Notes on Chromophores (Cutaneous Photobiology)

1. Definition

• Each chromophore has a characteristic absorption spectrum — usually with one peak wavelength that most efficiently excites it (absorption maximum), plus a range of less-efficient wavelengths.
• The term is used in two related but distinct contexts in dermatology: (1) endogenous photobiology — molecules in skin that absorb ambient UV/visible light and mediate biologic effects (sun damage, vitamin D synthesis, photoaging), and (2) laser/light-based therapeutics — molecules deliberately targeted by device wavelengths for selective photothermolysis.
• Fitzpatrick's Dermatology, p. 267-268

2. The Photobiologic Cascade (Fig. 17-2)

• Longer wavelengths penetrate matter (and skin) more deeply than shorter ones. UVA (315-400 nm) penetrates deeper into the dermis than UVB (280-315 nm), of which only a small fraction reaches beyond the epidermis; UVC has the shallowest penetration, reaching only the stratum corneum/upper epidermis.
• Because of scattering and absorption in the stratum corneum, the most effective wavelength for exciting a chromophore in situ can differ from its intrinsic absorption maximum (e.g., naked DNA peaks at 260 nm, but 300 nm is the most effective wavelength for DNA damage in basal keratinocytes, since shorter wavelengths are filtered out before reaching that depth).
• Fitzpatrick's Dermatology, p. 267-268

3. Major Endogenous Cutaneous Chromophores (Fig. 17-4)

• Fitzpatrick's Dermatology, Fig. 17-4, p. 268

4. What Happens After Absorption: Excited States (Fig. 17-5)

• Singlet excited state — no change in electron spin. Short-lived (nanoseconds).
• Triplet excited state — reached via intersystem crossing (isc), involves a change in spin. Longer-lived (up to seconds), and biologically more consequential because the longer lifetime allows more chemistry to occur (e.g., reaction with oxygen).

1. Return to ground state with heat release (internal conversion, ic) — no photochemistry.
2. Return to ground state with photon emission:
   • From singlet state → fluorescence (fast, nanoseconds). Per the Stokes law, fluorescent light is always of longer wavelength (less energetic) than the exciting light (e.g., blue-light excitation of protoporphyrin IX yields red fluorescence).
   • From triplet state → phosphorescence (slower, can persist seconds; emission wavelength longer than fluorescence).
3. Photochemical reaction → forms a new molecule, a photoproduct (e.g., photoexcited 7-DHC → previtamin D3; UV-excited DNA → cyclobutane pyrimidine dimer).
4. Photosensitized reaction → energy transfer from the excited chromophore to a different molecule. Classic example: excited porphyrin transfers energy to molecular oxygen, generating singlet oxygen, a reactive species that then damages other substrates (e.g., guanine bases in DNA). This underpins phototoxic drug reactions and photodynamic therapy.

• Fitzpatrick's Dermatology, Fig. 17-5, p. 268

5. Clinical/Physiologic Relevance of Cutaneous Chromophores

• Vitamin D photobiology: 7-DHC is photoexcited by UVB to previtamin D3, then thermally isomerizes to vitamin D3 — a direct example of a chromophore-driven photoproduct with major systemic (calcium/phosphorus metabolism) consequences.
• Photocarcinogenesis: DNA as chromophore → direct CPD formation (classic pathway) and an alternate melanin-dependent "chemiexcitation" pathway that generates CPDs even after UV exposure ends (dark CPDs), described by Premi et al.
• Photosensitivity/phototoxicity: Porphyrins and drug-derived chromophores absorbing UVA/visible light and generating reactive oxygen species (e.g., porphyria cutanea tarda, drug-induced photosensitivity).
• Sunscreen chemistry: Organic UV filters (e.g., avobenzone, oxybenzone) are themselves synthetic chromophores engineered to absorb UVA/UVB and dissipate energy as heat rather than transferring it to skin biomolecules. Newer research explores chromophore-based and nanoparticle sunscreen formulations for broader-spectrum, more photostable protection (Rajasekar et al., 2024, PMID 38226149).
• Pigmentation chemistry: Pheomelanin's benzothiazine chromophore is being studied for its dual role in photoprotection versus phototoxic/pro-oxidant potential in fair, red-haired phenotypes (Alfieri & Panzella, 2023, PMID 37687069) — relevant to why red-haired/fair-skinned individuals have higher UV-related skin cancer risk despite pigmentation.

6. Chromophores in Laser and Light-Based Therapy (Selective Photothermolysis)

• Melanin and oxyhemoglobin absorption spectra overlap substantially in the visible range, so lasers must be chosen to maximize selectivity for the intended target while minimizing competing absorption (e.g., 755 nm alexandrite has good melanin selectivity with relatively low oxyhemoglobin absorption).
• Beyond ~1064 nm, both melanin and hemoglobin absorption fall off sharply and water becomes the dominant chromophore, which is why longer-wavelength/ablative devices are water-driven.
• Longer wavelengths generally penetrate deeper (consistent with the same physical principle noted in solar UV/visible penetration), allowing depth-selective targeting of superficial versus deep vascular or pigmented structures.
• Adequate target chromophore must be present for treatment to work — e.g., laser hair removal requires melanized hair in the follicle, so treatments are timed to the hair growth cycle (6-8 week intervals) to ensure chromophore ("target") availability.
• In darker skin types, epidermal melanin acts as a competing chromophore, increasing risk of epidermal thermal injury/dyspigmentation when using melanin-targeting lasers — necessitating longer wavelengths, longer pulse durations, and lower fluences.
• Fitzpatrick's Dermatology (Laser and Light Sources chapter); Pfenninger and Fowler's Procedures for Primary Care; Andrews' Diseases of the Skin

7. Quick Summary Table