Necesidad de un procedimiento que permita utilizar la imagen como variable estratégica en hoteles cubanos

In order to develop carbohydrate microarrays, carbohydrates have to be attached to a surface. There are many different methods for this and broadly speaking they can be split into four categories (Table 3.1).8 There are many published examples of all of these methods for attachment and each category has advantages and limitations when compared to the others.

Table 3.1 Generalised methods of surface functionalisation in the development of carbohydrate microarrays. The pentasaccharide displayed is drawn using standard CFG nomenclature. Covalent Non-covalent Site specific Site non-specific x x x x x x x x x y

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3.2.2.1 Site non-specific and non-covalent attachment

At first glance this is a very attractive technique as it allows the carbohydrates to be in their natural form and does not require any expensive or laborious preparation of the surface. Many examples of this type of method exist in the literature including the non-covalent absorption of polysaccharides to black polystyrene and the immobilisation of polysaccharides on nitrocellulose.9, 10

However, it has been observed that the immobilisation process is dependent on the molecular weight of the compound being immobilised, where immobilisation is most efficient for larger molecules.9 This is a major drawback of this method as it means that the technique is only applicable to large polysaccharides and so smaller mono- or disaccharides cannot be probed. It also means that differential protein binding between polysaccharides may be caused by differences in polysaccharide immobilisation.

Non-covalent (or passive) absorption of polysaccharides onto a surface also has another drawback; deposition of the polysaccharide will be random and us such the molecule will be presented on the surface in a variety of orientations the majority of which will not be biologically relevant thus a lower signal may just be as a result of a low number of molecules presented in the biologically relevant conformation.11

3.2.2.2 Site non-specific and covalent attachment

This approach is appealing as it allows the use of unmodified glycans, which are bound to a modified surface. An example of this mechanism is the attachment of free

glycans to a surface modified with 4-azido-2,3,5,6-tetrafluorophenyl under UV light or the photochemical ligation of perfluorophenylazide derivatised carbohydrates to a poly(ethylene oxide) surface.12, 13 Whilst several other examples of this technique exist in the literature, one of the main limitations of this technique is the site non- specific nature of the attachment, which means that the glycans can attach in any orientation to the surface.

In 2012, Liang and Chen described the attachment of unmodified mono- di- and polysaccharides to modified glass and gold slides.14 The process they described initially involved the hydroxylation of either a glass or gold surface followed by addition of cyanuric chloride, this compound contains three chlorine atoms all of which can undergo hydrolysis and bind to hydroxyl containing molecules. The cyanuric chloride binds to the surface and then carbohydrates will then bind to the remaining binding sites of the compound in any orientation.14 Whilst this means that any orientation of molecule can be presented on the surface, lectin specificity was retained implying that the natural presentation was present on the surface and this means that this technique may overcome the challenges of the other methods that utilise the site non-specific, covalent attachment of glycans.

3.2.2.3 Site specific and non-covalent attachment

Examples of this approach include the use of biotin-conjugated glycans coupled with a streptavidin surface,15 the attachment of fluorous tagged carbohydrates to fluorous derivatised glass slides16 and the attachment of carbohydrates conjugated to either bovine serum albumin or human serum albumin to epoxide functionalised microscope slides.17

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Whilst this technique ensures that orientation of the glycan is controlled, it has the key disadvantage that it requires both a chemically modified surface and modified glycans, which can be expensive, and synthetically challenging, thus limiting the use of techniques based on this approach. Also the storage of protein functionalised surfaces is crucial to avoid protein denaturation and loss of surface functionalisation.

3.2.2.4 Site specific and covalent attachment

This method is the most desirable as it ensures that all glycans are presented in the same orientation and whilst many methods require both surface and glycan modifications there are some that can utilise unmodified glycans. Many methods that involve the modification of the surface and the glycans have been described, for example, the attachment of modified carbohydrates in the form of glycosylamines or malemide conjugated carbohydrates onto thiol modified glass slides.8 This approach is also utilised by the CFG in the production of a carbohydrate microarray, which uses glycans with an amino linker, which are covalently attached to NHS coated glass slides.1, 15, 18

However these methods require the modification of carbohydrates which is challenging, time consuming and often requires multiple purification steps.19 Site- specific and covalent immobilisation of chemically unmodified carbohydrates on a modified surface has also been described and this eliminates the need for modification of the glycan thus making it the most desirable technique. However, this technique is also the most challenging and few examples exist in the literature. Reductive animation is the most common choice for attaching unmodified carbohydrates to a

surface, this involves the use of a hydrazide functionalised surface which is commercially available in the form of 96 well plates.20 The reducing end of the sugar reacts with the hydrazide on the surface, after which the Schiff base is reduced. One example of this method is the attachment of unmodified oligosaccharides to hydrazide modified self-assembled monolayers on a gold surface.11 This method has also been used in the attachment of unmodified glycans to hydrazide functionalised polymers.21 Whilst this technique allows the site-specific attachment of unmodified glycans (and thus allows biological orientation to be conserved) its key disadvantage is that only reducing sugars can be attached to the surface, which limits the number of glycans that can be analysed.

Recently a new technique has come to light in which unmodified carbohydrates were attached to a silicon nanowire (SiNW). This has the advantage that interactions can be detected in a label free manner and real-time binding can be observed. The technique was also significantly more sensitive than any other techniques currently available (the authors were able to detect lectin binding down to 100 fg.mL-1). Currently the synthesis of these SiNW biosensors is challenging, making the technique inaccessible, but there is the potential to commercially produce arrays of these sensors, which would greatly improve the sensitivity of protein-carbohydrate interaction detection that is currently observed.22