March 29, 2024, 04:39:28 AM
Forum Rules: Read This Before Posting


Topic: Disaccharides  (Read 5039 times)

0 Members and 1 Guest are viewing this topic.

Offline orgo814

  • Full Member
  • ****
  • Posts: 412
  • Mole Snacks: +11/-6
Disaccharides
« on: October 20, 2014, 08:22:12 PM »
"In addition to lactose, how many other heterodisaccharides can D-galactose and D-glucose form?"

The answer is 39 in addition to lactose. But, I'm struggling to find a logical way to solve this. I look at number of chiral stereocenters to see stereoisomers but that did not help. I looked at beta and alpha combinations but that also falls way short of 39. Please help

Offline Dan

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 4716
  • Mole Snacks: +469/-72
  • Gender: Male
  • Organic Chemist
    • My research
Re: Disaccharides
« Reply #1 on: October 21, 2014, 03:33:40 AM »
I get 20 (including lactose) considering pyranose forms only.

If I consider two disaccharides different if they vary only by the configuration at the non-glycosidic anomeric centre, which would not be correct to do, I get 36. If I forgot to remove degenerate pairs from that list, which would not be correct to do, I would have 40 (which matches the answer, that may be coincidence though).

If I consider combinations of pyranose and furanose forms I get 80.

So I'm stumped as well... I can only get 40 by making two mistakes.

If it doesn't get solved on here with someone else's help, please post the solution when you have it.
My research: Google Scholar and Researchgate

Offline orgo814

  • Full Member
  • ****
  • Posts: 412
  • Mole Snacks: +11/-6
Re: Disaccharides
« Reply #2 on: October 21, 2014, 04:27:56 PM »
My professor confirmed it was a book mistake. Thanks anyways though... I was there for a half hour trying to solve it!

Offline billnotgatez

  • Global Moderator
  • Sr. Member
  • ***
  • Posts: 4399
  • Mole Snacks: +223/-62
  • Gender: Male
Re: Disaccharides
« Reply #3 on: October 21, 2014, 06:56:43 PM »
@butlerw2 Out of curiosity
What should have the answer been according to your professor?

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 5592
  • Mole Snacks: +319/-22
Re: Disaccharides
« Reply #4 on: October 21, 2014, 07:23:18 PM »
I was also interested in that question.  After some thought, I agree with Dan that the number is probably 20, as long as we restrict ourselves to pyranoses.

Offline orgo814

  • Full Member
  • ****
  • Posts: 412
  • Mole Snacks: +11/-6
Re: Disaccharides
« Reply #5 on: October 21, 2014, 09:06:22 PM »
He said about half that number so probably around 20

Sponsored Links