalanine: Weak 280 nm band (ε low but present) from benzene.

• Tyrosine: Strong phenolic peak ~275-280 nm.

CSIR NET Exam Tips

Practice distinguishing 260 nm (nucleic acids) vs 280 nm (proteins/aromatics); ratios like A260/A280 assess purity. Phenylalanine contributes least among aromatics but still absorbs.