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Paul S
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Chances are the main reason carbohydrate polymers have been neglected is that they're less versatile than polypeptides and nucleic acids. Also, if you want to make a complex carbohydrate sequence (multiple different monomers in a specific arrangement) there isn't any pre-existing biochemical machinery (e.g. genes) that you can co-opt to do it consistently every time. It's not the ability to create side-chains. You can do that with amino acids. It's not the ability to cross-link. One amino acid - cysteine - is specialized for exactly that role. Nor is cross-linking all that difficult in wet chemistry: we do it in polymer products ranging from paint to tires. Chitin, BTW, is not heavily cross-linked. Its durability comes in part from extensive hydrogen bonding between polymer chains, and hydrogen bonds aren't "chemical bonds" in the usual sense of that term. Stiffness may or may not scale well - I'm not enough of a materials chemist to know. But I do know that the stiffness of chitin in arthropods comes from the addition of a strong protein matrix to the chitin - at which point you've got a much more complex material. Pure chitin is tough, but not very stiff at all. In fact it's used as surgical thread. So my guess is that it's simply that carbohydrates aren't as generally interesting and useful as peptides. Basically, there's very little that a carbohydrate polymer could do that a peptide could not. The main advantages carbohydrates have as biological building materials (chitin, lignin, etc.) are simplicity and ubiquity (carbohydrates are produced from air by photosynthesis).