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Human hair curl has been widely explored, with dominating theories focusing on follicular shape, ultra-structural cortex patterns/ratios and intra-cellular interactions (Cloete et al., 2019Cloete E. Khumalo N.P. Ngoepe M.N. The what, why and how of curly hair: a review.Proc. R. Soc. A Math. Phys. Eng. Sci. 2019; 475 (Available from:)20190516https://royalsocietypublishing.org/doi/10.1098/rspa.2019.0516Google Scholar). A few studies have observed hydration-driven shape memory in synthetic and animal keratin fibres, resulting in temporary, reversible shape changes (Quan et al., 2021Quan H. Kisailus D. Meyers M.A. Hydration-induced reversible deformation of biological materials.Nat. Rev. Mater. 2021; 6: 264-283Crossref Scopus (63) Google Scholar). Hydrogen bonds (H-bonds) have been implicated but molecular level mechanisms and direct measurement techniques remain open challenges. We observed hydration-driven shape memory in curly, but not straight, human scalp hair fibres while conducting tensile tests (Cloete et al., 2020Cloete E. Khumalo N.P. Ngoepe M.N. Understanding Curly Hair Mechanics: Fiber Strength.J. Invest. Dermatol. Elsevier. 2020; 140 (Available from:): 113-120https://linkinghub.elsevier.com/retrieve/pii/S0022202X19325424Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar). Proof-of-concept investigations, presented here, lead to the postulation of an extraordinary network of weak H-bonds that arises from distinct arrangements of amino acid residues in the matrix of curly fibres. H-bonds are electrostatic interactions capable of symbiotic hydrogen atom donation or acceptance between independent molecules. Although weaker than permanent (covalent) bonds, H-bonds may be weak or strong. Hydrogen-oxygen and hydrogen-nitrogen associations are examples of strong H-bonds, whereas hydrogen-sulfur and hydrogen-carbon associations are examples of weak H-bonds. Cooperative synergy among these weak bonds yields a bond strength comparable to strong H-bonds (Mundlapati et al., 2015Mundlapati V.R. Ghosh S. Bhattacherjee A. Tiwari P. Biswal H.S. Critical Assessment of the Strength of Hydrogen Bonds between the Sulfur Atom of Methionine/Cysteine and Backbone Amides in Proteins.J. Phys. Chem. Lett. American Chemical Society. 2015; 6: 1385-1389Google Scholar; Wennmohs et al., 2003Wennmohs F. Staemmler V. Schindler M. Theoretical investigation of weak hydrogen bonds to sulfur.J. Chem. Phys. American Institute of Physics. 2003; 119: 3208-3218Google Scholar). Curly hair has dominant concentration levels of amino acid residues that contribute to weak H-bond formation, while non-curly fibres have higher levels of amino acid residues that do not contribute to weak H-bonding (Table 1) (Desiraju and Steiner 2001; Franklin and Slusky, 2018Franklin M.W. Slusky J.S.G. Tight Turns of Outer Membrane Proteins: An Analysis of Sequence, Structure, and Hydrogen Bonding.J. Mol. Biol. Academic Press. 2018; Crossref Scopus (15) Google Scholar; Robbins, 2002Robbins CR. Chemical and physical behavior of human hair Internet. 5th Editio. Chem. Phys. Behav. Hum. Hair. Berlin Heidelberg: Springer-Verlag; 2002. Available from: http://www.loc.gov/catdir/enhancements/fy0814/00059475-d.html%5Cnhttp://www.loc.gov/catdir/enhancements/fy0814/00059475-t.html%5Cnhttp://www.springerlink.com/index/10.1007/978-3-642-25611-0Google Scholar; Wolfram, 2003Wolfram L.J. Human hair : A unique physicochemical composite.J. Am. Acad. Dermatol. 2003; 48: 106-114Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar; Zhou et al., 2009Zhou P. Tian F. Lv F. Shang Z. Geometric characteristics of hydrogen bonds involving sulfur atoms in proteins.Proteins Struct. Funct. Bioinforma. 2009; 76: 151-163Crossref Scopus (205) Google Scholar). Biochemical and mechanical wet and dry investigations further support our hypothesis. Hair donors gave written informed consent in accordance with the University of Cape Town's Faculty of Health Sciences Human Research Ethics Committee (Ref:130/2018).Table 1Comparison of amino acid composition between curly and straight hair to determine the potential of secondary H-bond formation. Data from (Wolfram, 2003b).C1C2C3C4Amino acid residuesAFRICAN (Prototype for curly fibers)ASIAN (Prototype of straight fibers)Contribution to fiber bonds that may add to curvatureAlanine370-509370-415support hydrophobicityArginine482-540492-519support hydrophobicityAsparagine436-452456-500-- H-bonding not anticipated --Glutamine915-10171026-1082-- H-bonding not anticipated --Glycine467-542454-498facilitates β-turns in combination with ProHistidine60-8557-63Indole ring: facilitate SCHBs + higher cooperativity + support hydrophobicityIsoleucine224-282205-244support CHHBLeucine484-573515-546-- H-bonding not anticipated --Lysine198-236182-196support CHHB + hydrophobicityMethionine6-4221-37-- H-bonding not anticipated --Phenylalanine139-181129-143π-system: facilitate SCHBs + higher cooperativityProline642-697615-683facilitates β-turns in combination with GlySerine672-1130986-1101-- H-bonding not anticipated --Threonine580-618568-593support hydrophobicityTyrosine179-202131-170π-system: facilitate SCHBs + higher cooperativityValine442-573421-493support hydrophobicity½ Cystine1310-14201175-1357Thiol group: facilitate SCHB + higher cooperativity + bifurcationTryptophanno datano dataπ-system: facilitate SCHBs + higher cooperativity Open table in a new tab FTIR facilitates investigation of biochemical changes, through monitoring of functional group bond extension (red-shifting) or contraction (blue-shifting), and changes in absorption intensity. Three repeats were conducted. After hydration, red-shifting occurred in curly, not straight, fibres. Both fibre types showed reduced intensity, more so in curly fibres (Figures 1A,1B,1C,1E). Hair's water content means that water influx may not necessarily induce O-H bond frequency shifts. Differences between curly and straight hair may suggest distinct nano O-H environments. The lack of red-shifting in straight fibres may indicate a water-saturated environment and/or a lack of extraordinary H-bonds. Curly fibre red-shifting may result from competing bond effects to maintain the nano environments' thermodynamic stability. Intensity changes may be trivial, yet prominence in curly hair could be due to a more lipophilic environment. Hydration induced no shifting in curly fibres, with trivial intensity change. Small blue-shifts occurred in two C-H signature frequencies for straight hair, with notable intensity increase in the third (Figures 1B,1C,1E). Water influx can polarise nearby C-H bonds in a hydrophilic environment, inducing blue-shifts and intensity changes, as observed for straight fibres. Shifting absence in curly fibres suggests C-H bond and C-H···H-bond thermostatic stability in an environment not prone to polarisation, i.e., hydrophobic. After hydration, curly fibres showed small blue-shifting and intensity reduction, with insignificant changes in straight hair (Figure 1D,1E). The absence of changes in straight hair, may be suggestive of pre-existing H-bonding in the nano C=O environment of curly fibres. After hydration, a small and a larger blue-shift were observed for curly fibres (Figure 1D,1E). Straight fibres showed no shift in the first frequency and a double-sized blue-shift compared to curly hair in the second amide frequency. Intensity reduction was minimal for both fibre types, but in curly hair, it was more than double that in straight hair. The single large blue-shift in straight fibres implies strong bond contraction. Combined with the minimal intensity changes, results allude to direct activity on the N-H bond. Intensity reduction and two blue-shifts in curly fibres suggest neighbouring H-bond activity around the covalent N-H bonds. Stress relaxation tests are a mechanical approach for observing the contribution of different strength mechanisms. Twenty-five samples were tested for each hair type. Average peak stress is 75.1 MPa and 69.9 MPa for dry straight and curly hair, respectively, and 70.6 MPa and 71.8 MPa for wet straight and curly hair, respectively (Figure 1F). To account for the contribution of different bonds during the relaxation process, the Maxwell model was used to fit mechanical parameters to the experimental results (Figure 1G). A three-branch Maxwell model was the base configuration required to fit the experimental data. The first three mechanisms were present in all fibres, while the fourth mechanism is absent in the wet straight samples. The branch is relatively weak compared to the other mechanisms and is particularly attenuated for the straight dry samples. Finally, the fifth mechanism is present in the wet curly samples only. The mechanism is comparable (in order of magnitude) to that of the fourth branch for all the curly samples. To interpret experimental findings, we differentiate between two types of H-bonds. Type I represents strong H-bonds, spanning the cortex at KAP-KAP and KAP-KIF interfaces. They are present in all hair types regardless of curl and demonstrate a typical response during hydration and dehydration: bonds dissolve upon water influx, resurging upon dehydration. Type II is specific to curly fibres, comprising weak H-bonds formed in a lipophilic-dominating biochemical environment. Upon hydration, the competitive nature of the strong cooperative type II H-bond network is anticipated to counteract water influx, thereby reducing dissolution of type I bond dissolutions. Consequently, curly hair is expected to demonstrate resistance to wetting and a lesser reduction in overall bond strength compared to straight hair. Considering differentiation between H-bond types, stress relaxation experiments suggest enhanced, rather than reduced, strength in wet conditions for curly fibres. FTIR experiments partially validate the biochemical distinctions between wet curly and wet straight hair. Finally, the integration of existing amino acid data into the explanation of the H-bond mechanism indicates that the prevalent concentration of specific residues in curly hair supports the establishment of an extraordinary H-bond network, encompassing type II H-bonds. Our hypothesis warrants further exploration, particularly in terms of direct measurement of the two types of H-bonds for a statistically significant sample size and elucidating a mechanism for collaboration between type I H-bond, disulphide, and type II H-bond networks. Datasets related to this article can be found at https://doi.org/10.25375/uct.11790978.v2, hosted at ZivaHub UCT (Cloete, Elsabe; Khumalo, Nonhlanhla; Ngoepe, Malebogo (2020): Wet and dry test for hydrogen bond cross linkers in curly hair. University of Cape Town. Dataset.) None Conceptualisation: EC, MNN, NPK; Data curation: EC, MNN; Formal analysis: EC, MNN, EI; Funding acquisition: NPK; Investigation: EC, MNN; Methodology: EC, MNN, EI, NPK; Project administration: EC, MNN, NPK; Resources: NPK; Review and editing: EC, MNN, EI, NPK; Supervision: MNN, NPK; Visualisation: EC, MNN, EI; Writing: EC, MNN Desiraju et al., 2001Desiraju GR (Gautam R., Steiner T. The weak hydrogen bond : in structural chemistry and biology. Oxford University Press; 2001.Google Scholar. Grants held by NPK: The South African Medical Research Council (mid-career award) and the National Research Foundation SARChI Chair for Dermatology and Toxicology
Cloete et al. (Sun,) studied this question.