Stem-length requirements for chain folding of periodic polypeptides

Proteins of uniform sequence and stereochemistry can be produced in bacterial cells for use in fundamental studies of polymer morphology. In earlier work, a copolypeptide with repetitive sequence 1, designed to adopt a lamellar architecture upon crystallization, was synthesized using genetic engineering techniques. Solid state studies indicated that the material did not adopt the desired architecture, perhaps because the odd number of amino acids in the repeat unit prevented the formation of the appropriate chain trajectory for chain folding, disrupting the normal hydrogen bond pattern associated with $\beta$-sheets (a key component of the desired chain-folded assembly). It was asserted that this energetic liability would be offset by adding hydrogen-bonding pairs to the bulk structure, by insertion of additional alanylglycine dyads in sequence 1.$$\lbrack\rm (AlaGly)\sb3ProGluGly\rbrack\sb{\rm m}\eqno{\bf 1}$$ To test this premise, solid state analysis of a series of repetitive copolypeptides with increasing numbers of alanylglycine dyads (sequences 2) was accomplished.$$\lbrack\rm (AlaGly)\sb{\rm n}ProGluGly\rbrack\sb{\rm m}\quad n = 3, 4, 5, 6\eqno{\bf 2}$$In a preliminary study, a copolypeptide with repetitive sequence ((AlaGly)$\sb4$ProGluGly) $\sb{14}$ was synthesized. X-ray diffraction and infrared analysis of this material indicated an increase in solid state order over that of polypeptides with sequence 1. Mass analysis of this material using matrix-assisted laser-desorption/ionization (MALDI) mass spectrometry prompted the discovery of sequence errors in the DNA code for the protein. This analysis and the analysis of other chain-length variants of sequence 2 (n = 4, m = 10, 12, 16, and 20) highlighted the utility of the MALDI technique for the assessment of protein sequence, molecular weight and purity. Analysis of proteins with sequence 2 (m = 16, n = 3, 4, 5, 6) using differential scanning calorimetry, infrared spectroscopy, electron microscopy and X-ray diffraction was consistent with an increase in solid-state order with an increase in the number of alanyl-glycine dyads in the repetitive sequence. A model is presented featuring folded chains, which is rationalized on the basis of comparison of the X-ray scattering patterns of the materials with those of poly(L-alanylglycine)