Turbidity and light scattering measurements, along with phase contrast microscopy, were used to follow the processes leading to coacervation when aqueous solutions of bovine serum albumin (BSA) and poly-(dimethyldiallylammonium chloride) (PDADMAC) were brought from pH ) 4 to 10. The state of macromolecular assembly of complexes formed between BSA and PDADMAC prior to and during the pH-induced coacervation could be characterized by specific pH values at which recognizable transitions took place. In addition to the two characteristic pH values (pH crit and pH φ ) previously identified through turbidimetry, other transitions were explicitly established. On the basis of the pH-induced evolution of scattering intensity measurements, we concluded that the formation of soluble primary protein-polymer complexes is initiated at pH crit and proceeds until "pH′ crit ". A subsequent increase in scattering intensity at "pH pre " may arise from the assembly of quasi-neutralized primary complexes as their net positive charge decreases with increase in pH. Subsequently, a maximum in scattering intensity at pH φ is observed coincident with the appearance of turbidity and also corresponding to the first microscopic observation of coacervate droplets. The temperature independence of pH crit and pH φ suggests that hydrophobic contributions are negligible for the initial BSA-PDADMAC interactions and the subsequent coacervation process. The pH dependence of scattering intensity profiles allowed the identification of two other transitions beyond pH φ . Spherical microcoacervate droplets first observed around pH φ subsequently displayed morphological changes at "pH morph ", followed by the transformation to solid or flocculant substances at pH precip.

Ion transport characteristics across a macrocoacervate layer membrane composed of aqueous elastin model polypeptides with a specific repeating pentapeptide sequence, H‐(Val‐Pro‐Gly‐Val‐Gly)n‐Val‐OMe (n ≥ 40), were investigated. Transmembrane potential responses for NaCl, MgCl2, and CaCl2 concentration‐cell systems were measured and examined systematically by comparing with those across a coacervate membrane composed of bovine neck ligamental α‐elastin. In the case of the NaCl and MgCl2 systems, potential responses across these protein liquid membranes were different noticeably from each other depending upon the molecular structure with and without charged peptide side chains, whereas in the CaCl2 systems the transmembrane potential responses across the noncharged polypentapeptide coacervate membrane were comparable with those across the α‐elastin coacervate membrane carrying both the positively and negatively charged amino acid residues as an amphoteric ion‐exchange membrane. These results indicated that mechanisms of major Ca2+ ion transport are based on the specific and selective interactions with electrically neutral sites of elastin, such as the polypentapeptide backbone chain. © 1996 John Wiley & Sons, Inc.

The elastin precursor tropoelastin possesses a number of polymeric peptides with repeating 3-9 mer sequences. One of these is the pentapeptide Val-Pro-Gly-Val-Gly (VPGVG) present in almost all animal species, and its polymer (VPGVG)n coacervates just as does tropoelastin. In the present study, in order to explore the structural requirements essential for coacervation, (VPGVG)n and its shortened repeat analogs (VPGV)n, (VPG)n, and (PGVG)n were synthesized and their structural properties were investigated. In our turbidity measurements, (VPGVG)n demonstrated complete reversible coacervation in agreement with previous findings. The Gly(5) -deleted polymer (VPGV)n also achieved self-association, though the onset of self-association occurred at a lower temperature. However, the dissociation of (VPGV)n upon temperature lowering was found to occur in a three-step process; the Val(i) (4) -Val(i+1) (1) structure arising in the VPGV polypeptide appeared to perturb the dissociation. No self-association was observed for (VPG)n or (PGVG)n repeats. Spectroscopic measurements by CD, FT-IR, and (1) H-NMR showed that the (VPGV)n and (VPG)n both assumed ordered structures similar to that of (VPGVG)n. These results demonstrated that VPGVG is a structural element essential to achieving the β-spiral structure required for self-association followed by coacervation, probably due to the ideal spatial arrangement of the hydrophobic Val residues.

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