C39. Why is the Human Visual System Sensitive Only to Light of Wavelengths from about 760 to 380 nm? – An Answer from Thermochemistry and Kinetics

Yu-Ran Luo

Molecular Energetics Consultant

125 5th Avenue, NE, Room 310, St. Petersburg, Florida 33701-3034

Fax: 813-550-9071; E-mail: luo97@hotmail.com

The range of visible light has been explained by the knowledge of thermochemistry and chemical kinetics. The C, C-B bond dissociation energy at the 11 and 12 positions of the rhodopsin complex is estimated by

DH° (C,C-B , at 11-12) = {DH° (H-C, at 11) + DH° (H-C, at 12) - DH° (H-H)}

+ {D _fH° (I)- D _fH° (rhodopsin)}

 There DH° (H-C, at 11) and DH° (H-C, at 12) are the C-H bond dissociation energies at 11 and 12 positions in the hydrogenated rhodopsin (I), respectively; {D _fH° (I) and D _fH° (rhodopsin) are the heats of formation of species I and rhodopsin complex, respectively. The values of DH° and D _fH° can be easily predicted by means of the author approach for the BDEs (Chem. Phys. Lett. 1994, 228, 329), and Dr. Benson group method, respectively.

 The value of DH° (C,C-B , at 11-12) estimated is 37.4± 1.5 kcal/mol. The energy is equivalent to the wavelength of 760± 30 nm. This wavelength is just the red limit observed by the naked eye.

 The photons of the violet limit are enough to break weakest C-C and H-C bonds in important species involved in the photo-induced cis-trans isomerization cycle, and to stop the visual cycle. The ultarviolet photons can demolish biological molecules, initiate many free radicals, and result in further harmful chemical reactions.