We report studies of the structure and packing of Langmuir monolayers (LMs) of polypeptide poly(gamma-4-(n-hexadecyloxy)benzyl alpha,L-glutamate) (C16-O-PBLG) on the surface of water. The molecule is a "hairy rod" and consists of side attachments of hexadecyloxy chains (-O-C16) to the rigid rod-like core made up of alpha-helical poly(gamma-benzyl L-glutamate) (PBLG). Measurements include surface pressure (Pi) versus area/monomer (A) isotherms, x-ray specular reflectivity (XR), and grazing incidence diffraction (GID). In contrast to the LM of bare PBLG on water, which undergoes a monolayer/bilayer transition with increasing Pi, monolayers of C16-O-PBLG remain stable up to the highest densities. On the basis of XR and GID results, the structure of the C16-O-PBLG monolayer is characterized by the following main features. First, hydrophobicity causes the -O-C16 chains to segregate towards the film/gas interface and away from water and the PBLG cores, which sit parallel to and near the water/film interface. Since the attachment position of some of the side chains is at the core/water interface, the segregation forces these chains into the space between neighboring core rods. Compression associated with increasing Pi thickens the film but the internally segregated structure is maintained for all Pi (i.e., >similar to30 dyne/cm). Second, the C16-O-PBLG rods form domains in which the rods are aligned parallel to each other and to the interface. The correlation length for the interhelix positional order of the rods is short and typically comparable to or less than the length of the rods. With increasing Pi the spacing d between nearest-neighbor rods decreases linearly with A at high Pi, indicating a direct correspondence between the macroscopic compressibility and the microscopic interhelix compressibility. Third, as Pi increases past similar to5 dyne/cm, the local packing of tethered -O-C16 chains displays the same herringbone (HB) order that is common for high-density bulk and monolayer phases of alkyl chains. Various features of the observed GID peaks also imply that the HB order of -O-C16 chains is oriented with respect to the helical axes of aligned PBLG cores. We propose that the HB order is established initially by one-dimensionally confined chains between aligned rods at low Pi and grows laterally with compression. (C) 2003 American Institute of Physics.