From: Tom Laue <Tom.Laue@unh.edu>
  To  : Borries Demeler <demeler@bioc09.v19.uthscsa.edu>
  Date: Sun, 03 Oct 1999 13:32:51 -0400

Re: "fringe" extinction coefficient

Ooops, I sent that last e-mail prematurely. Try this:

Bo-
The equivalent is the refractive increment, dn/dc, and can be expressed as
fringes/mg/ml. The value depends on the cell pathlength and (inversely) on
the wavelength of light. Since the refractive index depends largely on the
difference in polarizability of the solvent and the solute, and since
biomolecules are largely made of C,N,O and H, in dilute-moderately dilute
aqueous solvents, dn/dc is reasonably constant. For the XLI using 670 nm
light dn/dc is 3.2-3.3 fringes/mg/ml. The reason for wanting to use a green
laser is that this value will be closer to 4, and will be close to 5 with a
blue (440) laser... hence increased sensitivity. There is some discussion
of this in the App. Dats. attached. Papers by Schachman and Richards
(hopefully referenced in the app. dat.!) and a book by Heller discuss dn/dc
in more detail. There is a table in the app. dat. with 'typical' values of
dn/dc. You will notice that the value also depends on the solvent. Since
Heller's time, there has been a quest to work from compositions to dn/dc,
similar to what has been done with vbar. Les Holladay released a program
that can make the calculations, but such work is fraught with pitfalls
since dn/dc is an electrical property, not a mass property. Hence, the
error on the calculations has not been adequately tested nor documented.

Best wishes,
Tom




--------------------------------------------
Tom Laue
Professor and Director of the Center to Advance Molecular Interaction Science
University of New Hampshire
Biochemistry and Molecular Biology
Rudman-379
46 College Rd.
Durham, NH 03824-3544

Phone: 603-862-2459
FAX:   603-862-4013
---------------------------------------------