Biomedical subjects

Marcus R Helfrich

Publications and source records attributed to Marcus R Helfrich.

4 recordsLinked to original sources

PubMed2005

Partitioning and assembly of metal particles and their bioconjugates in aqueous two-phase systems.

The behavior of metal nanospheres and nanowires and their bioconjugates in aqueous two-phase systems (ATPS) is described. The ATPS used in this work comprised poly(ethylene glycol) (PEG), dextran, and water or aqueous buffer. Au and Ag nanospheres less than 100 nm in diameter partition between the PEG-rich and dextran-rich phases on the basis of their surface chemistry and can be separated on this basis. Larger Au nanospheres and wires accumulate at the interface between the two aqueous phases. The influence of polymer molecular weight and concentration on interfacial assembly of Au wires is described. DNA-derivatized nanowires at the aqueous/aqueous interface retain the ability to selectively bind to fluorescent complementary DNA. In addition, Au nanoparticles have been bound to Au wires via selective DNA hybridization at the ATPS interface. Transmission electron microscopy and thermal denaturation experiments confirm that DNA-driven assembly is responsible for the formation of the nanosphere/wire assemblies. These results demonstrate the biocompatibility of the two-phase interface and point to future use as scaffolding in biorecognition-driven assembly.

DNA↗

PubMed2005

Dynamic microcompartmentation in synthetic cells.

An experimental model for cytoplasmic organization is presented. We demonstrate dynamic control over protein distribution within synthetic cells comprising a lipid bilayer membrane surrounding an aqueous polymer solution. This polymer solution generally exists as two immiscible aqueous phases. Protein partitioning between these phases leads to microcompartmentation, or heterogeneous protein distribution within the "cell" interior. This model cytoplasm can be reversibly converted to a single phase by slight changes in temperature or osmolarity, such that local protein concentrations can be manipulated within the vesicle interior.

Base Sequence↗

PubMed2004

Effect of macromolecular crowding on DNA:Au nanoparticle bioconjugate assembly.

DNA:Au nanosphere bioconjugates have applications in biosensing and in the bottom-up assembly of materials. These bioconjugates can be selectively assembled into three-dimensional aggregates upon addition of complementary DNA oligonucleotides and can be dissociated by heating above a melting transition temperature at which the DNA duplexes are denatured. Herein we describe the impact of polymeric solutes on the thermal denaturation behavior of DNA:Au nanoparticle bioconjugate assemblies. Polymeric solutes can dramatically impact biochemical reactions via macromolecular crowding. Poly(ethylene glycol)s (PEGs) and dextrans of varying molecular weights were used as crowding reagents. While both PEG and dextran increased the stability of DNA:Au aggregates, melting transition temperatures in the presence of PEG were impacted more significantly. Polymer molecular weight was less important than polymer chemistry and weight percent in solution. For a high (15%) weight percent of PEG, aggregation was observed even in the absence of complementary oligonucleotides. These results underscore the importance of polymer chemistry in addition to physical volume exclusion in macromolecular crowding and point to the importance of understanding these effects when designing biorecognition-based nanoparticle assembly schemes in complex matrixes (i.e., any involving polymeric solutes).

Base Sequence↗

PubMed2002

Aqueous phase separation in giant vesicles.

We report the synthesis and initial characterization of approximately 10 mum diameter lipid vesicles that contain two distinct aqueous phases. The aqueous two-phase system is a dextran/poly(ethylene glycol) solution that exhibits temperature-dependent phase behavior. Vesicles were prepared above the phase transition temperature of the polymer solution. Upon cooling to room temperature, the polymer solution phase separated both within the vesicles and in the bulk solution. The location of poly(ethylene glycol)-rich and dextran-rich phases was determined by fluorescence microscopy. These structures are exciting in that they enable for the first time the interior volume of liposomes to be structured.

Dextrans↗