Solubilization of Charged Porphyrins in Interpolyelectrolyte Complexes: A Computer Study

• Main Authors: Karel Šindelka, Zuzana Limpouchová, Karel Procházka
• Format: Article
• Language: English
• Published: MDPI AG 2021-02-01
• Series: Polymers
• Subjects: computer simulations; porphyrin; electrostatic co-assembly; IPEC; solubilization
• Online Access: https://www.mdpi.com/2073-4360/13/4/502

Using coarse-grained dissipative particle dynamics (DPD) with explicit electrostatics, we performed (i) an extensive series of simulations of the electrostatic co-assembly of asymmetric oppositely charged copolymers composed of one (either positively or negatively charged) polyelectrolyte (PE) block A and one water-soluble block B and (ii) studied the solubilization of positively charged porphyrin derivatives (P<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mo>+</mo></msup></semantics></math></inline-formula>) in the interpolyelectrolyte complex (IPEC) cores of co-assembled nanoparticles. We studied the stoichiometric mixtures of 137 A<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mrow></mrow><mrow><mn>10</mn></mrow><mo>+</mo></msubsup></semantics></math></inline-formula>B<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mn>25</mn></msub></semantics></math></inline-formula> and 137 A<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mrow></mrow><mrow><mn>10</mn></mrow><mo>−</mo></msubsup></semantics></math></inline-formula>B<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mn>25</mn></msub></semantics></math></inline-formula> chains with moderately hydrophobic A blocks (DPD interaction parameter <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>a</mi><mrow><mi>A</mi><mi>S</mi></mrow></msub><mo>=</mo><mn>35</mn></mrow></semantics></math></inline-formula>) and hydrophilic B blocks (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>a</mi><mrow><mi>B</mi><mi>S</mi></mrow></msub><mo>=</mo><mn>25</mn></mrow></semantics></math></inline-formula>) with 10 to 120 P<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mo>+</mo></msup></semantics></math></inline-formula> added (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>a</mi><mrow><mi>PS</mi></mrow></msub><mo>=</mo><mn>39</mn></mrow></semantics></math></inline-formula>). The P<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mo>+</mo></msup></semantics></math></inline-formula> interactions with other components were set to match literature information on their limited solubility and aggregation behavior. The study shows that the moderately soluble P<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mo>+</mo></msup></semantics></math></inline-formula> molecules easily solubilize in IPEC cores, where they partly replace PE<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mo>+</mo></msup></semantics></math></inline-formula> and electrostatically crosslink PE<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mo>−</mo></msup></semantics></math></inline-formula> blocks. As the large P<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mo>+</mo></msup></semantics></math></inline-formula> rings are apt to aggregate, P<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mo>+</mo></msup></semantics></math></inline-formula> molecules aggregate in IPEC cores. The aggregation, which starts at very low loadings, is promoted by increasing the number of P<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mo>+</mo></msup></semantics></math></inline-formula> in the mixture. The positively charged copolymers repelled from the central part of IPEC core partially concentrate at the core-shell interface and partially escape into bulk solvent depending on the amount of P<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mo>+</mo></msup></semantics></math></inline-formula> in the mixture and on their association number, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>A</mi><mi mathvariant="normal">S</mi></msub></semantics></math></inline-formula>. If <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>A</mi><mi mathvariant="normal">S</mi></msub></semantics></math></inline-formula> is lower than the ensemble average <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow><mo>⟨</mo><msub><mi>A</mi><mi mathvariant="normal">S</mi></msub><mo>⟩</mo></mrow><mi mathvariant="normal">n</mi></msub></semantics></math></inline-formula>, the copolymer chains released from IPEC preferentially concentrate at the core-shell interface, thus increasing <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>A</mi><mi mathvariant="normal">S</mi></msub></semantics></math></inline-formula>, which approaches <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow><mo>⟨</mo><msub><mi>A</mi><mi mathvariant="normal">S</mi></msub><mo>⟩</mo></mrow><mi mathvariant="normal">n</mi></msub></semantics></math></inline-formula>. If <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>A</mi><mi mathvariant="normal">S</mi></msub><mo>></mo><msub><mrow><mo>⟨</mo><msub><mi>A</mi><mi mathvariant="normal">S</mi></msub><mo>⟩</mo></mrow><mi mathvariant="normal">n</mi></msub></mrow></semantics></math></inline-formula>, they escape into the bulk solvent.