Spectroscopy
Charged amino acids absorb past 250 nm
Open access · cc by · source: Europe PMC
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.
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.
Key findings
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.
Methodology
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.
Limitations
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.
How this study connects
Role on claims
Each row is a claim on a concept or method page where this paper supports, challenges, or qualifies the statement. Roles are hand-checked — not a model guess.
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.
Evidence for the claim as stated.
A designed three-helix protein with no Trp/Tyr/Phe still absorbs past 250 nm because 54% of its residues are charged (17 Lys, 17 Glu, 2 Arg). α3C has 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 on MD snapshots assigns charge-transfer bands between NH₃⁺/COO⁻ side chains and the backbone (ProCharTS), modulated by 4–6 Å Lys–Glu contacts. The visible tail is modest; it will not replace tryptophan assays for every protein.
Evidence for the claim as stated.
Absorption near 400 nm is not one phenomenon. GSH and glycerol silver papers read it as a plasmon that tracks particle size; the titanyl paper reads a Ti–peroxo charge-transfer colour for an H₂O₂ LOD; ProCharTS reads a weak protein CT tail (ε hundreds M⁻¹ cm⁻¹) out to 800 nm; the Fe paper needs 370–1200 nm transient spectra to assign PALCT versus ⁵MC. A student who says 'the UV-vis peak proved it' has not chosen among those assignments.
Evidence for the claim as stated.
TDDFT spectra computed on MD snapshots can assign a surprising UV-vis tail. Protein α3C (54% charged residues) has ε ≈ 7338 M⁻¹ cm⁻¹ at 250 nm and ε ≈ 964 and 501 M⁻¹ cm⁻¹ at 450 and 800 nm; 4–6 Å Lys–Glu contacts modulate ProCharTS charge-transfer bands. Computed CT energies inherit both the TDDFT functional and the MD ensemble.
Evidence for the claim as stated.
Open questions
Tensions this paper is part of
From concept pages' “where studies disagree.” Disagreement means the same question; scope means different assays, populations, or outcomes.
Absorption near 400 nm is not one phenomenon. GSH and glycerol silver papers read it as a plasmon that tracks particle size; the titanyl paper reads a Ti–peroxo charge-transfer colour for an H₂O₂ LOD; ProCharTS reads a weak protein CT tail (ε hundreds M⁻¹ cm⁻¹) out to 800 nm; the Fe paper needs 370–1200 nm transient spectra to assign PALCT versus ⁵MC. A student who says 'the UV-vis peak proved it' has not chosen among those assignments.
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