Using solid-state 1H NMR relaxation methods to quantify small molecule interactions and damage in hair
Podium 24
Presented by: Jennifer Marsh
Introduction
Understanding interactions of small molecules in hair is of great interest to the industry. Quantifying penetration of materials and measuring their impact on single fiber properties such as tensile strength has been well studied but understanding specific locations and interactions that influence the measured fiber benefit has proved to be challenging. This work 1H NMR to study these keratin interactions with a series of small molecules and in a range of different hair damage levels. The 1H NMR method has been used to study hair but its use to study small molecules and their interactions has been extremely limited. These data can inform us whether small molecules behave in the same way through the hair fiber, how these interactions change in the presence of water and as hair is damaged.
Methods
T1 and T2 relaxation measurements were measured using a solid-state Bruker Avance 200 MHz 1H NMR spectrometer for a series of small molecules (methanol, acetic acid, ethyl acetate, isoamyl acetate and benzyl acetate, panthenol) that vary in molecular weight and Log P values. Relaxation times as a function of humidity (0%, 33%, 75%, 95% RH) were measured to understand the role of water in these interactions. Changes in the relaxation times were also measured after hair had undergone a series of pretreatments – exposure to different pH buffers, heat, and oxidative damage. Single fiber measurements were made using a Dia-Stron MTT 686 instrument (Andover, Hampshire, UK) with an extension rate of 40mm/min. Fiber diameters were measured with a Dia-Stron Fiber Dimensional Analysis System (FDAS 770), which incorporates a Mitutoyo laser micrometer (LSM-6200) (Malborough, MA, USA).
Results
Static 1H T1 relaxation times for a series of model small molecular compounds were measured in hair. Two populations of compound were identified with a rigid bound population identified with shorter relaxation times (T1Hb) and a labile free population with longer relaxation times (T1Hf). Comparison of solvent populations in hair to bulk solvent was used as a measure of interaction. Small values of ΔT1Hb and ΔT1Hf indicated reduced mobility and increased interactions with hair. Compounds with low molecular weights and low Log P values were shown to have greater interactions with hair. The relative sizes of the bound and free populations were also determined, demonstrating that for large, high Log P compounds, the free population dominates the overall behavior of the compound. For smaller, hydrophilic compounds, the size of the bound population increases considerably and becomes more relevant to understanding the total behavior of the compound in hair. The interaction between small molecules and hair has been correlated with changes in single fiber tensile strength.
The effect of humidity on interactions between solvents and water is complex. In this investigation, the solvents investigated all showed an increase in interaction as humidity increased. The increase in interaction suggests that hair swelling allowed greater penetration of larger solvents to active sites in hair for binding and increased interactions. To examine the effect of swelling, time dependent saturations of solvent in humidified hair was examined. As compounds diffuse into hair over time, interactions with hair increase. Comparing the changes in benzyl acetate mobility over time at low and high humidity, demonstrated that diffusion between the free population to the bound population was possible at higher moisture content fibers, whereas low moisture content fibers have separated populations.
Hair samples were damaged to different degrees using thermal, oxidative and pH treatments. The 1H T2 times of water in these samples was measured to examine the mobility of the bound and free populations of water in the hair. The free population showed the greatest difference, with decreasing ΔT2H values as hair became more damaged, demonstrating the reduction of mobility in the water molecules as they were able to access more binding sites.
Conclusions
1H NMR has been shown to be a method that can yield valuable information in the study of interactions between small molecules and hair and how these interactions change as a function of humidity and hair damage. This interaction has been linked to changes to single fiber tensile strength.
Understanding interactions of small molecules in hair is of great interest to the industry. Quantifying penetration of materials and measuring their impact on single fiber properties such as tensile strength has been well studied but understanding specific locations and interactions that influence the measured fiber benefit has proved to be challenging. This work 1H NMR to study these keratin interactions with a series of small molecules and in a range of different hair damage levels. The 1H NMR method has been used to study hair but its use to study small molecules and their interactions has been extremely limited. These data can inform us whether small molecules behave in the same way through the hair fiber, how these interactions change in the presence of water and as hair is damaged.
Methods
T1 and T2 relaxation measurements were measured using a solid-state Bruker Avance 200 MHz 1H NMR spectrometer for a series of small molecules (methanol, acetic acid, ethyl acetate, isoamyl acetate and benzyl acetate, panthenol) that vary in molecular weight and Log P values. Relaxation times as a function of humidity (0%, 33%, 75%, 95% RH) were measured to understand the role of water in these interactions. Changes in the relaxation times were also measured after hair had undergone a series of pretreatments – exposure to different pH buffers, heat, and oxidative damage. Single fiber measurements were made using a Dia-Stron MTT 686 instrument (Andover, Hampshire, UK) with an extension rate of 40mm/min. Fiber diameters were measured with a Dia-Stron Fiber Dimensional Analysis System (FDAS 770), which incorporates a Mitutoyo laser micrometer (LSM-6200) (Malborough, MA, USA).
Results
Static 1H T1 relaxation times for a series of model small molecular compounds were measured in hair. Two populations of compound were identified with a rigid bound population identified with shorter relaxation times (T1Hb) and a labile free population with longer relaxation times (T1Hf). Comparison of solvent populations in hair to bulk solvent was used as a measure of interaction. Small values of ΔT1Hb and ΔT1Hf indicated reduced mobility and increased interactions with hair. Compounds with low molecular weights and low Log P values were shown to have greater interactions with hair. The relative sizes of the bound and free populations were also determined, demonstrating that for large, high Log P compounds, the free population dominates the overall behavior of the compound. For smaller, hydrophilic compounds, the size of the bound population increases considerably and becomes more relevant to understanding the total behavior of the compound in hair. The interaction between small molecules and hair has been correlated with changes in single fiber tensile strength.
The effect of humidity on interactions between solvents and water is complex. In this investigation, the solvents investigated all showed an increase in interaction as humidity increased. The increase in interaction suggests that hair swelling allowed greater penetration of larger solvents to active sites in hair for binding and increased interactions. To examine the effect of swelling, time dependent saturations of solvent in humidified hair was examined. As compounds diffuse into hair over time, interactions with hair increase. Comparing the changes in benzyl acetate mobility over time at low and high humidity, demonstrated that diffusion between the free population to the bound population was possible at higher moisture content fibers, whereas low moisture content fibers have separated populations.
Hair samples were damaged to different degrees using thermal, oxidative and pH treatments. The 1H T2 times of water in these samples was measured to examine the mobility of the bound and free populations of water in the hair. The free population showed the greatest difference, with decreasing ΔT2H values as hair became more damaged, demonstrating the reduction of mobility in the water molecules as they were able to access more binding sites.
Conclusions
1H NMR has been shown to be a method that can yield valuable information in the study of interactions between small molecules and hair and how these interactions change as a function of humidity and hair damage. This interaction has been linked to changes to single fiber tensile strength.