bio-TINMAN - 28-10-2006 at 21:00
This questions' been bugging me for a few days now. I tried searching online, but havent found anything satisfactory.
The question is about whether there is a difference between mutarotation and optical isomerism and a about few details in between
===========
Optical isomerism as you folks know shows two rotations, dextrotation and levorotation and this is based on whether the plane polarised light is
shifted to the left or the right.
Now its been explained in my book that mutarotation is the phenomenon used to explain the shift in the optical rotation. ANd there are two terms to
explain mutarotation [alpha] and [beta]
So does this mean that [alpha] form is actually dextrorotation and [beta] is levorotation?
or can i use the terms -
[alpha]D-glucose and D(+)glucose
interchangably?
plz help me out here
nitro-genes - 29-10-2006 at 04:13
IIRC, alpha and beta isomers relate to the cis and trans configuration of the hemiacetal carbon group, so this is only for sugars. The cis and trans
form interchange constantly and probaly cause a shift in the optical rotation, which can be negative (levorotation) or positive (dextrorotation)
matei - 29-10-2006 at 04:37
Glucose has 2 enantiomers (optical isomers): D-Glucose and L-Glucose (the D-L notation is a convention, for the structures of the two enantiomers you
should check any undergraduate orgo book). Now, each of the two enantiomers exists in two forms called anomers (alpha and beta).
For example, the usual crystallized D-glucose is in the alpha form, but in solution an equilibrium exists between the alpha and beta forms. The alpha
form has a greater specific rotation (circa + 112 deg.) than the beta form (+ 18.7 deg.). If you dissolve D-glucose in water and you immediately
measure the specific rotation, it will be +112 deg., but if you measure it again after a few hours, you will see it is now + 52,5 deg. . This
phenomenon is called mutarotation.
In the D-series, alpha monosaccharides have a greater specific rotation than the beta ones. In the L-series the situation is reversed. (e.g.
alpha-L-glucose has a specific rotation of - 112 deg. and beta-L-glucose has -18.7 deg.).
The two anomers have opposite configurations of the OH group at the C1 carbon.
[Edited on 29-10-2006 by matei]
bio-TINMAN - 29-10-2006 at 17:40
thx folks. i was confused because they (the textbooks) were talking about alpha and beta forms and were showing their structures, and there were none
for the dextro and levorotation or why it was dextro once and levorotation once. i read through the text and have now shifted to reading polarisation
of light.....
thanks budds!
Ozone - 5-11-2006 at 13:48
And I thought that you might be into bio-tin-ylation...
Nevetheless, I thought is was important to mention that the rate of mutarotation is also *very* dependant upon both the temperature and the pH.
Addition of +H or -OH will greatly accelerate the rate of mutarotation (almost instantly, in some cases).
Increasing the temperature will enhance the rates significantly; this is most obvious in the case of D-fructose, which exhibits and *anomalous
mutarotation* (there is conversion from furanose to pyranose form *and* interconversion between a and b anomeric forms). In this case, the rotatory
contribution (at equilibrium) is ~-92°; heating this mixture changes the form and the [a]D20 can range from -92 ro -132, when this occurs, the sugar
is no longer sweet--this is why high fructose corn syrup is only used in cold drinks (or other applications where heating is a dissallowed).
Try it!
At rt, the + rotation of glucose drops over time (fairly rapid), and the - rotation of fructose does not. It will change, however when heated. Compare
this with the result seen with the addition of a trace of -OH at rt and as short a time as is feasible.
A crude polarimeter can be manufactured using a white light behind a K2Cr2O7 solution (more-or-less monochromatic source) and a couple of scavenged
polaroid sheets.
cheers,
O3
[Edited on 5-11-2006 by Ozone]
not_important - 5-11-2006 at 23:36
Note that LEDs, while not monochromatic, sometimes have fairly narrow emission spectra; you can look at its reflection at a shallow angle off a CD to
get a real rough feel for the LED's spectrum.
Ozone - 6-11-2006 at 04:34
Very true! I forgot about those. (thanks!) The Sodium D line is 589 nm, so, to be true to modern saccharimeters (automatic polarimeter), a
yellow-orange LED with this wavelength should be pretty close. Also, newer instruments also look at NIR at 880; in my experience, the results are
essentially identical (and they sell tons of *high* power diode lasers in the range on EBAY...
).
The homebrew polarimeter can be calibrated using a quartz plate (i'll get the values for quartz today).
must hit the lab,
O3
bio-TINMAN - 10-11-2006 at 07:14
hi place is so amazing. i really didnt expect that people would discuss this topic so much. In other forums i had exactly, on thorough counting and
rechecking - "0" replies.
You folks are really getting me interested in bio-chem. So teh values for quartz? (it'll help me in my preparing my notes)
Ozone - 10-11-2006 at 17:59
Apologies, I have been insanely busy (quals...). I have the plates at work (actually, they are packaged into a plastic assembly that looks like a
standard polarimeter tube; this is so they will fit in the machine). I will write down the values for you monday (hope this doesn't set you back).
The basic idea behind quartz is that it has the same dispersion characteristics as sucrose, and hence, is used to calibrate the instrument (for sugar
degrees °Z). It also, of course, reads in ° when calibrated against the plates. I'll also include some of the not-so-fundamentals behind how this
works.
Until then,
Cheers
O3
Ozone - 13-11-2006 at 18:44
Oh yeah, it's monday 
.
Here's the values from my quartz plate:
Rudolph Research Analytical quartz control plate, rotation at 20°C.
nm: °:
325 128.47
365 98.24
405 77.48
436 66.09
546 40.63
589 34.57
633 29.74
589/ISS 99.83 °Z
880/ISS 100.00 °Z
Granted, this is for this particular plate, but the values for quartz from a controlled manufacturer should not vary by too much.
Sorry, not enough time for the *fun*damentals. If anyone is interested, check:
http://museum.nist.gov/panels/bates/improve.htm
especially, Bates, F.J., et al., Polarimetry, Saccharimetry, and the Sugars, Nat. Bur. Stand. (U.S.), Circ. 440, (1942).
(this is hard to find, though), or, let me know, and i'll throw together an informative synopsis.
Hope this helps,
O3
[Edited on 14-11-2006 by Ozone]
bio-TINMAN - 14-11-2006 at 01:00
thanks. i think this time ill take the help of books, wikiz etc, and if that still doesnt help me out 
ill have to come back to you... not to say that ill instantly type off another message saying "nothing understood,
back ta ya"
thanks again.
Ozone - 14-11-2006 at 18:32
No problem 
.
good luck,
O3