Fundamentals of Photobiology

Fundamentals of Photobiology: EMS & Light Physics - Radiant Rules

• Electromagnetic Spectrum (EMS): Relevant portions include Ultraviolet (UV), Visible Light, and Infrared (IR).
  • UVA: 320-400 nm (📌 A=Aging, Antigenic)
  • UVB: 280-320 nm (📌 B=Burning, Blistering, Barrier function)
  • UVC: 100-280 nm (📌 C=Carcinogenic, Can't reach Earth)
• Light Physics & Properties:
  • Light consists of photons (energy packets) behaving as waves.
  • Energy ($E$) is inversely proportional to wavelength ($\lambda$): $E = hc/\lambda$.
  • Shorter $\lambda$ = Higher $E$ (e.g., UVC > UVB > UVA).
• Fundamental Laws (Radiant Rules):
  • Grotthuss-Draper Law: Light must be absorbed by a chromophore to initiate a photochemical reaction.
  • Bunsen-Roscoe Law of Reciprocity: Biological effect is proportional to total energy dose (Intensity × Duration of exposure).
  • Inverse Square Law: Radiant intensity decreases proportionally to $1/d^2$ (d=distance from source).

⭐ UVC radiation, though most potent in causing erythema and DNA damage, is almost entirely absorbed by the ozone layer and does not reach the Earth's surface.

Fundamentals of Photobiology: Skin Chromophores - Dermal Light Traps

• Chromophores: Molecules that absorb specific wavelengths of light (UV, visible, infrared).
  • Absorption initiates photochemical reactions.
• Major Endogenous Skin Chromophores:
  • Melanin: Epidermal; absorbs broadly (UVB > UVA > visible).

    ⭐ Melanin is the most important endogenous chromophore, providing natural photoprotection by absorbing UV radiation broadly, particularly UVB and UVA.

  • Hemoglobin (Hb): Dermal; Oxy-Hb & Deoxy-Hb absorb visible light (peaks in blue, green, yellow).
  • Porphyrins: Absorb Soret band (~400 nm) & Q bands (visible).
  • Nucleic acids (DNA/RNA): Epidermal; absorb UVB (260 nm peak).
  • Urocanic acid: Epidermal; absorbs UVB (270-290 nm).
  • Amino acids (tryptophan, tyrosine): Absorb UVB.
  • NADH/NADPH, Flavins: Absorb UVA & visible light.
• Dermal Light Traps: Dermal structures (e.g., collagen, elastin) scatter light, ↑ path length, ↑ chance of chromophore absorption.

Mnemonic: 📌 "My Heavy Protein Nucleus Usually Absorbs Nicely" (Melanin, Hemoglobin, Porphyrins, Nucleic acids, Urocanic acid, Amino acids, NADH/NADPH).

Fundamentals of Photobiology: Photochemical Reactions & Laws - Photon Power Plays

• Laws of Photochemistry:
  • Grotthuss-Draper Law (1st): Light must be absorbed by a chromophore for reaction.

  ⭐ The Grotthuss-Draper Law is fundamental: light must be absorbed by a molecule (chromophore) for a photochemical reaction to occur.

  • Stark-Einstein Law (2nd): One photon activates one molecule (primary step).
    • Quantum Yield ($ ext{Φ}$): $\frac{\text{Molecules reacted}}{\text{Photons absorbed}}$.
• Photochemical Reactions:
  • Type I (O$_2$-independent): Excited chromophore reacts directly with substrate (electron transfer).
  • Type II (O$_2$-dependent): Excited chromophore reacts with $ ext{O}_2$ forming ROS (e.g., $ ext{}^1 ext{O}_2$, $ ext{O}_2^{\cdot-}$).
    • Crucial for Photodynamic Therapy (PDT).
• Key Players & States:
  • Chromophores: Absorb light (melanin, porphyrins, psoralens).
  • Energy States: Ground ($S_0$), Excited Singlet ($S_1$), Triplet ($T_1$). $S_1 \rightarrow T_1$ (Intersystem Crossing).

Fundamentals of Photobiology: Skin's Photobiologic Responses - Sunburn & Beyond

• Acute Responses (Hours to Days):

  • Sunburn (Erythema):
    • Mediators: Prostaglandins (PGs), histamine, cytokines.
    • Peaks 12-24 hrs, resolves ~72 hrs.
    • Minimal Erythema Dose (MED): Lowest UV dose for erythema at 24 hrs.
  • Tanning:
    • Immediate Pigment Darkening (IPD): UVA/Visible; melanin photo-oxidation.
    • Delayed Tanning (Melanogenesis): UVB; ↑melanin synthesis; starts 48-72 hrs.
  • Epidermal Hyperplasia: Protective thickening.
  • Vitamin D Synthesis: UVB converts 7-DHC to Vit D3. $7\text{-DHC} \xrightarrow{UVB} \text{PreVitD}_3 \rightarrow \text{VitD}_3$.

• Chronic Responses (Years):

  • Photoaging (Dermatoheliosis): Wrinkles, solar elastosis, pigmentary changes.
  • Photocarcinogenesis:
    • UVB is primary carcinogen.
    • DNA Damage: Cyclobutane Pyrimidine Dimers (CPDs), (6-4) Photoproducts (6-4PPs). $DNA + UVB \rightarrow CPDs + (6\text{-}4)PPs$.
    • Leads to Actinic Keratosis, BCC, SCC, Melanoma.

  ⭐ UVB-induced cyclobutane pyrimidine dimers (CPDs) are the most common type of DNA damage leading to mutations and skin cancer if not repaired.

High‑Yield Points - ⚡ Biggest Takeaways

• UV radiation: UVA (320-400nm), UVB (290-320nm), UVC (200-290nm); UVB most erythemogenic.
• Skin chromophores (DNA, melanin) absorb UV light, initiating reactions.
• UVB causes direct DNA damage (pyrimidine dimers) and sunburn.
• UVA penetrates deeper, causes photoaging, and indirect DNA damage (ROS).
• Sunburn cells (apoptotic keratinocytes) are a key feature of UVB injury.
• Minimal Erythema Dose (MED) quantifies individual UVB sensitivity.
• Grotthuss-Draper Law: Light must be absorbed for photochemical effect.