Investigation of Single Chain Sugar-based Glycolipid

Review on Investigation of Single Chain Sugar-based Glycolipid Self-Assembly in Lyotropic Phase Using Fluorescence Spectroscopy

• M.Faisal Khyasudeen*

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ABSTRACT: Glycolipid in lyotropic phase has been widely study due to its nature similar to those in biological membrane. This uncharged amphiphilic molecules has been used in cosmeceutical industry as an emulsifier and as nano-carrier in pharmaceutical industry. Some of the reason on its application is because of its features such as low toxicity on environment, biodegradable, specific sugar-cell recognition as well as relatively cheap in production. Vesicle formation by the single chain glycolipid allow high efficiencies of drug delivery by the effective encapsulation of the active material until it reached the specific target cells. In this report, fluorescence studies using different probe were discussed to understand the stability and degree of flexibility of the lipid for its self-assembly. Tryptophan (Trp) with its ester derivatives (Trp-C4 and Trp-C8) are examples of probes to study the glycolipid hydrophilic head while pyrene molecules to gain insight of the hydrophobic group. Steady-state fluorescence indicate a reduction in polarity gradient from polar domain to the non-polar domain resulted from sugar head group and hydrocarbon tail respectively. While fluorescence lifetime measurement for the probes uncover an extra information such as ability to form two different rotamer or any heterogeneity arises due to flexibility of lipid self-assembly. Specifically, this review will be emphasized on characterization of glycolipid with probes component and how in turn these are related to the nature of self-assembly for the glycolipid.

INTRODUCTION

Amphiphiles are molecules that have a hydrophilic head and a hydrophobic tail. Because of this, amphiphiles when mixed with water is capable of self-assembly into a wide range of different structures with a variety of properties. Shown in Figure 1 are some examples of such structures [1, 2]. They include the lamellar structure, where the amphiphiles self-assemble to give a sheet like structure; the micellar phase where the amphiphiles form a spherical structure with a hydrophilic exterior and a hydrophobic interior; the hexagonal structure where amphiphiles form micellar cylinders that are stacked in a hexagonal lattice; and a whole family of cubic structures, which resembles pipe joints linking cylindrical micellar units. This lyotropic mesomorphism is exhibited by many amphiphiles [3]. The amphiphile systems can transform between these mesophases depending on the relative amphiphiles/water concentrations, salt concentrations, pH, temperature and pressure.

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Apart from the aesthetics of the various structures, the versatility of some of these amphiphiles structures has also been of great scientific importance and utility. For example, our body is made of 10^13 cells that are constructed from complex membranes self-assembled from amphiphilic phospholipids [4]. Lipids such as monoolein in its cubic phase has been used to crystallize membrane proteins, thus making possible their x-ray diffraction structure determination [5]. Similar lipids have also been used for drug delivery purposes [2]. Another example is the glycolipids [1], which form minor but integral components in the cell membranes of prokaryotes and eukaryotes, and are responsible in maintaining structure integrity in cells.

Figure 1. Phase change in the Glycolipid self-assembly by changing the temperature of the system

Glycolipids are one of the examples in large family of glycoconjugates in which the sugar head group is bonded to hydrocarbon chain. It is an amphiphilic molecule derived from the carbohydrate headgroup whereas the hydrocarbon tail builds up the hydrophobic part of the molecule. The glycolipids can exhibit liquid crystals properties and have mesophase behaviour. The advantages of using sugar-based non-ionic surfactants compared to the usual surfactants because they are highly biodegradable, environmentally friendly, and less toxic [2,3]. Besides that, they are also much cheaper, and have diverse biological activity.

Figure 2. Example of probe used

Alkyl glycoside has been used for numerous surfactant application. Microemulsion form by these nonionic surfactant will help in dispersing the water and oil in continuos phase. Direct application such as detergent and cleaner has been use including alkylpolyglucoside (APGs) where it is reported that APGs improves the foam properties of the product according to IPP quality standard. In addition, one development of alkyl glycoside which is still in its infancy is on its application as vesicles for drug delivery carrier as reported by Kiwada and co-workers. It is reported that high entrapment efficiencies of the anticancer drug with prolonged in vitro drug released particularly in spleen and liver were shown using alkyl glycoside as nano-carrier. Furthermore, it reported by Rauzah.H and coworker, the use of similar vesicle to enhance the drug skin penetration using maltosylated and lactosylated alkyl glycoside.

Understanding the various factors that contribute to the stability of these different lipid phase structures will be important in further applications of these fascinating objects. One important factor that determines the stability of the various phases in different conditions are the hydrogen bonding networks of the water environment surrounding the amphiphile’s hydrophilic headgroup.

In this report, fluorescence studies using steady state and time-resolved measurement were discussed in guiding to correlate the behavior of the probes with the local environment of the glycolipid. Columnar phase of an aqueous formulation of the single chain alkyl glycoside such in the example depict in figure 3 were reviewed due to the ability of it to form in wide range of concentration and temperature (cite the phase diagram).

Since lipid membrane usually do not possess any intrinsic fluorescence, it is common to used extrinsic associating probe which strongly sensitive to its surrounding environment. One of the most common membrane probe is tryptophan and pyrene. Trp is used as a local reporter for hydrophilic head and pyrene which is non-polar molecule to probe in hydrophobic region.

Figure 3. Example of chemical structure of single chain glycolipid.

DISCUSSION

One of valuable property of fluorescence probe is in their sensitivity towards different polarity gradient. Fluorophore such as trp shows a shift spectrum in steady state measurement correspond to change in dielectric constant of solvent. Spectrum shift as in Figure 3 reported by (Idayu paper) for both glycolipid, shows that the trp peak maxima shift towards the blue side by decreasing the solvent polarity. This is due to the solvent effect which cause the stabilization of excited state by the higher dielectric constant of solvent molecules. As explained in (prof Lakowicz), normally, fluorophore possess a higher dipole moment in the excited state compared to in the ground state. Thus the solvent dipole can relax to lower the energy of the excited state after the absorption process. The reported unstructured fluorescence is due to the 1La state while the structured fluorescence is reported to be in 1Lb state. The state in 1La is a solvent sensitive state towards the polar nitrogen atom indole group in tryptophan.

(1a) Steady-state and lifetime measurement for hydrophilic head using tryptophan with its derivatives

Steady state spectrum of tryptophan and its ester derivative (Trp, Trp-C4, and Trp-C8) together with the lipid embedded were reported by (idayu paper) in n-dodecyl β-D-maltoside(βMaltoOC12) and octyl β-D-glucoside(βGlcOC₈). From the fluorescence spectrum, it is clear that when the tryptophan and its derivative embedded in lipid shows almost a similar pattern as depict in figure 3. The spectra observed shift to the blue side relative to that in buffer. This indication explain the different local environment senses by tryptophan and its derivatives in the lipid system. By comparing the two spectra, tryptophan molecule shows less polar environment as compared to bulk water. This can be seen across the increasing chain length of the tryptophan ester. Due to increasing hydrophobic nature of the tryptophan moiety (trp C-4 and Trp C-8), it is pronounced that the tendency of the tryptophan penetration across the membrane will be also increase. Consequently, indole group in tryptophan will be attracted closer to the amphiphilic head of the glycolipid. Correlation between the fluorescence peak of tryptophan in lipid and in solvent predict the different in water profile across the aqueous nanochannel. The observation complement to the result that are reported by Dongping zhong and coworker, showing confined water in aqueous nanochannel can be divided into three distinct time scale;(1) ~100-150 ps correspond to two layers of well-ordered interfacial water which dynamically is a rigid water molecule. In this region, the solvent network which allow for water molecule to form intermolecular interaction with more than one water molecule were perturbed. This reduced the local polarity senses by the tryptophan ester similar to the peak maximum in 1,4-dioxane which is highly non-polar solvent. (2) 10-15 ps, correspond to quasi-bound water motion. This can be observed similar to the previous case where the tryptophan moiety resembles to those in methanol and ethanol. (3) 1 ps and lower, showing the bulklike near the channel centre where the environment shows the highest polarity similar to that in buffer. Basically the (1) and (2) is important in maintaining the global structure stability of the glycolipid and the flexibility to adapt different structure such in phase diagram. On lifetime of Trp in 10^-9 magnitude (1 ns or higher), two different lifetime component is usually correspond to the exhibition of different rotamer (rotational isomer) by the Trp moiety. These result point to a degree of flexibility of the lipid self-assembly that allow the Trp side chain to adapt two different rotamer

(1b) Steady State and lifetime measurement for hydrophobic tail using pyrene as probe.

Investigation of tail region using pyrene as a probe were reported to be sensible in less polar environment due to the non-polar characteristic of the pyrene. The favour of pyrene can be explained by observing the spectrum shift in different dielectric constant of solvent as reported by (idayu) in figure 6. Solvent such as cyclohexane shows a blue shift as compared to that in buffer. The peak between 360nm-450nm indicate formation of monomer by the pyrene while the peak maxima around 465nm is due to the formation of excited dimer (or excimer) (cited Lakowich ngan paper lain) even at a very low concentration (0.05mM of pyrene). In a polar condition, it tend to form dimer because of the hydrophobic nature of pyrene which disliked polar solvent and thus forming a cluster to reduce the surface contact with the molecule of the solvent. The spectrum as shown in figure 7 in (idayu paper) shows a complete absent of dimer in the lipid. Pyrene molecule was said to be dispersed in the tail region as monomer and tend to isolate from each other. Additional information from the fluorescence spectrum of pyrene is the (I1/I3) ratio which correspond to the vibronic transition of 0-0 band. The value for the ratio of peak 1 and 3 indicate local polarity of the environment that the pyrene experienced. In principle, high value of (I1/I3) indicate more polar environment. Some examples for the values of (I1/I3) in different solvent and lipid composition were shown in the table 1,

Table 1. Value of ratio (I

The result of pyrene in lipid shows an intermediate between the value as in buffer and cyclohexane. However, the ratio shows a closer number towards the buffer solution which further indicate the favorable of pyrene to locate near the lipid head group. This was proved by the simulation worked done by (Prof rauzah. Embedding the tryptophan and pyrene together result in reduction of polarity based on the value dropped by 0.02 as compared with only single component of trp-C₈ in lipid. The reduction of polarity in presence of trp-C₈ is because of the result in increasing hydrophobicity around the pyrene molecule by the C₈ chain in tryptophan. Lifetime measurement were conducted for both βMaltoOC12 and βGlcOC₈ in hexagonal phase and reported in (paper idayu). In both case, pyrene shows two distinct lifetime correspond to short component (0.87 – 0.97ns) and long component (11 – 51 ns) as an indication for the heterogeneity that the pyrene adopt in the system . Furthermore, lifetime of βMaltoOC12 which is 51 ns shows a higher isolation (caging effect) of pyrene in tail region as compared to βGlcOC₈ which has decay component of 11 ns. Comparison between the two type of lipid shows the effect of adding Trp-C₈ with pyrene in βMaltoOC12 (27 ns) is more prominent than in βGlcOC₈ (11.3 ns) system. In both cases implied the presence of C8 chain in Trp to cause a reduction in the local viscosity of tail region. However, in table 1, it is further observed that, the value for βGlcOC₈ is actually smaller than βMaltoOC12. Since the smaller value of this ratio correspond towards more non-polar environment, it leads to a certain assumption that βGlcOC₈ were actually interact more with the pyrene due to shorter alkyl chain compared to βMaltoOC12 which has more random and wobbling motion in the longer chain.(C₈ vs C₁₂). This increased the diffusion of oxygen and hence reduced the lifetime of pyrene

CONCLUSION

While it is tempting to cover all the other information about the unique nature of glycolipid self-assembly, this review provide a valuable insight on using fluorescence probes to investigate the local environment around two distinct group which is the hydrophilic head and hydrophobic tail of glycolipid. By comparing the fluorescence behavior of probes in solvent and in lipid, we will able to correlate those two in order to discover the local polarity in the lipid. While lifetime measurement in fluorescence uncover the hidden information that the steady-state measurement would not be able to obtain such as heterogeneity, flexibility and etc. These properties are crucial for biological processes such as ability of lipid in allowing different size of molecule to accommodate in the lipid. Understanding molecular self-assembly in microscopic scale and its behavior in different structure and phases will allow us to construct and produce efficient product for industrial application such as emulsifier and nano-carrier for drug delivery.

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ABBREVIATIONS

CCR2, CC chemokine receptor 2; CCL2, CC chemokine ligand 2; CCR5, CC chemokine receptor 5; TLC, thin layer chromatography.

REFERENCES