Ranalexin is a 20-amino acid antimicrobial peptide isolated from the skin of the American bullfrog, Rana catesbeiana. Its sequence is Phe-Leu-Gly-Gly-Leu-Ile-Lys-Ile-Val-Pro-Ala-Met-Ile-Cys-Ala-Val-Thr-Lys-Lys-Cys, featuring a single intramolecular disulfide bond that forms a heptapeptide ring. Structurally, Ranalexin resembles the bacterial antibiotic polymyxin, which also contains a similar heptapeptide ring. The peptide is initially synthesized as a propeptide with a putative signal sequence and an acidic amino acid-rich region at its amino-terminal end.

Ranalexin mRNA expression begins during metamorphosis in R. catesbeiana and persists into adulthood, primarily in the granular glands of the skin. The peptide exhibits broad-spectrum antimicrobial activity, with its hydrophobic amino-terminal region and the integrity of the disulfide-bonded ring being critical for its function. The structural and functional similarities between Ranalexin and polymyxin suggest a conserved mechanism of membrane interaction in antimicrobial peptides.

• Clark DP, Durell S, Maloy WL, Zasloff M. Ranalexin. A novel antimicrobial peptide from bullfrog (Rana catesbeiana) skin, structurally related to the bacterial antibiotic, polymyxin. J Biol Chem. 1994 Apr 8;269(14):10849-55. PMID: 8144672.

三氟乙酸（TFA）是一种强酸，常用于从固相树脂中裂解合成的多肽，也可用于提高多肽纯化步骤中的HPLC性能。默认情况下，定制的多肽是以冻干的TFA盐的形式交付的，其中TFA的含量可能高达10-45%。

定制多肽中的TFA可能会导致后续检测数据出现无法解释的差异。例如，nM浓度的TFA已被证明会干扰细胞实验，在某些情况下会抑制细胞增殖，而在其他情况下会增加细胞活力。

TFA去除服务推荐用于:

• 将用于细胞试验的多肽
• 将被用作原料药或制成品的多肽
• 含有大量碱性残基的亲水多肽

干品多肽的重量中不仅仅包含多肽，还包含有一些非肽的组份，如水、被吸收的溶剂、配位离子和盐等。肽的净含量是指肽在其中的重量百分比，这个百分比的数值范围很大，可能从50%到90%，取决于纯度、序列以及合成和纯化的方法，不要将肽的净含量和肽的纯度混为一谈，他们是两个完全不同的概念。纯度通常是通过HPLC测定的。纯度定义的是多肽样品中含正确序列的组分的百分比，而肽的净含量是指样品中肽类物质相对于总物质所占的百分比，肽的净含量通常是氨基酸组分分析或紫外分光法测定的，这个信息主要是在一些对肽的浓度很敏感的实验中，对计算肽的浓度是很重要的。