Some people have curly hair, as well as fur of some animals can be curly. However, shapes which are curly are not mirror symmetric, much alike screws or chiral molecules.

The question is whether curly hair in a given species (say, humans, but I don't stick to this example) are in the same proportion left-handed and right-handed?

One can easily imagine that chirality of molecules (e.g. keratin) gives rise to macroscopic chirality of curly hair. However, it is far from obvious whether this effect is strong enough.


I am interested mostly in data. It might be easy (yet misleading) to convince oneself that there should (or there shouldn't) be relation of microscopic to macroscopic mirror symmetry (or lack of it).

(E.g. in some other cases there is such effect - e.g. with left/right orientation of body organs like heart, or asymmetry of left and right brain.)

  • $\begingroup$ curliness of hair is not because of an enantiomeric form of keratin. It is dependent on the way keratin is packed in the intermediate fibers. $\endgroup$
    Apr 8 '13 at 12:51
  • $\begingroup$ @WYSIWYG a link might help the OP... $\endgroup$
    – MattDMo
    Apr 8 '13 at 15:18
  • $\begingroup$ Please refer this research done by L'oréal on keratin organization and hair texture and shape. I believe one can also find an answer in classical textbooks like MBOTC-Bruce Alberts. This is a commonly asked question in cell bio/protein structure exams. $\endgroup$
    Apr 8 '13 at 15:55
  • $\begingroup$ Plus.. all amino acids exist as L-enantiomers in proteins. Few exceptions in cell wall peptides of gram +ve bacteria $\endgroup$
    Apr 8 '13 at 16:00
  • $\begingroup$ @WYSIWYG Here keratin was only a guess. Chirality of intermediate fibers equally interesting to me. However the main question is: is proportion of left- and right-handed curly hair 1:1 in humans? $\endgroup$ Apr 8 '13 at 20:34

Microscopic chirality is not at all necessary to explain curly hair (or other objects). All that is required is flexibility. Once hair gets long enough, it collides with itself, and is pushed off to one side or another, forming a chiral curl.

For some anecdotal evidence against microscopic chirality, find someone with large diameter hanging curls in their hair. Those seem to often curl straight back on to themselves. A more compelling anecdote is that curled objects, including hair and telephone cords (if anyone still remembers those) can be pulled through themselves and chirally inverted. Sometimes there will be some resistance, but that could be the keratin or plastic or whatever having set in its shape over time, rather than being preferentially formed that way.

These are obviously both anecdotal though, and the idea that there is some slight chiral preference isn't ruled out. I'm not even opposed to the fundamental idea of transitive chirality through several orders of superhelicity (look at DNA), but it just hasn't been my empirical experience. As Ryan points out, if it were a strong effect, all hair would curl the same way, and straight hair wouldn't exist at all.

Just to provide a bit of data, this paper demonstrates that hair curvature is based on the distribution of 4 different cell types in a follicle. The keratin (and many other proteins associated with) intermediate fibrils were associated with curvature, but only to the extend that their compositions were different in the 4 cell types. But the curvature was based on the distribution of the cells themselves. Chirality wasn't specifically studied in the paper, but I think (given that they were staring at individual fibers with an electron microscope) that they would have noticed it had it existed.

Cortical cell types and intermediate filament arrangements correlate with fiber curvature in Japanese human hair http://www.sciencedirect.com/science/article/pii/S1047847708002980

Naturally straight and curved human scalp hairs were examined using fluorescence and electron microscopy techniques to determine morphological and ultrastructural features contributing to single fiber curvature. The study excluded cuticle and medulla, which lack known bilateral structural asymmetry and therefore potential to form curved fibers. The cortex contained four classifiable cell types, two of which were always present in much greater abundance than the remaining two types. In straight hair, these cell types were arranged annularly and evenly within the cortex, implying that the averaging of differing structural features would maintain a straight fiber conformation. In curved fibers, the cell types were bilaterally distributed approximately perpendicular to fiber curvature direction with one dominant cell type predominantly located closest to the convex fiber side and the other, closest to the concave side. Electron tomography confirmed that the dominant cell type closest to the convex fiber side contained discrete macrofibrils composed of helically arranged intermediate filaments, while the dominant cell type closest to the concave side contained larger fused macrofibrils composed of intermediate filament arrangements varying from helical to hexagonal arrays approximately parallel to the longitudinal fiber axis. These findings concur with the current hypothesis of hair curvature formation and behavior.

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    $\begingroup$ I know that microscopic chirality is not needed to explain curly hair. The question is whether it plays any role, e.g. in shifting the ratio of clockwise and counter-clockwise curly hair from 1:1. (Thanks for the link, anyway; too bad for me that it is under a paywall). $\endgroup$ May 6 '13 at 18:35

Keratin's handedness does not directly give rise to the curls of hair. One has to cross many many length scales to move from the keratin scale to the whole hair fiber. There's an animation here that shows those many length scales. But here are some other arguments if you're inclined...

If the superhelicity of keratin effected the curling of hair, all hair would twist in the same direction, because the keratin superhelix always twists in the same (left-handed) direction (for the naturally occurring proteins.) But that isn't the case. At least not for my hair!

Or you can ask, if the keratin superhelix gives rise to curly hair, then how is this effect mitigated in straight hair? The structure of keratin isn't directly giving rise to the long-range patterns of hair. The keratin filaments are being arranged in particular ways, and the long-range behavior is independent of the short-range superhelicity.

  • $\begingroup$ Orders of magnitude difference of scale is not itself and argument against (see edit of my question). $\endgroup$ Apr 10 '13 at 8:14

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