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Retrogradation kinetics for a potato starch‐water system (10% w/w gel) was monitored by Fourier Transform Infrared spectroscopy and compared with waxy maize starch. The spectra showed the C‐C and C‐O stretching region (1300‐800 cm−1) to be sensitive to the retrogradation process. A multi‐stage process was observed during the retrogradation of potato starch and characterized as the formation of short‐ and long‐range order. The first stage was characterized as the formation of helices and the fast formation of crystalline amylose regions. The second stage was described as the induction time for amylopectin helix aggregation. Stage three was described as the helix‐helix aggregation and the crystallization of amylopectin. The overall‐first order calculated rate constant of potato starch was (9.6±1.4) 10− 3h−1. The calculated rate constant were in agreement with the known difference in retrogradation kinetics of waxy maize and potato starch. The effects were explained by the differences in retrogradation rate of amylopectin and amylose. Potato starch consists of amylose as well as amylopectin. Whereas amylose crystallization occurs within a few hours, amylopectin crystallization is slow and takes a few weeks.

Incorporation of the channel-forming antibiotic gramicidin into the membrane of human erythrocytes highly (up to 30-fold) enhances rates of reorientation (flip) of lysophosphatidylcholine and palmitoylcarnitine to the inner membrane layer after their primary incorporation into the outer layer. Despite the high increase of flip rates by gramicidin, the asymmetric orientation of the inner membrane layer phospholipids phosphatidylethanolamine and phosphatidylserine is stable as demonstrated by the lack of accessibility of these lipids toward cleavage by exogenous phospholipase A2. On the other hand, gramicidin enhances the rate of cleavage of outer membrane layer phosphatidylcholine by phospholipase A2, which indicates changes in the packing of phosphatidylcholine following gramicidin binding. The increase of flip becomes detectable when about 10(5) copies of gramicidin per cell have been bound (gramicidin to membrane phospholipid ratio of 1:2000). This is a 1000-fold higher concentration than that required for an increase of K+ permeability mediated by the gramicidin channel. Acceleration of flip is thus not simply correlated with channel formation. The enhancement of flip is markedly dependent on structural details of gramicidin. Formylation of its four tryptophan residues abolishes the effect. Even at high concentrations of formylated gramicidin at which the extents of binding of native and of formylated gramicidin to the membrane are comparable, no flip acceleration is produced. Enhancement of flip by gramicidin occurs after a temperature-dependent lag phase. At 37 degrees C, flip rates begin to increase within a few minutes and at 25 degrees C, only after 3 h. This lag phase is most likely not due to limitations by the rate of binding of gramicidin to the membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

It is shown by 3t P-NMR and small angle X-ray scattering that induction of an hexagonal H n phase in dioleoylphosphatidylcholine model membranes by external addition of gramicidin A' depends on the solvent which is used to solubilize the peptide. Addition of gramicidin from dimethylsulfoxide or trifluoroethanol solution leads to H n phase formation whereas addition of the peptide from ethanol does not. This solvent dependence is shown by circular dichroism to be correlated with the peptide conformation. The channel conformation appears to be responsible for H n phase formation by gramicidin.Gramicidin A is an effective promoter of hexagonal HII phases in model and biological membranes [1]. This effect is a consequence of the specific chemical structure of this pentadecapeptide, in particular with respect to the presence of the four tryptophans at the C-terminal part of the molecule [1][2][3]. It was proposed that these bulky residues would provide a pronounced cone shape to the molecule which, within the shape-structure concept of polymorphism [4], together with the peptide's tendency to self associate into cylindrical structures [5][6][7] would explain the Hll phase-inAbbreviations: CD, circular dichroism; DMSO, dimethylsulfoxide; DOPC, dioleoylphosphatidylcholine.