We have found that tri- L -alanine (Ala 3 ) can crystallize in a parallel-chain Β structure in addition to the previously known antiparallel-chain Β structure. Although the chain conformations in each structure are essentially similar, their and Raman spectra are distinctively different. We have calculated the normal modes of each structure, and can account in significant detail for these differences. This demonstrates the essential validity of our empirically refined force fields, as well as showing that deeper insights into polypeptide and protein structure can be achieved through the rigorous analyses of normal mode calculations.
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http://deepblue.lib.umich.edu/bitstream/2027.42/37859/1/360310208_ftp.pdf