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LETTERS TO EDITOR
Year : 2019  |  Volume : 11  |  Issue : 3  |  Page : 141-142  

Enthalpy–entropy compensation in the melting of human hair


Department of Biology, Valdosta State University, Valdosta, Georgia, USA

Date of Web Publication12-Jun-2019

Correspondence Address:
Prof. Jonghoon Kang
Department of Biology, Valdosta State University, Valdosta, Georgia
USA
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ijt.ijt_21_19

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How to cite this article:
Cowan ML, Kang J. Enthalpy–entropy compensation in the melting of human hair. Int J Trichol 2019;11:141-2

How to cite this URL:
Cowan ML, Kang J. Enthalpy–entropy compensation in the melting of human hair. Int J Trichol [serial online] 2019 [cited 2019 Jun 20];11:141-2. Available from: http://www.ijtrichology.com/text.asp?2019/11/3/141/259985



Sir,

A recent paper published by Kunchi et al. examines thermal properties of the human hair, such as melting enthalpy (ΔHm) and melting temperature (Tm) by thermal scanning of hair obtained from 13 individuals.[1] According to their data, the value of ΔHm is positive for all 13 individuals, indicating that the melting of the human hair at the melting temperature is an endergonic process. While both thermodynamic parameters, ΔHm and Tm, provide valuable information for the thermodynamic basis for the melting phenomena exhibited by human hair, another fundamental thermodynamic parameter, entropy, is not discussed in the original paper. In this letter, we show our analysis of their results to provide melting entropy (ΔSm) of the human hair, its correlation with ΔHm, and a potential mechanism for the correlation between ΔHm and ΔSm.

As melting is a phase transition process, ΔSm can be calculated using the following equation:



Where Tm is the melting temperature in Kelvin.[2] Numerical values of both ΔHm and Tm for 13 individuals obtained from the original paper were used for the calculation of ΔSm using Equation (1).

The resulting values of ΔSm and its corresponding ΔHm are shown in [Figure 1]. The plot elucidates two thermodynamic features in the melting of human hair for 13 individuals. First, the melting of human hair at the melting temperature is an entropy-driven process, that is, to say ΔSm >0. Second, linear regression identified that there is a high correlation between ΔHm and ΔSm, which is known as enthalpy–entropy compensation, often observed in a weakly-coupled system.[3] The compensation means that as enthalpy increases the corresponding entropy also increases so that the resulting free energy change differences are minimized. For example, we showed in our earlier work that a large variation in ΔH associated with point mutations in peptides was compensated by its corresponding ΔS in their binding to the target proteins.[4] [Figure 1] clearly indicates that the melting of human hair exhibits compensatory behavior, suggesting that the molecular components responsible for the melting in human hair may exhibit the property of a weakly-coupled system.
Figure 1: Enthalpy–entropy compensation in the melting of human hair at Tm. Solid line is a linear fitting equation, y = 2 × 10-3 (x − 10-2), (R2: 0.9967). Linear regression was performed using SigmaPlot (Systat Software, San Jose, CA, USA)

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The major proteins responsible for the mechanical and thermodynamic features in the melting of human hair are keratin.[1] It is known that the human hair expresses at least 17 different keratin proteins.[5] Each type of keratin has a various number of natural variants [Table 1]. According to our search of a bioinformatics database, UniProt (https://www.uniprot.org/help/uniprotkb), the possible number of combinations of keratin compositions that one can have is enormous, as it is 1.76 × 1011. Each combination of keratin proteins will have its own unique value of ΔHm, and variation in ΔHm between individuals can be significant as observed in the paper.[1] However, the variation in ΔHm can be compensated by the corresponding ΔSm as shown in this letter. This compensatory behavior between enthalpy and entropy could be a mechanism for the limited variation in Tm observed in the human hair.
Table 1: Human hair keratins

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Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
   References Top

1.
Kunchi C, Venkateshan KC, Reddy ND, Adusumalli RB. Correlation between mechanical and thermal properties of human hair. Int J Trichology 2018;10:204-10.  Back to cited text no. 1
    
2.
Chang R. Physical Chemistry for the Chemical and Biological Sciences. Sausalito, (CA): University Science Books; 2000.  Back to cited text no. 2
    
3.
Haynie DT. Biological Thermodynamics. UK: Cambridge University Press; 2001.  Back to cited text no. 3
    
4.
Kang J, Auerbach JD. Thermodynamic characterization of dissociation rate variations of human leukocyte antigen and peptide complexes. Mol Immunol 2009;46:2873-5.  Back to cited text no. 4
    
5.
Moll R, Divo M, Langbein L. The human keratins: Biology and pathology. Histochem Cell Biol 2008;129:705-33.  Back to cited text no. 5
    


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