Role of Redox Metals in Color Formation in a Hair Colorant.

The objective of this work was to identify if low levels of redox metals such as copper would accelerate color formation on hair and to understand the consequent impact on initial color formation and color fade. Kinetics of color formation with oxidative dyes in solution in the presence of varying concentrations of copper ions were assessed via imaging and color measurements. Color uptake on hair and color fade were measured with a spectrophotometer, and copper levels in hair were measured with inductively coupled plasma atomic spectroscopy after hair digestion. In this work, the role of redox metal ions such as copper and iron on accelerating rates of oxidative dye formation was demonstrated. Kinetics of dye formation were measured in solution for three dye couples-p-phenylene diamine (PPD) plus resorcinol, PPD plus 5-amino-2-methylphenol (AHT), and 4,5-diamino-1-(2-hydroxyethyl) pyrazole sulfate (HDAP) plus AHT- in a solution that also contained ammonium hydroxide and hydrogen peroxide at pH 10. Low levels of copper were added at a concentration range from 0.01 µg/g to 0.1 µg/g and the rate of color formation measured over 2 h. All th ree dye couples showed signifi cant color acceleration that increased with increasing levels of copper. A mechanism where initial oxidation of primary intermediate PPD or HDAP is accelerated is proposed. This mechanism is demonstrated to become important when trace levels of copper are in hair and a hair colorant added. Color formation is accelerated outside versus inside hair, and ultimately, color uptake is reduced after the colorant is rinsed off hair. Noticeable color fade versus the starting hair color is also increased. This work provides evidence for the role of copper ions in color formation in hair and strategies to reduce copper levels in hair using a chelant such as histidine in a shampoo or conditioner before coloring.

High-resolution visualization of cosmetic active compounds in hair using nanoscale secondary ion mass spectrometry.

A wide range of small molecules are used in our daily hair products to improve the appearance of hair and to protect it from damage from the environment. In order to better design formulations of these products, an understanding of the partitioning and distributions of these small molecules in hair is critical. In this study, we used preferential extraction methods to measure the partitioning of active compounds commonly found in hair cosmetic products on the hair surface and inside hair, and investigated the use of stable isotope labelling, cryo-sample preparation and nanoscale secondary ion mass spectrometry (NanoSIMS) for high-resolution visualization of distributions of these compounds. With this approach, we quantified partitioning and directly visualized distributions at high-resolution of four molecules (e.g. resorcinol, salicylic acid, pentadecyl alcohol and pentadecanoic acid) in hair. This has not been achieved before and revealed distributions of high lipophilicity active compounds in the lipid-rich and hydrophobic cell membrane complex network in hair, while low lipophilicity ones distributed dispersedly.

Metal complexes with varying intramolecular hydrogen bonding networks.

Alfred Werner described the attributes of the primary and secondary coordination spheres in his development of coordination chemistry. To examine the effects of the secondary coordination sphere on coordination chemistry, a series of tripodal ligands containing differing numbers of hydrogen bond (H-bond) donors were used to examine the effects of H-bonds on Fe(II), Mn(II)-acetato, and Mn(III)-OH complexes. The ligands containing varying numbers of urea and amidate donors allowed for systematic changes in the secondary coordination spheres of the complexes. Two of the Fe(II) complexes that were isolated as their Bu4N+ salts formed dimers in the solid-state as determined by X-ray diffraction methods, which correlates with the number of H-bonds present in the complexes (i.e., dimerization is favored as the number of H-bond donors increases). Electron paramagnetic resonance (EPR) studies suggested that the dimeric structures persist in acetonitrile. The Mn(II) complexes were all isolated as their acetato adducts. Furthermore, the synthesis of a rare Mn(III)-OH complex via dioxygen activation was achieved that contains a single intramolecular H-bond; its physical properties are discussed within the context of other Mn(III)-OH complexes.

Synthesis, structure, and physical properties for a series of monomeric iron(III) hydroxo complexes with varying hydrogen-bond networks.

Mononuclear iron(III) complexes with terminal hydroxo ligands are proposed to be important species in several metalloproteins, but they have been difficult to isolate in synthetic systems. Using a series of amidate/ureido tripodal ligands, we have prepared and characterized monomeric Fe (III)OH complexes with similar trigonal-bipyramidal primary coordination spheres. Three anionic nitrogen donors define the trigonal plane, and the hydroxo oxygen atom is trans to an apical amine nitrogen atom. The complexes have varied secondary coordination spheres that are defined by intramolecular hydrogen bonds between the Fe (III)OH unit and the urea NH groups. Structural trends were observed between the number of hydrogen bonds and the Fe-O hydroxo bond distances: the more intramolecular hydrogen bonds there were, the longer the Fe-O bond became. Spectroscopic trends were also found, including an increase in the energy of the O-H vibrations with a decrease in the number of hydrogen bonds. However, the Fe (III/II) reduction potentials were constant throughout the series ( approximately 2.0 V vs [Cp 2Fe] (0/+1)), which is ascribed to a balancing of the primary and secondary coordination-sphere effects.

Comparison of a tartaric acid derived polymeric MRI contrast agent to a small molecule model chelate.

Contrast agents with high relaxivity are needed to increase the sensitivity of magnetic resonance imaging (MRI) for novel clinical and research applications. For this reason, polymeric structures containing multiple Gd(III) chelates are of current interest. Described in this communication are the syntheses and characterization of a glycopolymer derived from L-tartaric acid, Gd 4(H2O), as well as a low molecular weight compound, Gd 10(H2O), that models the Gd(III) chelate structure in the repeat unit of polymer Gd 4(H2O). Luminescence lifetime measurements in H2O and D2O for Eu(III) analogues of Gd 4(H2O) and Gd 10(H2O) [named Eu 4(H2O) and Eu 10(H2O)] reveal that the lanthanide in both structures likely has one water ligand in the primary coordination sphere. The relaxivity of the model chelate Gd 10(H 2O) at 400 MHz and 310 K was determined to be 4.7 mmol (-1).s (-1), representing a nearly 50% increase over Magnevist (3.2 mmol (-1).s (-1)). Relaxivity values on a per Gd basis for the polymeric structure Gd 4(H2O) prepared at two degrees of polymerization, n = 12 and 19, are similar, but slightly lower than Gd 10(H2O) (4.4 mmol (-1).s (-1) and 4.5 mmol (-1).s (-1), respectively). However, their molecular relaxivities of 51 mmol (-1).s (-1) and 80 mmol (-1).s (-1), respectively, provide a substantial increase over that of Magnevist.

A modular approach toward regulating the secondary coordination sphere of metal ions: differential dioxygen activation assisted by intramolecular hydrogen bonds.

Metal ion function depends on the regulation of properties within the primary and second coordination spheres. An approach toward studying the structure-function relationships within the secondary coordination sphere is to construct a series of synthetic complexes having constant primary spheres but structurally tunable secondary spheres. This was accomplished through the development of hybrid urea-carboxamide ligands that provide varying intramolecular hydrogen bond (H-bond) networks proximal to a metal center. Convergent syntheses prepared ligands [(N'-tert-butylureayl)-N-ethyl]-bis(N' '-R-carbamoylmethyl)amine (H(4)1R) and bis[(N'-tert-butylureayl)-N-ethyl]-(N' '-R-carbamoylmethyl)amine (H(5)2R), where R=isopropyl, cyclopentyl, and (S)-(-)-alpha-methylbenzyl. The ligands with isopropyl groups H(4)1iPr and H(5)2iPr were combined with tris[(N'-tert-butylureayl)-N-ethyl]amine (H6buea) and bis(N-isopropylcarbamoylmethyl)amine (H(3)0iPr) to prepare a series of Co(II) complexes with varying H-bond donors. [CoIIH(2)2iPr]- (two H-bond donors), [CoIIH1iPr]- (one H-bond donor), and [CoII0iPr]- (no H-bond donors) have trigonal monopyramidal primary coordination spheres as determined by X-ray diffraction methods. In addition, these complexes have nearly identical optical and EPR properties that are consistent with S=3/2 ground states. Electrochemical studies show a linear spread of 0.23 V in anodic potentials (Epa) with [CoIIH(2)2iPr]- being the most negative at -0.385 V vs [Cp2Fe]+/[Cp2Fe]. The properties of [CoIIH3buea]- (H3buea, tris[(N'-tert-butylureaylato)-N-ethyl]aminato that has three H-bond donors) appears to be similar to that of the other complexes based on spectroscopic data. [CoIIH3buea]- and [CoIIH(2)2iPr]- react with 0.5 equiv of dioxygen to afford [CoIIIH3buea(OH)]- and [CoIIIH(2)2iPr(OH)]-. Isotopic labeling studies confirm that dioxygen is the source of the oxygen atom in the hydroxo ligands: [CoIIIH3buea(16OH)]- has a -(O-H) band at 3589 cm-1 that shifts to 3579 cm-1 in [CoIIIH3buea(18OH)]-; [CoIIIH(2)2iPr(OH)]- has -(16O-H)=3661 and -(18O-H)=3650 cm-1. [CoIIH1iPr]- does not react with 0.5 equiv of O2; however, treating [CoIIH1iPr]- with excess dioxygen initially produces a species with an X-band EPR signal at g=2.0 that is assigned to a Co-O2 adduct, which is not stable and converts to a species having properties similar to those of the CoIII-OH complexes. Isolation of this hydroxo complex in pure form was complicated by its instability in solution (kint=2.5x10-7 M min-1). Moreover, the stability of the CoIII-OH complexes is correlated with the number of H-bond donors within the secondary coordination sphere; [CoIIIH3buea(OH)]- is stable in solution for days, whereas [CoIIIH(2)2iPr(OH)]- decays with a kint=5.9x10-8 M min-1. The system without any intramolecular H-bond donors [CoII0iPr]- does not react with dioxygen, even when O2 is in excess. These findings indicate a correlation between dioxygen binding/activation and the number of H-bond donors within the secondary coordination sphere of the cobalt complexes. Moreover, the properties of the secondary coordination sphere affect the stability of the CoIII-OH complexes with [CoIIIH3buea(OH)]- being the most stable. We suggest that the greater number of intramolecular H-bonds involving the hydroxo ligand reduces the nucleophilicity of the CoIII-OH unit and reinforces the cavity structure, producing a more constrained microenvironment around the cobalt ion.

Preparation of iron amido complexes via putative Fe(IV) imido intermediates.

The isolation and characterization of monomeric Fe(III) amido complexes with hybrid ureate/amidate ligands is described. An aryl azide serves as the source of the amido ligand in preparing the complexes from trigonal monopyramidal Fe(II) precursors. Aryl azides more commonly react with transition metal complexes by a two-electron oxidation process to yield imido complexes, suggesting that the Fe(III) amido complexes may be formed from high valent species by hydrogen atom abstraction from an external species. The mechanistic basis for formation of the amido complexes is investigated using substrates that readily donate hydrogen atoms. Results from these experiments suggest that the Fe(III) amido complexes are generated from Fe(IV) imido intermediates that can facilitate homolytic X-H bond cleavage. The Fe(III) amido complexes are high spin (S = 5/2) with a strong absorbance band at lambdamax approximately 600 nm and extinction coefficients between 2000 and 3000 M-1 cm-1. These complexes are hygroscopic, reacting with 1 equiv of water to produce the corresponding Fe(III)-OH complexes and p-toluidine.