I read in many journals that amino acids with branched and large aromatic R-groups have higher beta pleated sheet propensities. However, none really go in depth into the significance or reasoning behind this finding. I have made an educated guess based on my findings on alpha helix propensities:
In alpha helices, branched and aromatic R-groups are not favored as the g+ and g- isomers can extend into the helix and interfere with the hydrogen bonding. In beta pleated sheets, however, the R-groups are pointed perpendicular to the hydrogen bonds, and even large R-groups like that of phenylalanine or tryptophan will not able to reach the site of hydrogen bonding. Coupled with the fact that smaller amino acids will rather be more tightly bound to each other (so they prefer helices, e.g. collagen helix, alpha helix, save space, amide hydrogens and carbonyl oxygens closer together), larger amino acids therefore favor formation of beta pleated sheets.
I need a few key pieces of info to really accept my own explanation (if it is correct) as factual. First, I will need proof that beta pleated sheets are more structurally rigid than alpha helices (or else, smaller R-groups like that of alanine and cysteine will also prefer beta pleated sheets). I also kinda want to see the structure of beta pleated sheets with large R-groups to further ground the thought in my head, as I can't really see how R-groups like valine (heavily branched) will not affect hydrogen bonding. As an example, I attach one that I found for alpha helices.