Potato virus X (PVX) and narcissus mosaic virus (NMV) were studied using vibrational Raman optical activity (ROA) in order to obtain new information on the structures of their coat protein subunits. The ROA spectra of the two intact virions are very similar to each other and similar to that of tobacco mosaic virus (TMV) studied previously, being dominated by signals characteristic of proteins with helix bundle folds. In particular, PVX and NMV show strong positive ROA bands at approximately 1340 cm(-1) assigned to hydrated alpha-helix and perhaps originating in surface exposed helical residues, together with less strong positive ROA intensity in the range approximately 1297-1312 cm(-1) assigned to alpha-helix in a more hydrophobic environment and perhaps originating in residues at helix-helix interfaces. The positive approximately 1340 cm(-1) ROA band of TMV is less intense than those of PVX and NMV, suggesting that TMV contains less hydrated alpha-helix. Small differences in other spectral regions reflect differences in some loop, turn and side-chain compositions and conformations among the three viruses. A pattern recognition program based on principal component analysis of ROA spectra indicates that the coat protein subunit folds of PVX and NMV may be very similar to each other and similar to that of TMV. These results suggest that PVX and NMV may have coat protein subunit structures based on folds similar to the TMV helix bundle and hence that the helical architecture of the PVX and NMV particles may be similar to that of TMV but with different structural parameters.