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Analytical Techniques for Natural Product Research

By: Kumar, S.
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Plants are important source of lead molecules for drug discovery. These lead molecules serve as starting materials for laboratory synthesis of drug as well a model for production of biologically active compounds. Phytochemical processing of raw plant materials is essentially required to optimize the concentration of known constituents and also to maintain their activities. Extraction techniques and analytical techniques have played critical roles in phytochemical processing of raw materials. Extraction technologies from conventional extraction to green extraction as well as analytical techniques from single technique to hyphenated/coupled techniques most frequently used in phytochemistry laboratories are covered in the book.

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1: Analytical Techniques in Natural Product Research

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1

Analytical Techniques in Natural

Product Research

1.1  Introduction

Nature represents an extraordinary reservoir of novel molecules. Natural products have provided the inspiration for most of the active ingredients in medicines. Their high chemical diversity and the effects of evolutionary pressure to create biologically active molecules could be attributed to the success in drug discovery. Medicinal plants have played a key role in world health. Plants are rich sources of fine chemicals, largely unknown and explored, yet they still make an important contribution to health care in spite of the great advances in the field of modern medicine. Plants are a treasure trove of interesting and valuable compounds since they must glean everything from the spot on the earth where they are rooted. Also, they cannot escape when threatened; therefore, they have evolved a most impressive panoply of products to thrive in ever-changing environments despite these limitations (Newell-McGloughlin, 2008). There are about

 

2: Phytochemical Processing: Extraction Methods

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2

Phytochemical Processing:

Extraction Methods

2.1  Introduction

The plant kingdom represents an enormous reservoir of biologically active molecules, and plants with ethnopharmacological information have been the main source of early drug discovery. A large proportion of the drugs used in modern medicine have either been discovered directly from plants or modified synthetically from a lead compound. In addition, in the form of natural products or as functional foods, plants and their extracts offer an alternative to specifically targeted drugs in the treatment and prevention of many diseases. As diverse biological activities of plant extracts and phytochemicals are being reported, investigations of higher plants with known ethnobotanical information have attracted the attention of researchers. Phytochemicals have high commercial value in the local and global markets because of shifting from illness-oriented products to wellness-promoting products. Besides that, the prevalence of chronic diseases that cannot be cured by conventional drugs makes the phytochemical industry an upcoming industrial sector. However, a common pitfall associated with this sector is that the production of these phytochemicals is carried out mainly through various traditional methods, leading to high losses and low yield. To make the phytochemical industries viable and profitable, various transformations like suitable processes that include planting and harvesting, raw material preparation and value-added production are needed. Also, for the successful modernization of phytochemical processing, process technology needs to be optimized for extraction and product formulation. Several steps involved in phytochemical processing include size reduction through to chopping and grinding to good storage, to ensure that active phytochemicals are maintained before processing. Extraction is a key step in the iterative process of drug

 

3: Supercritical Fluid Extraction

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3

Supercritical Fluid Extraction

3.1  Introduction

Depending on temperature and pressure, substances can transform into three different states (gas, liquid and solid). Three phases of gaseous, liquid and solid coexist at the triple point. However, there is a region of pressure and temperature above this critical point to which neither liquid nor gas belongs. A supercritical fluid is liquid that is above its critical temperature and pressure (Bravi et al., 2007).

It has the unique characteristics of being a solvent that continuously varies in solubility. Charles Cagniard first discovered supercritical fluids

(SCFs) in 1822 and their high solvation power was first reported over a century ago. Demonstration of SFE technology for industrial applications was reported by Zosel at the Max Planck Institute für Kohlenforschung,

Germany in 1969 (Zosel, 1969).

SCFs have or display properties of both gases and liquids, diffusing into a solid like a gas while dissolving like liquids. Some of the physical properties of gas, liquid and SCFs are summarized in Table 3.1.

 

4: Chromatographic Techniques: High-performance Thin-layer Chromatography and High-speed Countercurrent Chromatography

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Chromatographic Techniques:

High-performance Thin-layer

Chromatography and High-speed

Countercurrent Chromatography

4.1  Introduction

Before the establishment of instrumental chromatographic methods such as gas chromatography and high-performance liquid chromatography, thin-layer chromatography (TLC) was the most common method used for the analysis of natural products. Still today, TLC is the most frequently used semi-quantitative method for the preliminary identification of constituents. Here, separation is achieved on the basis of partition or a combination of partition and adsorption, depending on the composition of the stationary phase and the mobile phase (Stahl, 1969). Highperformance thin-layer chromatography (HPTLC) is an automated form of TLC with better and more advanced separation capacity and detection limit. HPTLC is also known as planar chromatography or flat-bed chromatography (Srivastava, 2011; Sethi, 2013). Nowadays, HPTLC is used as an alternative analytical technique to gas chromatography (GC) and high-­ performance liquid chromatography (HPLC). Further, its hyphenation with other advanced techniques such as spectroscopic techniques to provide HPTLC-mass spectrometry (HPTLC-MS) and HPTLC-Fourier transform infrared spectroscopy (HPTLC-FTIR), scanning diode laser, etc., has made

 

5: Chromatography Techniques II: High-performance Liquid Chromatography and Ultra-performance Liquid Chromatography

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5

Chromatography Techniques

II: High-performance Liquid

Chromatography and

Ultra-performance Liquid

Chromatography

5.1  Introduction

Chromatography is an analytical technique based on the separation of compounds due to differences in their structure or composition. In general, chromatography involves moving a sample through the system over a stationary phase. Different affinities and interactions of molecules in the sample with the stationary support lead to separation of molecules. Sample components having stronger interactions with the stationary phase move more slowly through the column in comparison to components having weaker interactions. On this basis, different compounds can be separated from each other as they move through the column. High-performance liquid chromatography (HPLC) is a type of chromatography used to separate and quantify the compounds that have been dissolved in solution. HPLC is presently the most widely used method of qualitative and quantitative analysis in drug discovery from plant sources. Since the early 1980s, HPLC has been recognized as the most versatile technique for separating natural products directly in crude extract without the need for complex sample preparation.

 

6: Tandem Techniques

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6

Tandem Techniques

6.1  Introduction

Phytochemicals are formed as a result of biochemical transformations in plants. They are present often in low or very low concentrations in a complex matrix. The analysis of individual natural products in complex crude extracts requires efficient separation methods prior to their detection. According to the type of study (quantification, standardization, fingerprinting, screening, etc.), very sensitive and selective methods may be needed.

Hyphenated techniques combining different separation and detection methods were first introduced by Tomas Hirschfeld about three decades ago

(Wilson and Brinkman, 2003). Hyphenated techniques allow a rapid structural determination of known plant constituents with a minute amount of plant material (Wolfender et al., 2000). With such a combined approach, the time-consuming isolation of common natural products is avoided and an efficient targeted isolation of compounds presenting interesting spectroscopic or biological features is performed. To discover new bioactive products, the dereliction of crude extracts performed prior to isolation work is of crucial importance in order to avoid the tedious isolation of known constituents. Hyphenated analytical methods have been fully integrated into the isolation process and are used for chemical screening of crude plant extracts.

 

7: Non-destructive Techniques

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7

Non-destructive Techniques

7.1  Introduction

Rapid analytical techniques have been developed as alternatives to the traditional methods of analysis. The application of quick, solvent-free and on-field analytical techniques present an enormous advantage over the conventional wet chemistry approaches (Pedro and Ferreira, 2005). In particular, fast, reliable and non-destructive analysis with minimal sample preparation is required for the quantification of valuable components in screening a large number of samples. These techniques are also finding application in online, semi-continuous monitoring of composition, as sample preparation is minimal in these non-destructive techniques.

Furthermore, conventional chemical analysis such as high-performance liquid chromatography (HPLC) or gas chromatography (GC) may take several hours to generate results, which may delay the validation of quality control. Therefore, analytical techniques generating rapid analytical results as compared to conventional methods are required.

 

8: Antioxidant Assay

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8

Antioxidant Assay

8.1  Introduction

Nutrition and health are interconnected, and regular consumption of fruit and vegetables is associated with many health benefits (Pellegrini et al.,

2003). They contain a wide variety of biologically active, non-nutritive compounds known as phytochemicals. A great number of plants worldwide have proved to have strong antioxidant activity and powerful scavenger activity against free radicals. Medicinal plants have also been studied extensively for their antioxidant activity. Some widely consumed beverages like tea, red wine and cocoa are well known for their high antioxidant activities. Antioxidants stop the reactions of radicals. As there are many possible radicals formed during oxidation, antioxidants must be effective against all radical pathways. Synthetic antioxidants are very effective in blocking specific radical reactions. However, they are far less effective than the antioxidant systems that occur naturally in plants, because plant-derived antioxidants block both the major and minor oxidation pathways.

 

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