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08.01.2016
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Solar
UV filters in sunscreens Things we should know on their nature and functioning
Prof. Luca Valgimigli
Sunscreens or UV filters are natural or synthetic compounds that are included in cosmetic formulations to protect the skin from damages caused by solar exposure.
In the case of specific sunscreen formulas (to be used for sunbathing), they are responsible for the Solar Protection Factor, SPF, whose value indicates the degree of protection the formula will guarantee to our skin, thereby avoiding erythema and other damages like photo-aging.
What do UV filters exactly do and how do they act?
The principle is very simple: UV filters reduce the amount of solar radiation reaching the surface on which they are applied, for instance skin surface. The difference among the many UV filters lies in the mechanism by which they achieve this goal.
Physical filters reflect (back) a portion of solar radiation, letting only a limited fraction of light cross them and reach the skin. Therefore, they do not interact with solar radiation and they are not altered by it at all.
Chemical filters, instead, absorb a portion of solar radiation, and use it to achieve a higher quantum energy state. Immediately after, such excess energy is released to the environment in the form of heat, thereby making the filter ready to absorb solar energy again.
The different mechanisms of action are summarized in the picture on the left. In both cases, the amount of solar energy allowed to pass (which is not reflected or absorbed) depends on the amount of filters applied on the skin, which determines the solar protection factor.
Both types of filters offer advantages and disadvantages: physical filters have the advantage of being completely stable and not being damaged by solar radiation. Furthermore, compounds like Zinc oxide are totally inert and are very safe for skin. Their disadvantage lies in the difficulty to reach high SPF values without causing the so-called “white effect”. In order to reduce this effect, it is possible to use them in micronized form, which is much more precious (BeC uses this form in all sunscreen formulas). However, even micronized forms do not allow reaching high protection without becoming “visible” on the skin.
Therefore, in high protection formulas they are usually combined with chemical filters. Chemicals filters, infact, have the advantage of being very effective and of offering excellent performance while being completely invisible. They disperse very well in the formulation affording a more homogenous protection (corresponding to a more even tan!); however, since they absorb the solar energy, they are subjected to the risk of photo-degradation, or they could undergo photochemical reactions.
At this point, let’s better clarify a very important aspect. Often, the expression “chemical filter” is confused with “synthetic filter”, i.e. artificial, while it is generally believed that physical filters are natural. This is not necessarily true! Physical filters are normally inorganic, i.e. minerals, and normally they are natural, but they can as well be synthetic, i.e. artificially produced.
On the other hand, the most common chemical filters are often synthetic, i.e. man made; however, there exist also examples of natural chemical filters. Among the most important ones, there is certainly gamma-oryzanol, which is extracted from rice bran (see picture on the left), and other examples are cinnamic acid, typical of cinnamon but found also in Brassica vegetables (cabbage, broccoli, etc) and in shea butter, carotenoids(e.g. from carrots, tomatoes and several red-orange fruits), Vitamin E and many more.
These compounds have also the advantage of possessing additional properties (e.g. antioxidant, soothing, calming, anti-age); unfortunately, however, they are not as effective as synthetic chemical filters. BeC uses natural chemical filters when high SPF values are not requested (e.g. in Huile SolE’), but when these are needed, synthetic chemical filters must have been combinate.In conclusion, don’t be fooled by the words: chemical or physical filters indicate their mechanism of action not their origin. The origin can either be natural or synthetic, but the most important aspect is their stability toward solar radiation, which is fundamental for our safety under sunlight.
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.
