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18.01.2016
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Search, Solar
Photo-degradation of sunscreens and skin damaging
Prof. Luca Valgimigli
We have previously discussed the mechanism of action of sunscreens, underlining that chemical sunscreens absorb the energy of sunlight and subsequently re-emit it in the form of heat, possibly without any alteration in the structure of the sunscreen itself.
Does it always go this way?
Unfortunately, it doesn’t and we wish to discuss here a bit more on the photostability of sunscreens. Although this aspect is often overlooked, but it has major consequences on our health.
After having absorbed sunlight energy, chemical sunscreens are in a higher quantum energy state, from which they can undergo one of three processes:
- they can go back to the lower energy state by losing energy in the form of heat (which is often not perceived by our senses), thereby making ready to start over again and absorb more solar energy;
- they can release the excess energy by breaking their structure into fragments, i.e. they degrade and form free radicals or use the energy to react with other molecules (photochemical reactions);
- they can transfer the excess energy to another molecule by “hitting it”, i.e. they behave as photosensitizers.
Ideally, sunscreens should use only the first route; however, not all the sunscreens are identical and some of them, which are less stable than others, after several absorption-emission cycles, may take the second route (we will discuss in a following post of those sunscreens that take the third route and act as photosensitizers).
What happens if a sunscreen degrades?
If the sunscreen degrades, it most commonly produces free radicals and other dangerous species, and If the sunscreen has been absorbed deeply into our skin, those free radicals can attack and damage proteins and DNA, accelerating the photo-aging processes of the skin.In high quality formulations such damages can be prevented by the abundant presence of antioxidants (e.g. vitamin E) in the formula, so to block free radicals before they can cause any damage.
Therefore, in choosing a solar formulation is important to search high quality products after careful reading of their label!
What is the relation between the degradation of sunscreen and protection factor?
There is, however, another point to take into account: as the sunscreen degrades the solar protection factor (SPF) of the product progressively decreases. And antioxidants cannot help in this reguard.The loss effect of SPF during on exposing to sunlight depends on the photostability of the sunscreens: with highly photostable sunscreens the phenomenon has negligible relevance; however, with little photostable sunscreen molecules, which are unfortunately the most common in commercial formulas, the phenomenon is very relevant, as illustrated in the graphs on the left, displaying the comparison between two real formulations: a famous commercial products (don’t ask which one) and BeC sunscreen SPF15 cream.
Many of people think that “waterproof” sunscreen formulas which can resist for several minutes of swimming in seawater would provide a safer protection for the entire day, since the product would not be washed away.
A look at the graphs clearly tells that a waterproof sunscreen formula does not guarantee safer daylong protection.
First of all, we should consider whether the sunscreen contained in the formula is photostable. Moreover, we should consider that, even if the sunscreen is photostable, during a typical day at the seaside, we dry ourselves with a towel, we roll up in the sand which we clean up by rubbing or washing our skin, we sweat in the heat or during physical activity (e.g. beach sports). All such actions end up removing the sunscreen form our skin anyway. Therefore, a high quality sunscreen formula, based on photostable components, is the ideal choice for a safer protection, but we should not forget that it is wise to re-apply the product several times during the day, particularly in the case of children.
Cyclooxygenase, lipoxygenase and the inflammatory process
Cyclooxygenase and lipooxygenase are the two families of enzymes that are commonly involved in the inflammatory process, through a complex of reactions which is called arachidonic acid cascade. This complex of reactions develops as follows: a first enzyme, a phospholipase cleaves the phospholipids of biological membranes, releasing arachidonic acid, a polyunsaturated fatty acid with 20 carbon atoms (eicosa-5Z,8Z,11Z,14Z-tetraenoic acid ; C20:4; ω-6). The arachidonic acid is then transformed by two parallel enzymatic pathways, that is, by two families of enzymes: the cyclooxygenase which transforms it into prostaglandins and thromboxanes and the lipooxygenase which transforms it into hydroperoxides which in turn transform into leukotrienes .
There are two cyclooxygenase isoforms indicated with type 1 and type 2, briefly COX-1 and COX-2. COX-1 is the enzyme present in most cells (except red blood cells), and is constitutive, that is, it is always present. COX-2 is an inducible cyclooxygenase isoform: it is constitutively present in some organs such as brain, liver, kidney, stomach, heart and vascular system, while it can be induced (i.e. developed if necessary) following inflammatory stimuli on the skin, white blood cells and muscles.
There are various types of lipooxygenase that lead to different products, the most important in the inflammatory process is 5-lipooxygenase, 5-LOX.
Prostaglandins, Thromboxanes, and Leukotrienes
Prostaglandins, Thromboxanes, and Leukotrienes are chemical messengers or mediators, that is, molecules that bring a message to specific cells and activate or deactivate metabolic responses in these cells. They, therefore, have a function similar to hormones, only that, unlike what hormones do, the chemical message is carried only at a short distance, that is, only to the cells that are in the vicinity of the place where the mediators were produced. There are different prostaglandins, different thromboxanes and different leukotrienes that carry specific messages. In many cases these act as mediators of the inflammatory process , therefore they trigger all the events that are involved in inflammation:
– vasodilation with consequent blood supply (redness),
– increased capillary permeability with consequent fluid exudation (swelling or edema),
– stimulation of nociceptive nerve signals (pain),
– on-site recall of immune system cells that attack a possible invader (chemotactic action)
– activation of the biosynthesis of fibrous tissue to strengthen or repair the affected part (even if there is no need)
– generations of free radicals that can chemically destroy an invader (but also damage our tissues, i.e. they just “shoot in the middle”).
Prostaglandins and thromboxanes, however, also play important physiological roles in normal conditions, i.e. in the absence of inflammation. For example, they regulate the secretion of mucus that protects the walls of the stomach, they regulate the biosynthesis of cartilages and synovial fluid in the joints, they regulate vasodilation, hence the correct flow of blood in the various local districts, and more.
Triglycerides
Triglycerides are the main components of most oils and fats. These are heavy, non-volatile and little polar molecules, insoluble in water, made up of glycerol (or glycerin) esterified with three molecules of fatty acids: therefore, it is a tri-ester of glycerin, from which the name derives. Each fatty acid contains 8 to 22 carbon atoms (commonly 16 to 18) and can be saturated, mono-unsaturated or poly-unsaturated. The size of the fatty acids and their saturation determines the physical and sensorial properties of the triglycerides, which can appear as oils (liquids at room temperature) or fats (solid or semi-solid) and can have greater or less greasiness and smoothness on the skin. Unsaturated triglycerides or with shorter fatty acids are more fluid and have greater flowability.
Fatty acids (saturated, mono-unsaturated and poly-unsaturated)
The name fatty acids is commonly used to indicate those organic acids that are found in the composition of lipids, that is, in animal and vegetable oils and fats, both in the free form and in the form of esters with glycerol (e.g. in triglycerides), or they are esterified with “fatty” alcohols, that is, long chain alcohols, to form waxes. Fatty acids are carboxylic acids (formula R-COOH) which have a long carbon chain (R), unlike common organic acids such as acetic acid and propionic acid, which have 2 or 3 carbon atoms in total, respectively. Fatty acids are defined as saturatedif they do not have double carbon-carbon bonds, (called “unsaturations”), they are defined mono-unsaturated if they have only one, they are defined mono-unsaturatedpoly-unsaturated if they have two or more double bonds (see figure). The term omega-3 (ω-3) or omega-6 (ω-3), refers to the position of the first double bond starting from the bottom of the chain of carbon atoms: if the first double bond is encountered after 3 carbon atoms the fatty acid is classified as omega-3 , if after six carbon atoms omega-6 , as shown in the figure. The most common saturated fatty acids are palmitic acid (16 carbon atoms and no double bond, C16: 0) and stearic acid (18 carbon atoms, 18: 0), the most common mono-unsaturated is the oleic acid, typical of olive oil (18 carbon atoms and 1 double bond in position 9, C18: 1; ω-9), while the most common poly-unsaturated are linoleic acid and linolenic acid, progenitors respectively omega-6 and omega-3 (see figure).
Terpenes and terpenoids
Terpenes or terpenoids are a large family of natural molecules, typically containing 10 to 30 carbon atoms, which are biosynthesized from a common “brick”, isopentenyl pyrophosphate (IPP), containing 5 carbon atoms (see figure). The discovery that the repetitive brick consists of 5 carbon atoms is relatively recent, while it was once assumed that the entire family was created by repeating a brick of 10 carbon atoms, which was called “terpene”. Therefore, the molecules with 10 carbon atoms (such as limonene, see figure) were called mono-terpenes, i.e. composed of a single brick, diterpenes those with 20 carbon atoms (e.g. the cafestol that gives the aroma to the coffee), triterpenes those with 30 carbon atoms (e.g. beta-carotene). Since molecules made from 15 carbon atoms were also found (such as bisabolol), it was thought they contained a terpene and a half, and were called sesquiterpenes (from the Latin semis = half + atque = and). Today it is known that the repetitive unit is composed of 5 carbon atoms, therefore it is easy to understand how mono-terpenes contain two (see figure), sesquiterpenes three, diterpenes four, triterpenes six.
