Page 351 - Carrahers_Polymer_Chemistry,_Eighth_Edition

314 Carraher’s Polymer Chemistry

O
O

NH H O
O N
H N
2
OH

O
O O
O
N O O O O
H
O

N
O O H
O

O
H N
2
NH
O
O
OH
O

FIGURE 9.8 Representative structure of melanin.

photosynthesis. Radiation is used commercially to increase the wood pulp yield through cross-
linking and grafting of lignin and other wood-components onto cellulosic chains. Radiation is also
used in the synthesis and cross-linking of many synthetic polymers.
Radiation is also important in the synthesis and rearrangement of important “surface” macromol-
ecules. Tanning of human skin involves the activation of the polypeptide hormone beta melanocyte-
stimulating hormone (MSH) that in turn eventually leads to the phenomena of tanning. Exposure to
higher energy light from about 297 to 315 nm results in both tanning and burning, whereas exposure
to light within the 315–330 nm region results in mainly only tanning.
Ultraviolet radiation activates enzymes that modify the amino acid tyrosine in pigment-produc-
ing cells, the melanocytes. The enzyme tyrosinase, a copper-containing oxygenase, catalyzes the
initial step, which is the hydroxylation of tyrosine to 3,4-dihydroxyphenylalanine that is oxidized to
dopaquinone subsequently forming the melanins (Figure 9.8). The concentration of tyrosine is rela-

tively high in skin protein. These modified tyrosine molecules undergo condensation forming mac-
romolecules known as melanins. Melanins have extended chain resonance where the pi electrons
are associated with the growing melamine structure. As the melanine structure grows, it becomes
more colored giving various shades of brown color to our skin. This brown coloration acts to help
protect deeper skin elements from being damaged by the UV radiation. The absence of the enzyme
tyrosinase that converts tyrosine to melanin can lead to albinism.
At least two colored melanins are formed—a series of black melanins and a series of so-called
red melanins. Our skin pigmentation is determined by the relative amounts of these red and black
melanins in our skin.

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