Fourier transform infrared spectroscopy studies of lipid domain formation in normal and ceramide deficient stratum corneum lipid models

Lipid Biomimetic Models as Simple Yet Complex Tools to Predict Skin Permeation and Drug-Membrane Biophysical Interactions

The barrier function of the skin is primarily determined by its outermost layer, the Stratum Corneum (SC). The SC consists of corneocytes embedded in a lipid matrix composed mainly of ceramides, cholesterol, and free fatty acids in equimolar proportions and is organised in a complex lamellar structure with different periodicities and lateral packings. This matrix provides a diffusion pathway across the SC for bioactive compounds that are administered to the skin. In this regard, and as the skin administration route has grown in popularity, there has been an increase in the use of lipid mixtures that closely resemble the SC lipid matrix, either for a deeper biophysical understanding or for pharmaceutical and cosmetic purposes. This review focuses on a systematic analysis of the main outcomes of using lipid mixtures as SC lipid matrix models for pharmaceutical and cosmetic purposes. Thus, a methodical evaluation of the main outcomes based on the SC structure is performed, as well as the main recent developments in finding suitable new in vitro tools for permeation testing based on lipid models.

Quantification of Lipid Phase Order of In Vivo Human Skin Using Attenuated Total Reflection Fourier Transform Infrared (ATR FT-IR) Spectroscopy and Multivariate Curve Resolution Analysis

A new analysis methodology utilizing multivariate curve resolution (MCR) has been successfully combined with Fourier transform infrared (FT-IR) measurement of in vivo human skin to resolve lipid phase constituents in the spectra relative to high and low chain ordering. A clinical study was performed to measure lipid order through different depths of stratum corneum of human subjects. Fourier transform IR spectra were collected through the top 10 layers of the skin on four sites on the left and right forearm of 12 individuals. Depth profiling was achieved by tape stripping to remove layers of skin with 10 successive tapes from each site. In vivo ATR FT-IR spectra were collected after removing each tape. Three isolated spectral regions were analyzed, centered around 2850 cm^-1, 1460-1480 cm^-1, and 730 cm^-1, corresponding to stretching, scissoring, and rocking -CH₂ vibrational modes, respectively. Both traditional lipid conformation analysis and MCR analysis were performed on the same spectral data. The lipid order ratio, expressed as the fraction of highly ordered orthorhombic (OR) lipids to the total lipids content (orthorhombic + hexagonal [HEX] + liquid crystal [LC]), was assessed as function of depth. Lipid order depth profiles (LODP) show an increase in order with the stratum corneum depth which can be adequately described by an exponential function for the data obtained in this study. The LODP derived from the three vibrational modes show very similar trends, although the absolute order ratios are somewhat different. The variance of the skin LODP across individuals is much greater than between sites within the same individual. The higher arm sites closer to the elbow on the left and right arm show no statistically significant difference and are recommended for use in comparative studies. The scissoring mode shows the highest sensitivity for determination of LODP value.

FTIR Analysis of Proteins and Protein-Membrane Interactions

Fourier transform infrared (FTIR) spectroscopy has become one of the major techniques of structural characterization of proteins, peptides, and protein-membrane interactions. While the method does not have the capability of providing the precise, atomic-resolution molecular structure, it is exquisitely sensitive to conformational changes occurring in proteins upon functional transitions or intermolecular interactions. The sensitivity of vibrational frequencies to atomic masses has led to development of "isotope-edited" FTIR spectroscopy, where structural effects in two proteins, one unlabeled and the other labeled with a heavier stable isotope, such as ¹³C, are resolved simultaneously based on spectral downshift (separation) of the amide I band of the labeled protein. The same isotope effect is used to identify site-specific conformational changes in proteins by site-directed or segmental isotope labeling. Negligible light scattering in the infrared region provides an opportunity to study intermolecular interactions between large protein complexes, interactions of proteins and peptides with lipid vesicles, or protein-nucleic acid interactions without light scattering problems often encountered in ultraviolet spectroscopy. Attenuated total reflection FTIR (ATR-FTIR) is a surface-sensitive version of infrared spectroscopy that has proved useful in studying membrane proteins and lipids, protein-membrane interactions, mechanisms of interfacial enzymes, the structural features of membrane pore forming proteins and peptides, and much more. The purpose of this chapter was to provide a practical guide to analyze protein structure and protein-membrane interactions by FTIR and ATR-FTIR techniques, including procedures of sample preparation, measurements, and data analysis. Basic background information on FTIR spectroscopy, as well as some relatively new developments in structural and functional characterization of proteins and peptides in lipid membranes, is also presented.

Stratum corneum modulation by chemical enhancers and lipid nanostructures: implications for transdermal drug delivery

Skin is the outermost and largest protective covering of the body. The uppermost layer of the skin, stratum corneum also called the horny layer is composed of keratin-filled cells covered by a lipid matrix which shields the skin from physical and chemical entrants. The lipid lamellar structure comprises of ceramides, cholesterol, fatty acids and proteins. Chemical enhancers that mimic the lamellar chemistry, reversibly fluidize the latter can be utilized for enhancing transport of cargo across the epidermis into the dermis. This review deals with the stratum corneum chemistry, mechanisms to modulate its packing with the aid of chemical enhancers, biophysical techniques for characterization and applications in the design of nature-inspired biocompatible lipid nanostructures for transdermal delivery of drugs and bioactive agents.

Chain Length of Free Fatty Acids Influences the Phase Behavior of Stratum Corneum Model Membranes

The skin, the largest organ of the human body, forms a flexible interface between our internal and external environment that protects our organism from exogenous compounds as well as excessive water loss. The stratum corneum (SC), the outermost layer of mammal epidermis, is mainly responsible for the skin impermeability. The SC is formed by corneocytes embedded in a lipid matrix, which is mostly constituted of ceramides (Cer), free fatty acids (FFA), and cholesterol (Chol), organized in two coexisting crystalline lamellar phases. This arrangement of lipids is crucial to skin barrier function. The aim of this paper is to determine the impact of FFA chain length on the phase behavior of SC model lipid membranes using solid-state deuterium NMR and IR spectroscopy. We studied ternary mixtures of N-lignoceroyl-d-erythro-sphingosine (Cer24), cholesterol, and palmitic (FFA16) or lignoceric (FFA24) acid in an equimolar ratio. This proportion replicates the lipid composition found in the SC lipid matrix. Our studies revealed that the phase behavior of Cer24/FFA/Chol ternary mixtures is strongly affected by the length of the FFA. We found the formation of phase-separated crystalline lipid domains when using palmitic acid whereas the use of lignoceric acid results in a more homogeneous mixture. In addition, it was observed that mixtures with lignoceric acid form a gel phase, a very unusual feature for SC model mixtures.

The effect of the chain length distribution of free fatty acids on the mixing properties of stratum corneum model membranes

The stratum corneum (SC) plays a fundamental role in the barrier function of the skin. The SC consists of corneocytes embedded in a lipid matrix. The main lipid classes in the lipid matrix are ceramides (CERs), cholesterol (CHOL) and free fatty acids (FFAs). The aim of this study was to examine the effect of the chain length of FFAs on the thermotropic phase behavior and mixing properties of SC lipids. Fourier transform infrared spectroscopy and Raman imaging spectroscopy were used to study the mixing properties using either protonated or deuterated FFAs. We selected SC model lipid mixtures containing only a single CER, CHOL and either a single FFA or a mixture of FFAs mimicking the FFA SC composition. The single CER consists of a sphingoid base with 18 carbon atoms and an acyl chain with a chain length of 24 carbon atoms. When using lignoceric acid (24 carbon atoms) or a mixture of FFAs, the CER and FFAs participated in mixed crystals, but hydration of the mixtures induced a slight phase separation between CER and FFA. The mixed crystalline structures did not phase separate during storage even up to a time period of 3months. When using palmitic acid (16 carbon atoms), a slight phase separation was observed between FFA and CER. This phase separation was clearly enhanced during hydration and storage. In conclusion, the thermotropic phase behavior and the mixing properties of the SC lipid mixtures were shown to strongly depend on the chain length and chain length distribution of FFAs, while hydration enhanced the phase separation.

Current understanding of skin biology pertinent to skin penetration: skin biochemistry

The purpose of this review is to summarize some of the biochemical or chemical findings that have contributed most significantly to our current understanding of the permeability barrier of the skin. This literature survey covers the period from the 1970s up to the present. This seems appropriate since earlier progress was comprehensively covered in a 1978 review by Bob Scheuplein entitled 'Permeability of the skin: a review of major concepts' and in the earlier review by Scheuplein and Blank entitled 'Permeability of the skin'. Both of these review articles are still being cited, and the earlier one has been cited more than 800 times. Overlap with material covered in these earlier publications will be minimized. The overall significance of findings from some of the most recent years may not yet be determined. The emphasis will be placed on the determination of the composition and structures of the epidermal lipids, especially those of the stratum corneum, key enzymes in the biosynthesis of these lipids and some of the physical chemical properties of these lipids as revealed by X-ray diffraction, infrared spectroscopy and other physical methods.

Structural characterization of membrane proteins and peptides by FTIR and ATR-FTIR spectroscopy

Fourier transform infrared (FTIR) spectroscopy is widely used in structural characterization of proteins or peptides. While the method does not have the capability of providing the precise, atomic-resolution molecular structure, it is exquisitely sensitive to conformational changes occurring in proteins upon functional transitions or upon intermolecular interactions. Sensitivity of vibrational frequencies to atomic masses has led to development of "isotope-edited" FTIR spectroscopy, where structural effects in two proteins, one unlabeled and the other labeled with a heavier stable isotope, such as (13)C, are resolved simultaneously based on spectral downshift (separation) of the amide I band of the labeled protein. The same isotope effect is used to identify site-specific conformational changes in proteins by site-directed or segmental isotope labeling. Negligible light scattering in the infrared region provides an opportunity to study intermolecular interactions between large protein complexes, interactions of proteins and peptides with lipid vesicles, or protein-nucleic acid interactions without light scattering problems often encountered in ultraviolet spectroscopy. Attenuated total reflection FTIR (ATR-FTIR) is a surface-sensitive version of infrared spectroscopy that has proved useful in studying membrane proteins and lipids, protein-membrane interactions, mechanisms of interfacial enzymes, and molecular architecture of membrane pore or channel forming proteins and peptides. The purpose of this article was to provide a practical guide to analyze protein structure and protein-membrane interactions by FTIR and ATR-FTIR techniques, including procedures of sample preparation, measurements, and data analysis. Basic background information on FTIR spectroscopy, as well as some relatively new developments in structural and functional characterization of proteins and peptides in lipid membranes, are also presented.