Sofia Ahmed

Photo, thermal and chemical degradation of riboflavin

Summary Riboflavin (RF), also known as vitamin B2, belongs to the class of water-soluble vitamins and is widely present in a variety of food products. It is sensitive to light and high temperature, and therefore, needs a consideration of these factors for its stability in food products and pharmaceutical preparations. A number of other factors have also been identified that affect the stability of RF. These factors include radiation source, its intensity and wavelength, pH, presence of oxygen, buffer concentration and ionic strength, solvent polarity and viscosity, and use of stabilizers and complexing agents. A detailed review of the literature in this field has been made and all those factors that affect the photo, thermal and chemical degradation of RF have been discussed. RF undergoes degradation through several mechanisms and an understanding of the mode of photo- and thermal degradation of RF may help in the stabilization of the vitamin. A general scheme for the photodegradation of RF is presented.

Effect of borate buffer on the photolysis of riboflavin in aqueous solution

The photolysis of riboflavin (RF) in the presence of borate buffer (0.1-0.5M) at pH 8.0-10.5 has been studied using a specific multicomponent spectrophotometric method for the determination of RF and photoproducts, formylmethylflavin (FMF), lumichrome (LC) and lumiflavin (LF). The overall first-order rate constants for the photolysis of RF (1.55-4.36 x 10(-2)min(-1)) and the rate constants for the formation of FMF (1.16-3.52 x 10(-2)min(-1)) and LC (0.24-0.84 x 10(-2)min(-1)) have been determined. The values of all these rate constants decrease with an increase in buffer concentration suggesting the inhibition of photolysis reaction by borate species. The kinetic data support the formation of a RF-borate complex involving the ribityl side chain to cause the inhibition of photolysis. The second-order rate constants for the borate inhibited reaction range from 1.17-3.94 x 10(-2)M(-1)min(-1). The log k-pH profiles for the reaction at various buffer concentrations indicate a gradual increase in rate, with pH, up to 10 followed by a decrease in rate at pH 10.5 probably due to ionization of RF and quenching of fluorescence by borate species. A graph of second-order rate constants against pH is a sigmoid curve showing that the rate of photolysis increases with an increase in pH. The results suggest the involvement of excited singlet state, in addition to excited triplet state, in the formation of LC.

Effect of divalent anions on photodegradation kinetics and pathways of riboflavin in aqueous solution.

The present investigation is based on a study of the effect of buffer and non-buffer divalent anions (phosphate, sulphate, tartrate, succinate, malonate) on the kinetics, product distribution and photodegradation pathways of riboflavin (RF) at pH 6.0-8.0. RF solutions (5x10(-5)M) were photodegraded in the presence of divalent anions (0.2-1.0M) using a visible light source and the photoproducts, cyclodehydroriboflavin (CDRF), formylmethylflavin (FMF), lumichrome (LC) and lumiflavin (LF) were assayed by a specific multicomponent spectrophotometric method. RF degradation in the presence of divalent anions follows parallel first-order kinetics to give CDRF and LC as the final products through photoaddition and photoreduction reactions, respectively. The divalent anion-catalysed CDRF formation is affected in the order: phosphate>sulphate>tartrate>succinate>malonate, showing maximum activity of the anions around pH 7. The divalent anions cause deviation of the photoreduction pathway in favour of the photoaddition pathway to form CDRF. The first- and second-order rate constants for the reactions involved in the photodegradation of RF have been determined and the rate-pH profiles and pathway relationships discussed. The catalytic activity of the divalent anions appears to be a function of the relative strength and chemical reactivity of the RF-divalent anion complex acting as a mediator in the photoaddition reaction.

Photostability and Interaction of Ascorbic Acid in Cream Formulations

The kinetics of photolysis of ascorbic acid in cream formulations on UV irradiation has been studied using a specific spectrophotometric method with a reproducibility of ±5%. The apparent first-order rate constants (kobs) for the photolysis of ascorbic acid in creams have been determined. The photoproducts formed in the cream formulations include dehydroascorbic acid and 2,3-diketogulonic acid. The photolysis of ascorbic acid appears to be affected by the concentration of active ingredient, pH, and viscosity of the medium and formulation characteristics. The study indicates that the ionized state and redox potentials of ascorbic acid are important factors in the photostability of the vitamin in cream formulations. The viscosity of the humectant present in the creams appears to influence the photostability of ascorbic acid. The results show that the physical stability of the creams is an important factor in the stabilization of the vitamin. In the cream formulations stored in the dark, ascorbic acid undergoes aerobic oxidation and the degradation is affected by similar factors as indicated in the photolysis reactions. The rate of oxidative degradation in the dark is about seventy times slower than that observed in the presence of light.

Effect of pH, Buffer, and Viscosity on the Photolysis of Formylmethylflavin: A Kinetic Study

The kinetics of the photolysis of formylmethylflavin, a major intermediate product in the aerobic and anaerobic photolysis of riboflavin, was studied in the pH range 2.0–11.0. Formylmethylflavin and its photoproducts, lumichrome and lumiflavin, were determined in degraded solutions using a specific multicomponent spectrophotometric method. The photolysis of formylmethylflavin in alkaline medium takes place by first-order kinetics and the rate constants (kobs) at pH 7.5–11.0 range from 0.27 × 10–4 to 3.88 × 10–4 and 0.36 × 10–4 to 5.63 × 10–4 s–1 under aerobic and anaerobic conditions respectively. In acid medium, the photolysis involves a second-order mechanism and the rate constants at pH 2.0–7.0 range from 1.37 to 2.11 and 2.03 to 2.94 M–1 s–1 under aerobic and anaerobic conditions respectively. The rate–pH profiles for the photolysis reactions indicate the highest rate of formylmethylflavin degradation is at ~pH 4 and above pH 10. In the alkaline region, the increase in rate with pH is due to higher reactivity of the flavin triplet state. The photolysis of formylmethylflavin is catalyzed by phosphate ions and is affected by the solvent viscosity.

Photostability and Photostabilization of Drugs and Drug Products

Photostability studies of drugs and drug products are an integral part of the product development process in the pharmaceutical industry. These studies are carried out to ensure quality, efficacy, and safety of the formulated products during manufacture, storage, and use. This review deals with the concept of photostability and related aspects and the literature available in the field. It highlights the role of the photochemistry in the photostability studies, describes the functional groups important for the photoreactivity of drugs, explains photophysical processes, and deals with the kinetics of photochemical reactions. The various modes of photodegradation of drugs with examples of selected compounds are presented. The biological consequences of the effect of light on the drug degradation are described. The photostability testing of drugs and drug products and the requirements under ICH guideline are discussed. Some information on the packaging requirements for the formulated products is provided. The various methods used for the photostabilization of solid and liquid dosage forms are also discussed.

Formulation and Stability of Ascorbic Acid in Topical Preparations

Ascorbic acid (vitamin C) and its derivatives are known to perform various important physiological and metabolic functions in humans. In addition to dietary supplements, a number of topical formulations containing ascorbic acid and derivatives are now available that induce collagen synthesis, strengthening of skin tissues, reduction in pigmentation loss, and improved growth and health activities. It has also been used in a variety of cosmetic preparations as an antioxidant, pH adjuster, anti-aging and photoprotecting agent. Ascorbic acid is highly sensitive to air and light; and to achieve its stabilization in cosmetic preparations, it has been suggested to use ascorbic acid in microencapsulation form, in combination with other chemical moieties such as vitamin-E, by the control of pH and electrolyte concentration and use of stabilizing agents like citric, tartaric, or ferulic acids. A large number of cosmetic creams and lotions are available in the market containing the derivatives of ascorbic acid (e.g., sodium ascorbate, ascorbyl palmitate). Although these preparations are chemically stable, they lack the pharmacological activity of ascorbic acid. In the present review, it has been emphasized to consider the importance of various factors involved in the formulation of such preparations to achieve the stabilization of ascorbic acid as such, to maintain its pharmacological activity.

Photodegradation of levofloxacin in aqueous and organic solvents: A kinetic study

The kinetics of photodegradation of levofloxacin in solution on UV irradiation in the pH range 2.0-12.0 has been studied using a HPLC method. Levofloxacin undergoes first-order kinetics in the initial stages of the reaction and the apparent first-order rate constants are of the order of 0.167 to 1.807×10-3 min-1. The rate-pH profile is represented by a curve indicating the presence of cationic, dipolar and anionic species during the reaction. The singly ionized form of the molecule is non-fluorescent and is less susceptible to photodegradation. The increase in the degradation rate in the pH range 5.0-9.0 may be due to greater reactivity of the ionized species existing in that range. The rate appears to vary with a change in the degree of ionization of the species present in a particular pH range and their susceptibility to photodegradation. Above pH 9, the decrease in the rate of photodegradation may be a result of deprotonation of the piperazinyl group. The levofloxacin molecule is more stable in the pH range around 7, which is then suitable for formulation purposes. The photodegradation of levofloxacin was found to be affected by the dielectric constant and viscosity of the medium

Preparation and characterization of bioactive composites and fibers for dental applications.

OBJECTIVES The present study was carried out to create composites and fibers using polyurethane (PU) with hydroxyapatite (HA) that could be used for dental applications. METHODS Composites with varying HA concentration were prepared by solution casting technique. Similarly, PU-HA fibers with varying PU hard and soft segments and fixed HA concentration were also prepared. Various characterization techniques, such as, X-ray diffractometry, differential scanning calorimetry, scanning electron microscopy and Fourier transform infrared spectroscopy in conjunction with photo-acoustic sampling cell were employed to study the composites and fibers for changes in their physicochemical properties before and after immersion in artificial saliva at 37°C for up to 5 days. RESULTS The results indicated formation of amorphous apatite layers with maximum amorphicity in composites containing highest amount of HA with 5 days of immersion in artificial saliva. Similarly, fibers with more PU hard segment resulted in better transformation of crystalline HA to its amorphous state with increasing immersion time thus confirming the bioactive nature of the HA-PU fibers. SIGNIFICANCE Concentrations of HA and PU hard segment along with the duration of immersion in artificial saliva are two major factors involved in the modification of solid-state properties of HA. The amorphous apatite layer on the surface is known to have tendency to bind with living tissues and hence the use of optimum amount of HA and PU hard segment in composites and fibers, respectively could help in the development of novel dental filling material.

Multicomponent spectrometric analysis of riboflavin and photoproducts and their kinetic applications

Riboflavin (RF) is a light sensitive compound and is known to form a number of photoproducts. These photoproducts possess the same nucleus and may interfere in the analysis of RF by UV and visible spectrometry. Therefore, it is necessary to apply the methods of multicomponent spectrometric analysis to quantify the vitamin and its photoproducts accurately. Such methods are useful in the study of the kinetics of photodegradation reactions of RF to obtain accurate and reliable results. Any interference in these methods due to linear or nonlinear irrelevant absorption of the minor unknown products can be accounted for by the application of appropriate correction procedures prior to kinetic treatment. Various factors affecting the accuracy, precision and selectivity of these analytical procedures are also discussed. This review highlights the principles and applications of multicomponent spectrometric methods and their application to the simultaneous determination of RF and its major photoproducts in degraded solutions to evaluate the kinetics of degradation.