Statement

At 50 kGy gamma radiation, aromatic amino acid fluorescence (tyrosine, phenylalanine) is reduced to 9–25% of original intensity, while PAH fluorescence retains ~50%.

Evidence

Blanco et al. 2018, Astrobiology 18(12):1497–1516. DOI: 10.1089/ast.2016.1645

Quantitative data at 50 kGy gamma:

• Tyrosine-BSA: 25% retained (75% loss)
• Phenylalanine derivative: 9% retained (91% loss)
• Benzo-a-pyrene-BSA (PAH): 51% retained (49% loss)

Two-pathway radiolysis model: N/N₀ = A·exp(−k₁·D) + B·exp(−k₂·D), where A = 0.649, k₁ = 0.00288 kGy⁻¹, B = 0.351, k₂ = 0.202 kGy⁻¹ (gamma). The fast pathway (k₂) is exhausted by 50 kGy; from 50–100 kGy only the slow pathway (k₁) operates, reducing retention to approximately 87% of the 50 kGy values.

Cross-reference: Bhartia et al., 2010, Applied and Environmental Microbiology 76(21):7231–7237. DOI: 10.1128/AEM.00943-10 Established that DUV native fluorescence detection of bacteria depends on intact aromatic fluorophores (tyrosine, tryptophan at λmax 325–341 nm).

Argument

A1: Dose extrapolation is conservative. Blanco et al. measured at 50 kGy; the two-pathway model predicts continued degradation at 100 kGy. Since the fast pathway is already exhausted, the remaining slow pathway reduces 50 kGy retention values by an additional ~13%. The direction is unambiguous.

A2: Differential degradation creates systematic bias. Biological aromatics (tyrosine, phenylalanine) degrade 2–5× faster than abiotic PAHs. This asymmetry means post-irradiation fluorescence ratios systematically underrepresent biological contributions.

Implication

Biological aromatics degrade faster than abiotic PAHs. Supports "Affected" for:

• FLUORESCENCE SPECTROSCOPY × SCI 2.2 (Biosignatures)
• FLUORESCENCE SPECTROSCOPY × SSA 1.2 (Abiotic Baseline)

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