Monosaccharide templates for de novo designed 4-alpha-helix bundle proteins: template effects in carboproteins

De novo design and total chemical synthesis of proteins provide powerful approaches to critically test our understanding of protein folding, structure, and stability. The 4-alpha-helix bundle is a frequently studied structure in which four amphiphilic alpha-helical peptide strands form a hydrophobic core. Assembly of protein models on a template has been suggested as a way to reduce the entropy of folding. We have previously developed the concept of carbohydrates as templates in the de novo design of protein models termed 'carboproteins'. Here we present the chemical synthesis of three 8.1 kDa 4-alpha-helix bundles by oxime ligation of tetra-aminooxyacetyl functionalized D-galacto-, D-gluco-, and D-altropyranoside templates with an amphiphilic C-terminal hexadecapeptide aldehyde sequence. CD spectroscopy indicated that the choice of template has an effect on the overall structure of the carboprotein, as the altro-based carboprotein was found to be more alpha-helical than the corresponding galacto- and gluco-carboproteins. However, an influence on stability could not be detected in the present experiments, as the three carboproteins gave similar free energy of foldings (deltaG(F)H2O) and melting points in chemical and thermal denaturation experiments.