Charged amino acids absorb past 250 nm
A 67-residue protein with no aromatic side chains still absorbs from 250 to 800 nm because Lys/Glu charge-transfer transitions create Protein Charge Transfer Spectra.
Source
Near UV-Visible electronic absorption originating from charged amino acids in a monomeric protein
Study at a glance
- Design
- Other — UV-Vis and TDDFT of aromatic-free α3C protein attributing ProCharTS charge-transfer bands
- N
- Spectroscopy of a synthetic 67-residue protein — no cohort N
- Population
- Synthetic three-helix protein α3C and charged-amino-acid models
- Outcome
- Near-UV/visible absorption from charged side-chain charge-transfer transitions
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
They measured UV-Vis of synthetic three-helix protein α3C (no Trp/Tyr/Phe), compared concentrated non-aromatic amino acids and Lys peptides, and computed TDDFT spectra on MD snapshots of Lys and Glu. Temperature and pH were used to perturb the fold and the charged-side-chain distances.
What they found
α3C is 54% charged residues (17 Lys, 17 Glu, 2 Arg) yet shows ε ≈ 7338 M−1 cm−1 at 250 nm and a tail to 800 nm (ε ≈ 964 and 501 M−1 cm−1 at 450 and 800 nm). TDDFT assigns charge-transfer bands between NH3+/COO− side chains and the backbone; 4–6 Å Lys–Glu contacts modulate the visible tail. They name the phenomenon ProCharTS.
The limits
What it doesn't show
α3C is a designed mini-protein, not a typical enzyme active site. Extinction coefficients of the visible tail are modest, so ProCharTS will not replace tryptophan absorbance for every assay. Computed CT energies depend on TDDFT and the MD ensemble.
Key terms
- ProCharTS
- Protein Charge Transfer Spectra: 250–800 nm absorption from charged-residue CT transitions, not aromatic rings.
- Charge-transfer transition
- Electronic excitation that moves electron density between donor and acceptor groups (here NH3+/COO− and backbone).
- TDDFT
- Time-dependent density functional theory used to compute electronic absorption from MD snapshots.
- α3C
- 67-residue three-helix bundle without aromatic amino acids, used as the spectroscopic model.
- Molar absorptivity (ε)
- How strongly a 1 M solution absorbs at a stated wavelength, in M−1 cm−1.
Flashcards
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Quiz yourself
ProCharTS absorption is attributed to:
Common questions
Do proteins need Trp/Tyr/Phe to absorb above 250 nm?
Not always—charged Lys and Glu can produce CT bands and a visible tail (ProCharTS).
What structural motif is α3C?
A three-helix bundle previously solved by NMR.
What distances matter for the visible tail?
MD finds proximal 4–6 Å Lys amino–Glu carboxylate contacts that modulate CT.
How was the protein mass checked?
ESI-MS gave 7462.883 Da.
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