Abstract

Introduction

Badly healing wounds are still a big issue in clinical medicine all over the world. Especially inflamed wounds often exhibit impaired healing properties and are prone to infections of opportunistic pathogenic bacteria. On the one hand, wounds have to be screened for infections extensively, on the other hand, they have to be kept wet and in an Oxygen-free atmosphere for optimal healing conditions. Thus there is an obvious contradiction between the best healing conditions and the commonly used infection swab test, where you have to remove the wound coating. Furthermore, current swab tests needs a few days to evaluate the presence of pathogenic bacteria, but it is important to get this information as soon as possible and to start the suitable treatment. Ebrahimi and Schönherr developed a quick and non invasive detection method for wound infections without the necessity to remove the wound coating. The principle of the test is the modification of an amino group of chitosan with succinic anhydride to gain a carboxyl group that can be linked to the amino group of our alanyl-alanyl-phenylalanine peptide linker. This peptide linker is fused to our actual detectable unit, the methylcoumarin.

This linker was chosen due to the fact that chymotrysin cleaves peptides n-terminal of aromatic amino acids.

In our studies, we present an uncomplicated method to repeat the work of Ebrahimi and Schönherr in a way that works for iGEMers and FabLabers. For that, we combined a few instructions to guarantee a working product without the need of expensive instrumental analysis.

Methods

Material:

Chitosan (high molecular weight, 310-375 kDa, >75% deacetylated), succinic anhydrite, ala-ala-phe-7-amido-4-methylcoumarin, 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide and 1-Hydroxy-2,5-pyrrolidindion were purchased from Sigma-Aldrich. Acetic acid and Acetone were given from AK Fessner (TU Darmstadt).
We used 250 mL-Dreihalskolbenrührapparatur with a dropping funnel, a spin coater and laboratory shakers.
Laboratory and Lab equipment were provided by AK Fessner and AK Kolmar (TU Darmstadt) and the centrifuge was provided by AK Hausch. Spin-coating was performed in the Laboratory of Prof. Koeppl (TU Darmstadt).

Preparation of N-Succinic Chitosan (NSC)

Aqueous Acetic acid solution was put in the Dreihalskolbenrührapparatur. Chitosan was added in small doses through the flask neck under heavy mechanical stirring. Therefore a lab stirrer was needed, not just a stir bar, because the solution becomes very viscous. The solution was stirred until becoming a homogenous semi-fluid. Now 20 mL of a saturated succinic anhydrite solution in acetone were prepared and added drop-wise to the reaction flask under heavy mechanical stirring for 30 minutes. The reaction mixture was left overnight at room temperature. The following day, it was centrifuged to separate the precipitate from the left, non reacted Chitosan.

Preparation of thin-layer Chitosan/N-Succinic Chitosan

Spin coating is a suitable technique to prepare thin layers out of viscose fluids. In principle, a spin coater is a device where a surface you want to coat is clamped onto a centrifuge. Through fast rotation of the surface, a fluid is distributed evenly. We used a microscope slide as surface and coated it with our Chitosan/N-Succinic Chitosan mixture from the previous step. To determine the best fluid properties we performed four test with different amounts of chitosan and acetic acid concentrations. Every coating step with the spin-coater was performed at 400 rpm for 20 seconds and then 2000 rpm for 60 seconds.

Succinyl saturation of Chitin

To ensure a sufficient amount of N-Succinic Chitosan in our product we performed a second succinylation of the thin-layer. For this step, we tested two different procedures.
1. Succinic anhydride in Acetone
2,5 g Succinic anhydride was soluted in 100 ml Aceton and was shaked on 37 ° for 72 hours. The glas petri dish was sealed with parafilm and the evaporated Aceton was replaced every 12 hours.
2. Succinic anhydride in DMSO
We dissolved succinic anhydride in DMSO and shook it at 60°C for x hours.

Grafting of Ala-Ala-Phe-7-Amido-4-methylcoumarin (AAP-AMC) to N-Succinic Chitosan

The linkage of the AAP-AMC was performed in two steps. First 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) was dissolved in methanol and 1-Hydroxy-2,5-pyrrolidindion (NHS) was added. A separate solution of AAP-AMP in methanol was prepared. The immobilized NSC film was immersed in the EDC/NHS solution for 60 minutes while shaking. The film was rinsed with methanol. Afterwards, the NSC film was immersed in the AAP-AMP solution for 60 minutes. The film was rinsed again with methanol and immersed in methanol for 60 minutes, the methanol was replaced every 15 minutes. The film was rinsed for the last time with methanol an dried.

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