Raman optical activity of filamentous bacteriophages: hydration of α-helices

Abstract

We report the first observations of vibrational Raman optical activity (ROA) on intact viruses. Specifically, ROA spectra of the filamentous bacteriophages Pf1, M13 and IKe in aqueous solution were measured in the range ∼600–1800 cm−1. On account of its ability to probe directly the chiral elements of biomolecular structure, ROA has provided a new perspective on the solution structures of these well-studied systems. The ROA spectra of all three are dominated by signatures of helical elements in the major coat proteins, as expected from pre-existing data. The helical elements generate strong sharp positive ROA bands at ∼1300 and 1342 cm−1 in H2O solution, but in 2H2O solution the ∼1342 cm−1 bands disappear completely. The spectra are similar to those of polypeptides under conditions that produce α-helical conformations. Our present results, together with results from other studies, suggest that the positive ∼1342 cm−1 ROA bands are generated by a highly hydrated form of α-helix, and that the positive ∼1300 cm−1 bands originate in α-helix in a more hydrophobic environment. The presence of significant amounts of highly hydrated helical sequences accords with the known flexibility of these viruses. Differences of spectral detail for Pf1, M13 and IKe demonstrate that ROA is sensitive to subtle variations of conformation and hydration within the major coat proteins, with M13 and IKe possibly containing more non-helical structure than Pf1. The ROA spectra of Pf1 at temperatures above and below that at which a structural transition is known to occur (∼10 °C) reveal little difference in the protein conformation between the two forms, but there are indications of changes in DNA structure.