A linear peptide is an unbranched chain of amino acids joined by peptide bonds, carrying a free N-terminus at one end and a free C-terminus at the other, with no covalent ring closure. The great majority of research peptides fall into this category, including BPC-157, ipamorelin, the GHRH analogs and the incretin family.
Why linear structure matters in peptide research
The advantages are practical. Linear sequences are the natural output of solid-phase synthesis with no additional closure step, so yields are higher and cost per milligram lower; characterisation is simpler, because mass and retention time together largely settle identity without connectivity questions.
The cost is exposure. Free termini are substrates for aminopeptidases and carboxypeptidases, and an unconstrained chain samples many conformations, so intrinsic affinity is often lower than a constrained equivalent. That is why linear peptides are so often the starting point for modification rather than the endpoint: N-terminal acetylation and C-terminal amidation cap the termini, while PEGylation and DAC slow clearance. Contrast with the cyclic peptide entry; see how sequences are written.