Changing the polarity of the protein environment causes shift in the wave length of maximum absorbance. ie. less polar solvent causes red shift. This is usually along with a shift in molar absorbtivity. Does it necessary mean that red shift due to less polarity of protein environment is along with an increase in molar absorbtivity or heperchromic shift? why?
Thanks
If as substance shifts to a lower energy state with a longer wavelength, it is referred to as a Bathochromic shift or (also called) red shift. The color will move more toward the red. Conversely, something that moves to higher energy will be referred to as a hypsochromic shift. If there is an increase in the absorptivity or cause the spectrum to become more intense, it will be referred to as a hyperchromic shift. But a decrease is referred to as a hypochromic shift.
Many thanks for your replies.... Suppose the molecule is a protein with both tryptophan and tyrosine residues. Our investigation has been done in UV spectrum, and the only variable is the solvent polarity. As the solvent polarity was reduced (from water to ethanol) a batthochromic shift was appeared. This was with hyperchromic shift, too. How could the latter phenomenon (hyperchromic shift) be explained in UV-Vis absorption spectrometry?
The difference in absorbance with changing environmental polarity is due to the rigidity of the molecule, which effects the electrons ability to jump to a higher energy level (generally; lower polarity = greater rigidity = greater absorbance). The rigidity of the molecule explains both the shift to a lower energy wavelength, and the increased absorbance.
Many thanks Dr. Kirkham for your kind reply and logical explanation...
For me this explanation means higher polarity or permeability of water cause weakness of hydrogen and electrostatic interactions in proteins. So, it may cause changes in protein characteristics, which may cause a change, ie decrease in their molar absorptivity at desired and determined wavelength; while a less polar solvent with lower permeability constant cause such effects less, which maintain the protein structure better. This may refer as solvent rigidity.
I will be thankful to correct me in case my explanations are wrong or incomplete.
Peter, firstly, I'm a chemist, not a biochemist. Although I fail to see the relevance of the distinction.
If you want to understand the specifics of the quantum mechanical interactions, I'm probably not the best person to ask, but the way I understand it, is that the degree of polarity (i.e. the size and duration of the dipole) directly effects the various transitions between energy levels. A smaller polar moment results in less intermolecular force between the solvent and solute. I think that this is your "effect of solvent properties on the oscillator strength of a given electronic transition", but put into plain English.
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