Terms Of Packing Research Articles

Role of proline ... proline interactions in the packing of collagenlike poly(tripeptide) triple helices.

AbstractConformational energy computations were carried out on the packing of two identical collagenlike poly(tripeptide) triple helices in order to determine the energetics of favorable packing arrangements as a function of composition and chain length. The triple helices considered were [CH3CO‐(Gly‐Pro‐Pro)nt‐NHCH3]3 and [CH3CO‐(Gly‐Pro‐Ala)nt‐NHCH3]3, with nt = 3, 4, and 5. The packing arrangements were characterized in terms of their intermolecular energies and orientation angles Ω0 of the axes of the two triple helices. For short triple helices (nt = 3 or 4), many low‐energy orientations, with a wide range of values of Ω0, can occur. When the triple helices are longer (nt = 5), the only low‐energy packing arrangements of two poly(Gly‐Pro‐Pro) triple helices are those with a nearly parallel orientation of the two helix axes, with Ω0 ≈ −10°. This result accounts for the observed parallel (rather than antiparallel) arrangement of collagen molecules in microfibril assembly and stands in contrast to the preferred antiparallel arrangement of a pair of α‐helices. Since the preference for a parallel arrangement of these collagenlike triple helices is less pronounced in the case of poly(Gly‐Pro‐Ala), it appears that this preference is a consequence of the frequent presence of imino acids in position Y of the Gly‐X‐Y repeating triplet. In poly(Gly‐Pro‐Ala), most of the low‐energy packing arrangements are parallel, but a few arrangements with low energies and high values of |Ω0| occur. These packing arrangements have a high energy, however, when Pro is substituted for Ala, and thus they are not accessible for collagen with natural amino (imino) acid sequences. The computations reported here account for some of the characteristic features of collagen packing in terms of the local interaction energies of a pair of triple helices.

A shredded-gel bolus material for use with an EMI CT1010 brain scanner.

Current procedure for the EMI CT1010 brain scanner requires the application of packing around the patient's head prior to scanning. Such bolus material forms a supplement to the internal carbon wedges of the machine. Together these elements act in a similar manner to the “water bag” on the earlier generation machines, in both limiting the dynamic range of the intensity of the transmitted X-ray beam detected, and also providing additional pre-filtering of the polychromatic X rays to reduce beam hardening effects within the head itself. However, using the thin-rubber, powder-filled patient bolus bags presently supplied by EMI often leads to difficulties in achieving adequate packing in terms of distribution of bolus material around the head. As discussed briefly below, failure to attain proper bolussing can result in very serious artefacts in the CT image (termed “banding”, “streaking” and “shading”), which threaten the clinical usefulness of the scans (EMI, 1977). The purpose of this technical note is to describe an alternative bolus material which is especially suitable for rapid packing of small heads.