Loops are majorly random coil, turn are loops teminated by hydrogen dons at two ends. DSSP, Stride and so on on easily identify these two different structure motifs.

@Li- Thanks. But on DSSP or stride i find option only for turns and not for loops. 

Generally speaking, these two structural motifs cannot be classified as either strands or helices. They both are degenerate. They typically connect a {helix, strand} to another {helix, strand}. The main difference is the length; turns are short and loops are longer. A specific software tool might give its own definitions. To my knowledge, PyMoL and other software do not differentiate between them. The regular VIBGYOR color coding is followed from N to C terminal which encompasses both loops and turns where the prominence is given to helices and strands. Best regards.

@Chitturi - Thanks sir. You have mentioned  "turns are short and loops are longer". Do you have an idea about exact size that makes difference?

Normally, random coil is defined as anything that is not either helix, sheet or turn. beta turns normally are four-amino acid motifs, which stabilize a 180° change in chain direction by a hydrogen bond linking the backbone or residue I to i+3. https://en.wikipedia.org/wiki/Beta_turn

Dr Thirumal Kumar D, is there any free tool/server to locate the exact position of "loops" in a structure? i.e. from residue X to residue Y.

Please let me know.

Thanks!

@Puneet Kumar: No, loops are too heterogeneous to predict directly. Their boundaries are defined by the boundaries of the flanking regular secondary structure elements.

Are the "turns" the secondary structures?

Patrícia Pereira Duzi Carvalho well, yes and no. They are not essentially what makes up a secondary structure in proteins like helices and strands (sheets) but they are involved in make up of the secondary structures since they link the main constituents of these structures. So technically it is a part of secondary protein structures but not a major constituent.